Hafler DH 200 DC Voltage on Speaker Terminals

[@johnhermanson5249]

Years ago I built a couple of Dynaco ST120 kits. They were known for being "unstable" at low impedance. I love this kind of video, it's a real training course. Thank you, How could anyone (interested in electronics) not like this?

[@justicelut]

I never get tired of looking at old gear being resurrected!

[@selectorerwin1don12]

Great video in which i love how u highlight the technicalities of the amplifier issues which u fixed

[@sand0077]

Very informative video. Been working on a Crown DC-300A amp that has a 1v dc voltage on one channel that I haven't been able to find the cause of. After viewing your video I now have a new direction to pursue with a possible resistor issue. Thank you for the presentation!

[@MrMaxeemum]

Well done sir. I can't believe the previous people to work on it were so lazy, the schematics are so easy to find. Very servicable and should last another 30 years or so. Keep up the good work, I enjoy your videos.

[@12voltvids]

+MrMaxeemum The shop I used to work at I would have not been permitted to service this. The boss would storm around, and take things and put a DNR on it. (Do Not Repair) He was more interested in selling new, and no matter what the quality of the product, he didn't want to get married to an old unit, so if it was more than 10 years old he would condem stuff. Then it became 5 years. Anything more than 5 years old would get the DNR tag on it. Hard to make a living when the boss didn't permit me to service things no matter how bad the customer wanted it. Now I don't have to worry about that, because I am the boss. Don't make a living doing this anymore, as I have a real job. I probably could make a living doing vintage stuff, because the local vintage audio shop has offered me a job there a few times, but I decline because I enjoy my day job, and doing the odd repair on the weekend is enough for me.

[@martinda7446]

PS Been watching your repairs all day. Great with a cup of coffee on a Sunday. Thanks 12Vol.

[@dontcare563]

This was the very first power amplifier I owned and I purchased in a kit and assembled it around 1980. Great little amp for the time.

[@michaelwolak3098]

Really enjoy all your historical video’s and don’t find any of your video’s boring and always watch till the end

[@larryh8072]

I have a DH 200 that's been sitting on my work bench for about a year. I had tried it after I bought it and found I got a huge thump on the speakers when I turned it on. It was working but I didn't like the turn on transient. I was going to build in a relay turn on delay when I got a round to it. It had about a 250 mv offset after turn on. After I watched your video I checked R39 on both channels. Son of a gun.. both were open. Looks like maybe it's a common problem on the DH200. The offset is now about 50 mv and I will likely trim it out by fine tuning R14 or R12 which are part of the constant current sources for the input diff amps. Thanks for the tip! Saved me hours of troubleshooting...

[@nathanielreid4967]

Yes, one man boredom is another man excitement, I may not comment on all ur vids but I like them all.

[@GordonGidluck]

Insightful video. I have a DH-200 also and got a capacitor kit for it. This inspires me to actually open it up and work on it. These are great amps.

[@cjmmjc2131]

where did you get your cap kit?

[@stevefick3919]

Great video. I had one of these puppies a few years back. Started getting loud 60 Hz through the speakers. Turned out to be a grounding problem and some of the smaller caps were bad. Got it all fixed up and put in a set of gold plated input RCA's. These are really nice amps! I have moved on to greener pastures, however. My equipment is PS Audio, now. Stellar Gain Cell DAC preamp and the S300 power amp. Along with my Monitor Audio Silver 300's, it's an awesome set-up now! Thanks for the video! I call KZbin my "Video Encyclopedia".

[@pacofan1]

I inherited a Hafler amp and Pre amp. I have now reason to believe that it was ever misused. However I would like to have someone as knowledgeable as your shelf to look it over. I was my dad’s. He passed it along to whnenwhen he passed away Thank you, Dave

[@mrGoesto11]

I like watching you troubleshoot. Reminds me of something my electronics prof told me, "50%" of the time it's the connections".

[@williamchow1624]

Picked up a Hafler amp 10 yr ago for free on Craigslist. Been sitting on the shelf. Now I am motivated to fix it. Yes, you do know what you are doing.

[@12voltvids]

+William Chow Get the manual. It is free from Hafler. These were mostly kits so they have good instruction manuals.

[@williamchow1624]

Yes, I will do that. Thanks

[@Darryl603]

Great job! Always nice to have a reference unit on the side. Pretty rewarding to fix something in a few hours that someone else had for a month. When they ask how you fixed it, just say you ran a few tests, that should cover it... I appreciate you taking the time to share the fun with us. The video quality was excellent as well. THUMBS UP!

[@12voltvids]

+Darryl 603 The reference unit wasn't needed, as it was in for a minor repair itself, but was nice to compare a later model that had been updated with running changes, presumably to improve reliability.

[@treadmillrepair754]

I have the same Hafler amplifier in my Lab, maybe the next week I try to fix it. Do you know if someone sales the output fets? Cheers.

[@int53185]

A diagnostic tour de force sir. Very impressive knowledge and skill.

[@ricfair9919]

Not too many people around who would invest the time to get that amp working. You love your work and use a logical approach to repairing this vintage tech. Yes, you should tell customers "I know what I'm doing".

[@12voltvids]

My customers know i know what i am doing.

[@jwl9286]

Not boring to me! We need to get back to basics before we loose it. This of course is more then basics but much of the technology developed in the space age might be lost in chips, modules, and throw away units. This is like going into a chip and making repairs. Thanks again you do hang in there.

[@12voltvids]

jwl9286 There is nothing like working to component level on a discrete circuit. makes you think and use diagnostic skills.

[@MrDirect23]

I learn so much From you. I was along time dealership technician for toyota but love tinkering. With audio. Since I was a kid. Now I’m older and I’m just getting into repairing Some units I like pioneer sx series. 74-1979. I recently acquired a Yamaha cr-1040 and having trouble finding the issue. Turns on and lights all work No sound output at all And won’t come out of protection! I found excessive DC voltage from TP1. About 52.01volts dc and .010v @ TP2 Which is close to 0 the Schematics suck from hifiengine- All internal fuses good and I’m just stuck. Been reading a lot. But I’ve hear these units have ICs that go bad and there’s no parts. Wondering if you’ve dealt with these before. Thx. Anyways love the videos been going through a lot of your content. Thank you in advance.

[@pcno2832]

I always distrusted those bi-polar DC-coupled amplifiers, though I'm sure DC blocking caps big enough to prevent low frequency attenuation would add a lot to the price. These things are fine at long as nothing goes wrong.

[@krr711]

I see a lot of guys that tinker like this use their needlenose pliers as a nut driver. It makes me cringe but this guy certainly knows his business. Thank you for sharing!

[@12voltvids]

Not just the guys that tinker. Lots of shops used them as well because they were always at hand, and the nut driver was always across the shop on the other bench. I have a full set of sockets with a hand driver, but that requires that I get up, go pick up the tool box, break it open and fit the correct nut driver on the handle. Can't be bothered the pliers are right there within reach. Cringe on!

[@eolhcytoos]

I don't know how you do it, but you always do. Another excellent video! Thanks

[@kevinbeckenham3872]

Very good & informative documentary.I love Hafler amplifiers,they well engineered equipment,built to last.

[@ke2244]

I'm repairing my high school buddy's DH200 we built together from kit -new in 1980. I was in tech school at the time. Parts are not hard to find and what are NLA can be substituted with a little spec sheet study and maybe hardware mod. The electronic parts to replace everything in this device might cost $80 if you bought only the finest quality stuff. Mouser and DigiKey, but Mouser is my go-to. The internet is an extremely powerful tool in such endeavors. I'm sorry to say this 12voltvids but your work makes my skin crawl. Using fence pliers to diddle screws tsk tsk. invest in some better tools like a nut driver set and your work will improve -less broken screws and scratches. Remember that those of us who can are making example for those who want to also. That said, kudos to you for courage to video and overall success.

[@carlosmenendez8720]

Could not agree more..

[@mdzacharias]

I used to work on these at a dealer. I remember that one ohm resistor. Finding the first one ran me around. Of course, a lot younger and less experienced back then... good job!

[@12voltvids]

+Mark Zacharias It really didn't take me that long to find. It was the first component I looked at, as I remember it well. I worked on quite a few of these types of amps until my psycho boss decided that we would only work on the brands he sold, that being Yamaha, Technics, Sony, Bose. His reasoning was he didn't want to support other dealers customers, even though these people bought this expensive stuff because they wanted the quality, and these type of customers were extremely desirable because the "audiophools" that spent the kind of money these units cost were more than prepared to spend big dollars fixing the units. On the other hand a person that only spent 499 on a POS Yamaha would toss it if the bill was 100.00. A Hafler, Bryce, McIntosh owner, you could quote them 500 on the repair bill and they wouldn't bat an eye, and those customers become very loyal when they find someone that can service their gear. The guy that owns these pieces will certainly become a repeat customer, for me, because I did what his "own" guy could not. I fixed the problem, quickly, and didn't blow a bunch of smoke up his ass. I called him, and got right to the point. 3 resistors bad. Capacitors look OK at this point. If he decides that he wants to recap it, I would be happy to do it, but at this point I don't see any signs that they are failing.This is why, 13 years after I left the TV shop, people are still looking me up and bring me stuff to fix, and I have even had stuff sent in from as far away as Chicago to be repaired. Same with that big Toshiba that had an open resistor. First component I checked. I am pretty much sticking to vintage audio and video these days. No money in TVs and most LCD sets are a write off. Plasma I have had good success with, but LCD sets, the ones I have fixed I have featured on the channel, but there are many more that were not repaired due mostly due to bad screens.

[@mrGoesto11]

@@12voltvids Just curious but why didn't you test the filter caps for leakage before deciding they were good?

[@12voltvids]

@@mrGoesto11 Because filter caps are usually not the problem. Resistors are.

[@GammaSierra]

First I've heard of this brand. Looked up the specs and they are impressive. THD at 100W into 8 ohm speakers at 1KHz: 0.0015% or 10KHz: 0.005%

[@larrydering1598]

Another great repair by a pro technician. Love the work and your method.

[@isettech]

Properly sized speaker fuses or an amp with working speaker protection are essential. When bench testing, sometimes the DC is limited current such as one side of the the output pair failed and a load will bring the output to normal voltage, but with a signal will have positive or negative missing on the scope. Use a load resistor to test. I use 250 Watt 6.4 ohm for bench tests. This will quickly differentiate a DC offset issue from an open leg of the amp. Another essential for my bench is a power monitor. An amp with a shorted output transistor putting high DC power into the load resistor will first show up as high AC power draw. Very inexpensive AC wattmeters can be purchased from China for under $15 delivered. Many even have an adjustable alarm function that flash when the wattage setpoint is exceeded. Power on surge is OK, Continued high wattage is a sure indication of damaging current.

[@2tallB]

Very interesting point about testing signal with a load. I’ll keep that in mind. 👍🏼

[@MrTopsoil911]

i just replaced a power cord on a pioneer sa9900, i recapped,deoxit all the pots and controls and adjusted bias as someone attempted to fix. now it runs great and the customer is happy. i only charged $35

[@12voltvids]

+nigel mahabir This amp had been quoted at over 300 at a shop to repair.I was considering replacing all the caps, but those big American made caps looked to be in good shape so I saved the client some bucks, as those big ones are quite pricey these days.

[@thegreener196]

you are an electronics bad ass! just picked up a couple of haflers and i wish you were in the spokane, wa, area. I need to have them checked out. I ran across your vid looking for some help on checking these haflers out but this stuff is way over my head but i enjoyed watching very much. excellent content.

[@Bill_N_ATX]

I had a DH500 that was rock solid. You could damn near arc weld with it. Lost it in a house fire otherwise I’m sure it would still be working almost 40 years later.

[@dhpbear2]

I"m wondering if that R39 is under-rated at 0.5W, since BOTH of them failed!

[@scottk7679]

Awesome work as usual! I would have been stumped quite a bit longer than you with regards to the fact that BOTH channels had approx the same level of DC as on your initial checks.

[@12voltvids]

+Scott K It made it easy to fix the other channel as soon as I found it on the first!

[@nickbenke3306]

That mains cable would be illegal in Europe! It has to be double insulated and it's surprising how many units I get that still have it. Great clip as always!

[@12voltvids]

No kidding. You also have 240 volts on it vs 120 here.

[@nickbenke3306]

@@12voltvids Indeed! I accidentally got a full shock recently but survived and hid my discomfort by doing an Irish Jig! Ha ha!

[@felony53]

I built the same amp and matching preamp in 1979. I never tested the DCV at the terminals until today after viewing this video. I measured on one channel 0.067 and the other 0.5. I'm assuming those equate to 67mV and 500mV. The R39 spec in my build is 2.2Ohm/.5Watt. With the resistors connected I measured around 1.7Ohms for both sides. Not sure what to do about these numbers.

[@pliedtka]

The second Zobel with open 10ohm resistor doesn't really do much unless the amp is prone to oscillations. The termo cut-out out off circuit, really smart. BTW, you can't buy the mosfets out tranies for many of the amps, Toshibas, Hitachis no longer in production, too bad because some of the best power amps use them.

[@12voltvids]

Yes I know most of the old semis are non longer available and no sub available either.

[@waynewayne3709]

I've repaired many many amplifiers with large dc at the output and the dc sense circuit relay clicks in then back out.. Them big caps would need say 6k8 resistors plus 100nf caps put across them to bleed away the volts when the unit is powered down. Even a soft start circuit if there is space..

[@martinda7446]

I would say most voice coils will happily survive extended connection to 5v DC - and 3v is no problem really. Depending on your speaker design of course, most bass units will find that 3 or 5 volts directly across them. Tweeters and mid range units are usually protected against DC by their crossover filter network, the most basic tweeter having a series cap for 1st order roll off. Standard motor principles apply.and most hifi mid/bass units will not be anywhere near maximum deflection at 5v. So mechanically it's no problem. Power dissipated in a speaker is limited by heat removal from the coil. Continuous max power dissipation is specified and it is no different for DC. 5V will be 3 and a quarter Watts for an 8 ohm speaker. Just over a Watt for 3V. These are not insignificant numbers and will mess up the linearity of your speaker and have other effects - none that you want. But they will not kill the driver until you reach voltages which approach the limits of power dissipation. If you have a big amp like this short an output transistor it will dump the whole DC rail associated with that transistor across your speaker - 66v with the Hafler, and that is instant toast. A continuous 1100W into a 4 ohm speaker. I've seen it happen, PLOP, the drive unit shoots out, and before you can react, from behind the dust cap a ring of red begins to glow and POP that's it finished. Silence, apart from maybe a slow fizzling from amp and speaker.

[@eman59461]

Very nice video. Ive went somewhere thinking it was going to be a hard job just to find an open resistor replaced and all was well.

[@NinaYo401]

My dh-200 always had an idle hum. Great sound

[@VintageElectronicsGeek]

I'm surprised the factory mains cable was not thicker?

[@12voltvids]

This isn't that high a power amp. 100 watts per channel, When it comes to the gauge of the wire, remember that the wires inside the transistors that actually connect the pins to the silicon die are literally like threads. I think the 19Ga wire that supplies the board will handle more than enough current.

[@VintageElectronicsGeek]

The amp looks mean and ferocious and the cable looks weak and timid! :) But yes, I know it'll work! Glad to see you finally made your trip!

[@Mosfet510]

Yes, given the size of the amp, you would think they would put a heavier gauge or at least better insulated mains cable on there. It will carry the current, but looks like crap. Big mean looking amp and a little mouse tail lol The name takes me back to my teens though! (Audio store hopping)

[@12voltvids]

This was before monster cable came along (and other like them) and started ripping people off selling them hunks of solid copper wire and convincing them that it sounds better.It biggest bunch of idiots are the ones that customize car stereos. But you know the saying. There is a sucker born every minute. Even the mains feeding your house is typically not that heavy of gauge. #4, or if you have a higher capacity service perhaps #2 and that is aluminum to boot, and you need heavier wire when it is ALU as ALU is not as good of conductor as CU.

[@Mosfet510]

Yeah, some people in industry made a killing off those people who bought monster cable and similar brands. It's too bad, because people just wanted to get the best they could out of their system, and the snake oil salesman took full advantage of it.

[@jeremytravis360]

I had a Sansui AU101 amplifier that had a lot of leakage. Because it was a 240 UK model I changed the mains lead to a three core earthed cable. It was odd feeling the case when the amp was turned on.

[@benmeuninck7410]

marantz stereo receiver sr 325 have tin whiskers ? from 1982 with low humming and some cracking with the volume off when it cracks the hum get louder then lower some times goes away

[@timcolivet7343]

I worked on one of these a few years ago, they're quite rare over here in the uk . From memory I think the one that I repaired had gone unstable and was leaking D.C too. I did end up replacing the 2 core mains cable for a 3 core. These are nice amps.

[@rogerd4559]

Three core mains.....hmmm let me get out my English to american dictionary.... that must mean a three conductor powercord? Is that what your attempting to say?

[@billwilliams6338]

12VOLTVIDS, why is the circuit design using "thermal cutoff components" instead of using fuses? I'm guessing the fuses delay time is longer compared to the thermal cutoff components. All equipment has leakage current on the chassis, any reasons what causes leakage current in the microamps that is always on the chassis? It is the AC power transformers coupling to the chassis causing the leakage current or the transformers windings being coupled to the chassis causing the leakage current? What else can cause a DC voltage on the speaker terminals besides the output transistors being turned on always or biased circuit is biasing the output transistors to be turned on?

[@ranbymonkeys2384]

I'm just curious why you are putting your fingers on those caps like that? 3:34 3:50 I also see you touching the transformer and the frame I saw later on that there was juice going through these things. I'm just curious.

[@12voltvids]

It's not energized. I know what I can touch without risk. Even the mains on my bench is isolated from Earth for safety. Anything i touch is not energized

[@ranbymonkeys2384]

@@12voltvids You touched the caps then immediately measured them at 133 volts. You touched those caps not with one hand but with both hands at the same time, look for yourself. 3:34

[@ranbymonkeys2384]

@@12voltvids The only reason I bring it up right now I'm working on an amp and I'm new at this stuff and I'm scared to damn death, haha. Checking to make sure it's unplugged turn the pot plug it in, you would probably find it hilarious. I'm learning though. One time working on this amp every time the 10,000 uF caps discharged themselves so I'm in there and luckily it spared onto a screwdriver or something and not me. I guess it's good to be scared, somewhat.

[@12voltvids]

@@ranbymonkeys2384 I wasn't touching the metal conductor, I was touching the insulated plastic wrapper.

[@ranbymonkeys2384]

@@12voltvids I figured, it was either that or you edited out the part where your fillings flew out of your mouth.

[@johncunningham5435]

Some of these so called techs should be short; I spent years cleaning up other technicians work that in some cases were so dangerous that it presented a shock hazard to the customer. I agree with you that anyone puts their name on a repair better make damn sure that it is fixed or go home and leave it to a competent repair shop.

[@cjmmjc2131]

i have one of these, and im going to go check the bias voltage right now. I dont want it frying my infinitys. and in either case good or bad im going to replace the .5 ohm resistor Question, I want to put a 3 prong plug on this and my dh-101, I realize that the ground (green) goes to the chassis, is however the black or white critical in their placement/polarity

[@larryshaver3568]

i wonder if the power switch should be replaced

[@ignacioperez8358]

I like your videos. They are very Good lot's to learn. The only thing I noticed Your audio has a back ground hum. Or noise. Thank You

[@Unker_Spunkanathan]

12volt, why was there a coupling cap between the transformer primary to the chassis. I presume the chassis was unearth and floating

[@12voltvids]

Floating.

[@zx8401ztv]

Loverly job sir, your technical ability/skill is allways spot on :-D Now Thats An Amplifier, unlike the crappy ones ive seen in the past. The second you showed it, i thought "Loverly unit". I didnt know what it was designed for, but its so heavy duty, im well impressed :-). You did show the a.c mains on the case, i didnt see how you fixed that leak. Looking at those 0.5R earth to earth resistors, i thought the input drive device was referenced to mains earth, so real ground on one side of the resistor and the a.c leak on the speaker ground POP!!, that dead 10 ohm could be because of the massive drive earth difference + ac when the 0.5R popped. Its likely im wrong, but im interested enough to try to see why those 0.5R popped :-). See you get my brain rumbling again lol :-D

[@12voltvids]

+zx8401ztv The 10 ohm that popped was in the speaker dampening circuit.The only reason they are there is because this is a mosfet amp, which are high impedance devices. A speaker is basically like a motor/generator.When the coil moves out on the positive rise, and then the voltage goes off, as the coil returns it produces back EMF that would normally feed back at the outputs. On Bipolar transistors this isn't really much of an issue, because bi-polar transistors, besides being crazy ( grin ) are typically a low impedance device and will dissipate this unwanted energy as heat. Mosfet on the other hand can't easily handle this unwanted voltage ringing back on the wire, and it causes undesirable audible artifacts to be introduced. (distortion) so that resistor / capacitor network works to dump that unwanted current before it hits the outputs and causes unwanted noises.The AC leakage was the line cord damage. It was touching the metal cabinet.

[@12voltvids]

+zx8401ztv The back EMF won't hurt the mosfets, but will cause enough harmonics to cause nasty distortion that will hurt the listeners ears.

[@cyrenferolino1730]

hi sir I'm now your fan Im students in electronic s i like your video s especially in amplifier fixing.. I really learn a lot ...

[@lelandclayton5462]

Hey 12voltvids, Anyone ever tell you that you sound like John Candy?

[@rogerd4559]

looks somewhat like him as well

[@thomasdiyaudio5353]

I've seen quite a few other videos on this amp and resistor is the common problem. It seems to me higher W resistors should have been used in the original design. Maybe the transient exceeds the max of these resistors?

[@12voltvids]

You have to remember that the haffler dh200 was David hafflers first solid state design and many were kit amplifiers. There were factory assembled available too just like factory assembled Heathkit. It's a nice sounding amp but I would never run one because if the DC potential risk. No protection circuits whatsoever. Like the old crown amps. I would choose a bgw over a haffler or a sunn, crest, or Peavey. Any of the professional amplifiers will do. I have a crest v900 and it is just an amazing amplifier.

[@izzzzzz6]

What is up with all those blown resistors? Something (transistor) probably went short and was then replaced but they couldn't find the resistors that it took with it when it blew! I am looking for some tips on how to troubleshoot power amplifiers that have gone into protect mode. Any chance of a video on how to do this? Thanks

[@dhpbear2]

Did you cut out the caps that are connected to each side of the AC line cord to ground?

[@mortenrolsing7137]

Love your videos, especialy the amp repair ones, thanks. Watching this video while load testing a Denon PMA-700V i just repaired :-D Greatings from Denmark Morten

[@jptourigny222]

Good day! I am trying to fix a 75 watts, solid state guitar amplifier (Traynor TS-75) with no sound (and the speaker in the cabinet is good). I believe there is DC voltage on the speaker terminals as I already lost a test speaker when I initially examined the amp out of cabinet. I noticed that in your analysis of the Hafler, you did not connect an 8 ohm dummy load and directly measured the DC voltage at the speaker terminals. Would you please tell me whether, in your opinion, I could do the same on my amplifier so that I can proceed with further testing. By the way, excellent video! Many thanks.

[@12voltvids]

You do not need a load on a ss amp. Tube yes, because if the volume is cranked and no load very high voltages can be generated in the output transformer which can arc and burn our the transformer. Solid state should have 0 volts across the speaker terminals with no output. DC indicates a fault. Bad output transistor, bad driver transistor, or open resistor in bias circuit the most common. Leaky coupling cap also can use the balance.

[@jptourigny222]

@@12voltvids Many thanks! The four output transistors are good and all resistors on the one board are good. I'll confirm that there is DC voltage at the speaker output and investigate driver transistors and coupling capacitor.

[@walle637]

does the power transformer isolate your dc rectified side from the ac in as it steps down the voltage?

[@taxfreedollars]

Is it difficult to convert a Hafler DH200 or DH220 from 110V to 240V internally, therefore removing the need for an external step up transformer.

[@12voltvids]

Sure. Find a replacement transformer with the same secondary rating with a 240 primary. Of use a step down transformer. Just like i have to do with a few 240 devices i have. I either have to use a step up transformer or plug it i to the 240 outlet i charge my car from.

[@mohimadvani6425]

Finally opened up the hafler dh200. The voltage on one channel is 0 volts dc, and the other channel is 0.12 volts negative dc. adjusting the bias doesn't change anything. The right channel which has negative 0.12v is heating up . What could be the problem? I am using dh110 preamplifier with it.

[@rogerd4559]

That .5 ohm bias resister is a wire wound resistor is it not? If that AC leakage was added to the load on it woulnt that over current and perhaps blow it out?

[@crazymetallian]

i'm not even seen the video, is a issue within the input phase?? a short to ground, maybe a capacitor shorted to ground??

[@jcampbell2481]

Great video! Your trouble shooting skills are first rate.

[@benmeuninck7410]

aiwa cx nma845 tape decks not starting play or record unless hit on the side of the tape deck (just replaced the belts)was dusty when it was apart

[@DeadKoby]

Question... I know the DC offset is bad.... the DC offset accounts for about... 2-3 watts of DC power going through a speaker... that wouldn't be enough to cook a speaker... I imagine if it was ignored, it would become worse, and then a big problem. Am I wrong to say that the current DC offset level isn't going to burn up a 200watt speaker at this time?

[@abelabner]

Hello all! I can't find a service manual for a hafler se240 I own. It plays fine but has slight DC offset issue 20MV and 40 MV on each channel respectively. The unit has bias control but I have no idea what number to go for on my ammeter. I'm not an expert but not novice 12volt can you or any viewers help me with the numbers and or schematic. Thanks much eh!

[@jrg1man1978]

I know this is an old video but I'm having an issue with mine where neither channel will bias up past 6mv. Figuring both channels wouldn't go kaput but just being safe I went along both, testing all the resistors and replacing bad ones. Replaced Q9. Resoldered driver transistors. Still same issues. Kind of stuck. Do you know where I might go for a map of key voltage values. Had reservoir caps checked at my tech. Thanks for the videos. While I'm rarely looking at the same equipment, the troubleshooting techniques are always helpful.

[@manueldi_77]

@35:17 - are there two 5mm LEDs inside?

[@manueldi_77]

+Manuel Dibarbora Ok, On the right angle I saw they are not :-) Nice repair as always btw,

[@12voltvids]

+Manuel Dibarbora There are?

[@manueldi_77]

+12voltvids The soldering lugs looked like clear LEDs at the first look for me @35:17 in the video. Sorry for the confusion.

[@roncheaters]

Hi.. This is probably my favorite channel.. Along with AVE, and EEV Blog. But I think I learn more about the basics from your channel. I know people put you down and crap, but don't let that bother you.. I wish I could ask tour advice on a direction to go with a plasma TV that was given to me.. I'm used to LCD's and when I opened it up.. I had to step back and say "Oh My! Wow! There's a lot going on in there" Its a 42 inch Sony KDE-42XS955.. but it says Wega-powered on the front panel.. Its such a sleek, elegant machine, I don't want to toss it.. If you're interested doc.. I'll tell you the symptoms.. (I suspect bad cap's even though they're Nichicon's, and the set can't be more than 10 years old) Made In Japan. Don't let my question seem like a pest.. I love your channel man, stick with it, I learn so much. I Wish you'd do tutorials.

[@atestchannel8297]

Hi 12voltvids, you are really good at spotting the culprit. I was wondering why an open R39 would cause DC voltage on speaker terminals when watch your video. It is to feed the output signal to the differential amplifiers to reduce distortion. Perhaps when it is open, the 0V is maintained by the symmetry of the positive (push) cycle and the negative (pull) cycle.

[@12voltvids]

+ATestChannel Yup exactly.

[@12voltvids]

+ATestChannel positive voltage is fed in d4, 5, 6 and R13which forms a voltage divider, current limiter throughr39 which is the return to ground.if you look on the left side of r39 on the schematic, you will see that it is tied to the ground side of the audio input, and R3, connects between this ground and the audio input. Therefore any DC rise to the positive side on the left side of r39 will cause a voltage rise on the base of q1 through R3 and R4, which will turn on q1, q 7, q8, q12 and ultimately the positive outputs q14 and 15.If you guys would like a deep dive of the amplifier circuit and schematic I would be happy to do one. The first, and only component I checked and replaced on this unit was R39, and I came to that conclusion without making a single measurement. I checked my bias control, and it had no effect, and that was because the negative side of the amplifier, q5 through R5 was being driven off die to the imbalance on the input.Had I measured the shield from the audio input to amplifier ground I would have also seen a rise in voltage, and this is why the chassis was floating, is it was r39 that ties chassis ground to amplifier ground. Amp ground is t he center tap of the transformer secondary.This is basically the same design of amp that my high school electronics class had the option to build as their project. I got to troubleshoot all the other students failures. I on the other hand took on a real challenge, and that was building a black and white TV camera, that to this day I am pissed, because someone stole it from my parents house. I had it mounted up in the corner looking out in the front yard, and some goof scaled the fence and stole it in the middle of this night.

[@int53185]

+12voltvids Your explanation reminds me of why I dropped out of EEngineering. Your knowledge is quite impressive.

[@12voltvids]

+James Last I have probably forgotten more stuff. When I left the business in 2003 I was a lot quicker to the draw, but got pretty rusty in the10 years I was completely away from repairing anything. I retrained in a different field, telecommunications, so now I deal with VDSL and voice loops, fiber optics, and Ethernet/coax networking for IPTV.It took me a little while to get back into electronics diagnostics, and I basically just learned the basics of Plasma TV over the past few years by doing a lot of reading. I am more at home on the old school audio components, and you know what, this is where the money is in this industry anyway. Nobody will spend money fixing a TV, but they will on their classic old amplifiers, because everyone is figuring out that the old school stereo amplifiers are much better quality than new components, and vintage audio is now a hot topic.We even have a local business that does only vintage. The shop owner has offered me a job with his shop, but I turn him down, do my own thing and keep my day job, counting down the days to retirement.

[@atestchannel8297]

+12voltvids In new design, as in the case of my receiver, 0V is absolutely floating. The audio inputs, speaker output terminals, and headphone socket are insulated from chassis. Low voltage portions are grounded to chassis, like the display, control panel. Somehow there is a -.8.pV difference between 0V and chassis. I measured the DC voltage between speaker terminals. It is in mv range. The 10V standby power supply may be the problem, as the positive rail of the standby is connect to the chassis. If this is the case, how this thing worked for so many years without a problem in the first place? So it is still a myth.

[@ricfair9919]

Dave is this not a BAD design, an open resistor can cause voltage to leak to the speakers, I guess no circuit to shut down the amp. Is this why newer amps have a relay?

[@rogerd4559]

Dont good speakers systems include some type of fuse or circuit breaker to protect againd failing power amps?

[@12voltvids]

No not always. Some companies did but most did not. Even high end. The reason is because of nuicense fails. Many amps had speaker fuses to protect the amp in the event of a speaker wire short.

[@rogerd4559]

@@12voltvids ok so there is a fuse somewhere for the protection of the amp and speakers

[@12voltvids]

@@rogerd4559 Yes there are fuses on the outputs.

[@2nd_Channel]

Did you trace down the - probably construstion error - as why both 1 ohm's resistores was open?, or just go with the "what ever, it will last another 20 years"

[@12voltvids]

Originally it was a 1/2 ohm, but I had an identical unit that had been modified, and the 1/2 had been replaced with a 2, 1/2 in series (1 ohm) so I replaced with 1 ohm. The schematic shows 1/2, but likely they were failing in the field. The unit is much older then 20 years, probably closer to 40 years.

[@2nd_Channel]

ok, yeah so it will last a very long time

[@rogerd4559]

WOW! I nearly fell off my chair when I saw you use a heat gun and not a blow torch

[@mrGoesto11]

blow torch? seriously?

[@pliedtka]

@@mrGoesto11 Really, some people do it with mini torch. When I have nothing on hand I use cigarette lighter, mostly when working on cars - sometimes the space is small and mini torch does the job, just have to be careful with the flame.

[@SDsailor7]

Where did you find the schematic?

[@kennethmccloskey6883]

Did you ever re cap those amp boards?? and what of those old Sangamo Filter caps? At 40 years old is it safe to leave those and not replace?

[@12voltvids]

Owvwr didn't want to spend bug bucks. Once you start changing caps you are into hundreds. The big caps can in some cases run 100 each. I was paid about 60 to fix this amp.

[@kennethmccloskey6883]

@@12voltvids Understandable, just a shame to get it working so well and then leave those old caps in there that could possibly to issues very soon

[@12voltvids]

@@kennethmccloskey6883 I can only go as far as the owner wants to go. Caps are extremely overrated by amateurs. They read someone's opinion and think that every capacitor is going to fail in every circumstance. This is incorrect. Only certain capacitors are going to cause issues. Since amateurs don't know they are just following someone's advice. Any shops will also incorrectly tell their customers that they have to change all the capacitors as a way to jack up the price and again customers don't know. There were a few years when bad caps were everywhere and those years were in the late 80s into the 90s and this was manufacturing problems in Japan with small caps. They had another problem in the early 2000 when the Chinese companies were having issues. In these old amps the caps are not under stress. They are not running at 95% of their rated voltage and they were made before the EU started passing legislature limiting what chemicals were in components. You don't see experienced servicers changing every capacitor. The exception being old antique tube radios and amplifiers that used old paper capacitors. This is because the paper failed due to the bleaching process when the paper was made. Over the years these old caps became electrically leaky and this leakage would damage tubes. That doesn't happen in solid state devices even if caps become electrically leaky as trabsistors are low impedence devices and the voltages being low any slight leakage has very little impact. You also get lots of warning as they are going out. Again if a customer wants to spend the money I will do it, I had a guy about 10 years ago want his amp totally recapped which I did and he came back and had me put all the old ones back because he wasn't happy with the sound. He had become so accustomed to the sound his old amplifier was producing that changing the caps made it sound like a cheap Chinese amplifier as he put it and he paid me to replace them back with the old caps which i mistakenly returned. I should have thrown them all away and would have had I had any idea that i would be putting the old ones back in essentially for free. Another thing is many units I repair for people are going up for sale. So the more someone puts into something the less they will make on the sale. That puts pressure on me to change the bare minimum to get something back in service.

[@docfoot316]

Yes i would agree as well you do know what your doing always good to watch the experts ,and nice classic amp lol

[@bones007able]

did I miss? what was the cause of 90 volts on the chassis?

[@RalphRacc00N]

its one thing to add the service sticker to the back panel, but right on the front? unbelievable.

[@raultendillavences2587]

The mosfet transistor of this amp are very dificult to find. do you have another option or sustitute for the mosfets?

[@12voltvids]

No sorry. I know the difficulty in getting parts. I am going thought the same problem getting an STK power module. So far 2 counterfeit or bad ones received.

[@raultendillavences2587]

12voltvids I saw the video (i think was for a yamaha console) thanks friend

[@12voltvids]

That's right. Awaiting another part. Fingers crossed.

[@richardhill3531]

I know that for DH220 - mosfet output transistors are available from a company in England. Are DH200 different?

[@raultendillavences2587]

Richard Hill is haffler?

[@zurdoremi]

I don't know who to ask this question but here it is: Why would a speaker Balance control not work in any audio equipment? (home stereo, car stereo, etc.). Or in other words, the Speakers Balance ( Left / right ) control does nothing.

[@12voltvids]

Broken control. Broken ground going to control.

[@deangale4496]

Excellent wish you were in Australia

[@sux2bu883]

You just say I have been doing this for sometime. This ain't my first rodeo.

[@mohimadvani6425]

What if we put a blocking capacitor at the output?

[@12voltvids]

Well that would totally change the sound for starters. The DC voltage that appears during the discharge phase or power down isn't going to hurt anything. If you want to be totally safe, just put a speaker switch on the speakers and turn the speaker switch off before powering down the amp, and on after turning it on and letting it settle down.

[@mohimadvani6425]

Thank you, for your reply. I have a hafler,110,200 combo . one of the channels IS giving around 3v dc, maybe if I set the preset would solve the problem. I am not using it anymore, but will be interesting to open it up and work on it. I am energised watching your video.😀 BTW, I am subscribing.

[@HombreMusical4711]

Your camera has a "hum". at first it was not noted but as time went on the video, it started bothering me. I like your videos. Thanks

[@widecast]

What’s all the hum on your audio.

[@opshacom1]

Isn't that method risky, with measuring the value of a active component in a powered cct to an Analogue or cheap Multi meter.

[@geekyshrink]

So if you had to replace the cord, why not use a polarized power cord? You didn't use a 3-prong because you didn't have the right size strain relief (really??) But I would think a 2-wire polarized would be a good idea. edit: oops. Just saw the explanation below. Hmmmmm. I put a polarized cord on mine. You say it could cause problems. Should I re-replace it?

[@12voltvids]

It really makes no difference. The power cord is not in any way connected to the chassis. It goes to the primary of the transformer. The only thing a polarized cord would do is ensure that the switch is switching only the hot side, and not the neutral, but at the end of the day it really isn't an issue, as the unit is isolated by the transformer, and the original cord was not polarized to begin with.

[@andrewcrain5461]

I have a chance to buy a halter XL 600 or a adcom 555 witch one would you Recommend??.Just curious you seem to be a really good technician both of them are the same price $300

[@Zone1242]

Excellent repair as always - really enjoyed following along. Not sure those amps would be compatible with my neighbours :-) Cheers, John

[@12voltvids]

+Zone1242 The thing about power is you don't have to use it. It is nice to have all that headroom available so that sonically music can be reproduced as accurately as possible. I use a Luxman LV103, which is a tube buffered MOSFET output amp. Only 60 watts per channel, but it does sound very good. In my living room I have a yaqin tube amp, which is basically a clone of a Marshall guitar amp with a paid of 12AX7 and pair of EL34B per channel. That produces a very smooth sounding 35watts.

[@antigen4]

dim bulb tester - PERFECT use for it

[@izzzzzz6]

Not a good idea to fit an earthed power cord?

[@12voltvids]

This is isolated from mains by main transformer. Earthing only increased chance of him induction by means of ground loop. How many receivers and amps have a grounded plug? Very few. How many grounded plug receivers come in with the ground prong cut? Many!

[@izzzzzz6]

@@12voltvids Better hope that the live never comes loose and touches the chassis then :D

[@12voltvids]

@@izzzzzz6 Won't be coming loose. Heat shrink tubing is a good insulator. Nothing to break free, as the other end is soldered down to heavy lugs.

[@thomasraden]

I always thought having an amplifier with a DC output stage is like playing russian roulette. As soon as one of the transistors in the final stage blows up you also toast your speaker. Much better to use electrolyte capacitors in serial to your valuable speakers. Nobody will hear the difference.

[@64ledzep]

Just found your channel, do you know of anywhere I can get the mosfets for a DH220?

[@pliedtka]

Almost impossible to get them, try some people on DIYAudio.com or places that sell good amp kits. 220 was designed by Erno Borbelly He did bunch of designs for DIY, had businesses selling parts in Europe but he's gone to Earth like many good designers.

[@patricknicolucci5073]

wow a few resistors cheap repair great job

[@frankiemcgovernsnr1933]

Well it is because you know what you are doing

[@waynewayne3709]

Yep, I've been in the repair trade a long time and seen many many units just screwed up because some just have no clue in what there doing.. And its great to see a time serviced tech show he's skill.

[@paulwilson5227]

Nice job. would like more of this type of work........many thanks Paul

[@12voltvids]

+Paul Wilson I prefer to work on the pro stuff as opposed to LCD TV which is mostly not repairable.

[@DJCruicky]

I read the title as "200 DC Voltage on Speaker Terminals". I'm thinking phew, that's a lot :O :D.