#106

The amp had output stages in both channels shorted. It turned out that 5 out of 16 power transistors failed. Driver transistors on the negative side in each channel failed as well and killed a couple of 6.8 Ohm resistors around them. After unsuccessful attempt to get cheap transistors from China on eBay, the amp was tested with 8 out of 16 output transistors in the output stages. This configuration worked fine, and now the question is about money: if we tolerate mixing old and new transistors, 5 replacement transistors from DigiKey will cost about $40. Replacing all 16 will be (almost) beyond economical repair. Let me know what you think. Thanks.

[@monkeymhn7]

Let me tell you a couple QSC stores. The first is a series one 1200. This amp started life with a local rock band in 1985. Than was sold to Roman Hill a rock band I ran sound for for a couple of years. It latter was sold again to another band. Than came back into my life about 1995 ish while I was the sound man for Justified a local country band. It stayed with us up to the leaders death and to this day still preforms every week end with the former owners grand son. The only work ever done to this amp is cleaning. The second is a latter model 1400. It survived running near full power in to a shorted speaker system for about five hours. After the short was cleared the amp worked just fine and still does every night in a local theatre. The QSC series one amps were good preforming, durable,no frills amps manufactured at a good price point. They usually are easy to repair and generally are worth saving.

[@pascalphase2556]

I like your approach very logic, and clever, using only two pairs of transistors for testing, it's very smart ! Bravo !

[@Futschikatores]

Sorry I can't provide valueable input for this problem, just wanted to wish you happy new year and say thanks for all the *great* videos.

[@feedback-loop]

Thanks! Happy new year!

[@treadmillrepair754]

25 years ago I was fixing many of these amplifiers. After some failed attemp with China transistors I bought the output stage in Digikey with zero problems. Best Regards.

[@luvradios]

Superb troubleshooting & beautiful testing process, damn fine job Sir!

[@SoddingaboutSi]

You've got that far , you know what needs to be done. Repair it! Happy new year.

[@andybonneau9209]

Buying electeonic parts from Ebay (China) is an almost guaranteed tipoff. A bad deal at any price.

[@gorak9000]

Yup, I once bought large transistors like that from ebay to fix a power supply - they were definitely fakes as well.

[@sw6188]

The trouble with ebay is you don't know what you will get. Sometimes the devices are genuine, but most are fakes if you buy from chinese sellers. I bought some MJ15003 from one knowing they were probably counterfeit but thought I would try anyway as I was covered by the money back guarantee. I thought if they were genuine then I have just got a bargain! They turned out to be fakes. I confirmed that with a message to ON SEMI. I got the refund.

[@MarkTillotson]

Audio transistors from eBay are these days almost all fake - ask anyone on diyaudio.com....

[@sw6188]

@@MarkTillotson Yes, pretty much all of them are - but every now and then you can find some genuine NOS devices but it's hit and miss because you really don't know what you're getting.

[@MerlinMerlinL]

All chinese transistors are 2N3055 remarked

[@amirb715]

Happy New Year. Looking forward to the next one. specially if it is a test gear like old times...

[@tonycanning8518]

I have experience with QSC amp's and I have to say they are really good. Well worth repairing.

[@DumbSkippy]

Dear Mr FeedbackLoop, I learned so much from your video! I am so grateful for you sharing your knowledge. Spaceba tovarich ! - from Perth, Western Australia

[@carstendahlhjarup8783]

Happy new year!, thank you for interesting videos. They come in handy in the extra Covid time - we all have. Also help us all improve in trouble shooting.

[@petercornell2002]

Excellent vid, many thanks. All the best for 2021

[@juanea2eax521]

Thanks for these interesting videos and happy new year

[@terrymarky9996]

Thanks for posting this, this is invaluable troubleshooting for any of the tranditional QSC Class AB amps. If you look at the schematics of these amps ( USA series, MX series, RMX series) they are practically the same design that has been slightly changed over the years. Even the 1400 to USA850 to USA 900 are the same amp, the RMX1450 is the same amp design again just in 2U form.

[@alphabeets]

Your doing this work is adding great value to the amp. Be patient and I’m sure you can find someone looking for a rebuilt vintage amp.

[@_hackwell]

hi ! very interesting repair. Things can be tricky when the feedback loop ( no pun intended ) is unbalanced. Here's a trick I used for years as a service engineer : I replace the output power transistors by a diode between base and emitter connections to close the loop. This way I avoid blowing up 200$ of power stage to Mars and this allows easy measurements with nominal voltages. Hope this helps

[@DumbSkippy]

Wow @elecsys. I love your simple solution and lateral thinking. Kudos sir !

[@jpdesroc]

Indeed a very clever idea !!

[@MichaelBeeny]

I have used these QSC amplifiers in cinemas for many years. They are normally really reliable and do sound good, even in HiFi use. they are very conservatively rated and work well into low impedances and bridge mode.

[@rockman49er]

What size qsc amplifier would be good for cinema I just got the qsc 1400 and it's not enough power for what I want I'm currently pushing 2 260 watt 8 ohm floor speakers I'm going to get some huge 4 ohm speakers what's a good size for super loud bass 🔊 👍 thanks

[@DaveMcAnulty]

Be interesting to see the inside of one of those transistors.

[@clemensknieling453]

I have a picture of an opened faked power transistor

[@xyloeye]

Thanks, Alexander. Another really great video. I agree about the Chinese stuff. I quit ordering directly from China because of so much junk merchandise. Vintage transistors are especially difficult in my experience. Happy New Year!

[@v12alpine]

A cool feature of these capacitively coupled output stage is pretty much zero possibility of passing DC to the speakers and no DC offset adjustment needed. Every QSC amp I've worked on uses this even today. Keep in mind that testing without a load you will see some DC offset. Sort all your new/old transistors by HFe and try to keep them as matched as possible in each parallel bank. The old ones can vary by a large amount and is probably why it blew a few of them in the first place.

[@ronniebrown8775]

Learned a lot on this one thank you for sharing

[@ady3328]

Subscribed, very very good channel and also presented in such a way it is absolutely educational…..

[@TKomoski]

I had bought from Aliexpress some MOSFETS and they where good. I always check the seller feedback before I buy. I know you took a chance, but you warned yourself and you didn't listen. Happy New to you sir and I look forward to watching you this year. Mark 8.33k subs as of 01/02/2021 *CHEERS*

[@sw6188]

Beware. Some parts will actually work, but they may not last - especially under high load. They counterfeit the devices but use much smaller dies and at first the part appears to work but when you load it up or try to draw the rated current it goes BANG because it is severaly under-rated.

[@bartvandekeere740]

Nice repair! Repaired one of these more than 10 years ago. I remember the fan being not shielded and driven by high voltage, quite painful when accidentally touching it. There is a plastic shield missing in your amp diverting the airflow to the transistors but would not matter much i guess. In my case that shield broke off and blocked the fan, amp heated up until destruction with the entire output stage shorted. Keep up the good work!

[@feedback-loop]

The plastic thing is on the top cover. I just did not show it.

[@bartvandekeere740]

@@feedback-loop you’re right, i was mistaken by my loose one :-) love your videos! Thanks!

[@bfx8185]

As usually great video ! Thx ( I would rather check for some new alternative replacement if they are cheaper, if not forget it)

[@stephenbell9257]

That amplifier is drawing a lot of current from the mains supply for only 50W output on a single channel channel. I would be concerned were all that extra energy is going - probably into heating the output transistors.

[@oriole8789]

As much as I want to criticise the idea of buying transistors from China, this video ends up as being more helpful in making that argument more than any comment could. There are specific goods that I do buy from China, such as UV LEDs, kapton tape, stainless fasteners, teflon goods etc. For lots of specific types of products, quality will be "fine". For lots of others - ICs, transistors, capacitors, resistors, connectors, wires etc - stay away. ;) You've created great value in making this video series to educate people. In this way, your time wasn't wasted as it otherwise would have been. Thank you for making it!

[@tonybalm1513]

I'd say it's worth it. It looks very well made. That transformer is very nice!!!!!!

[@Amperekaefer]

*Vielen Dank* für dieses Video! 👍

[@kenwittlief255]

you get huge cred for having a Fluke 87 !

[@justinhiggins2210]

I tried to use some of those Chinese power mosfets in a ppi pc2350, and they lasted about 1.5 3 minute songs and smokomatic maximus. Ordered the part number from a reputable p0arts house and it fired right up and works to this day. The correct parts were expensive but well worth it.

[@borikmuha]

Очень познавательный ремонт, спасибо за контент

[@ydonl]

I have used a few QSC amps in live sound systems. I would generally describe them as "pretty okay" in terms overall quality and sound, in the middle-to-slightly-lower range of the market. They work; they sound reasonable. I want the bass to feel like a locomotive, and a QSC just doesn't quite, and I want the highs to be kinda open and shimmery, but the QSC can't quite do that, either. So... if the application is not critical, and the price is cheap, a QSC might do the job, but if I wanted *nice* sound, or if it was expensive, I might look for an alternative. For a *good* amp, my first choice would be a Crown (but not a PowerBase). The Micro Tech and Macro Tech amps are heavy, almost never break, sound great, and just do all the things you want; they are a little upscale on ebay. They probably powered most of the concerts you went to back in the day. Stacks of 'em. Movie theaters, and so on. For portable systems, any of the newer switching amps do pretty well, and weigh almost nothing. The problem with Crown is you're less likely to get a chance to make a repair video. :) The old Mackie FR power amps were *great*, except for a slight tendency to blow fuses when you turned them on. (Huge inrush current! Big, mean power supplies.) But they were gutsy and sparkly and wonderful; they made really nice, solid bass guitar amplifiers, or clean and clear mains amplifiers. There are other amps for reasonable prices; I would guess most of them are somewhere around QSC quality, a little lower or higher, depending. I would avoid a particular "B" brand, though.

[@H-77]

The better QSC (like the Powerlights) tend to be upper-end of the market (about on par with Crown, though IMO a better product), their lower-end stuff (GX series) tends to be lower end of the market. The older analog Powerlights were great amplifiers. They're some of the most reliable amps around, and they actually sound really pretty good. From a measurement perspective, they get fairly close to the MC2 amplifiers (and that's saying something), though I haven't had a chance to do a real A-B listening test between the two. Fans are a little noisy, but not bad. As a side note, when we're talking about companies that have made some really lackluster stuff over the years, Crown might just take the cake. The old Macro-Techs were probably some of their better products, though they are definitely not my cup of tea. The fan in the Macro-Tech really doesn't move as much air as you'd hope given how loud it is. Many will say that nothing touches a BSS EPC780. Of course, there's also almost nobody around who can fix an EPC780, so they aren't very practical.

[@Centar1964]

What is your opinion in ASHLEY amps?

[@ydonl]

@@Centar1964 Me? I've never used one.

[@analogaudiorules1724]

The body is in mint condition, those amplifiers sound truly exceptional and can easily go toe to toe with the best of the best, even things that cost 4 times the price, it is also made in USA and is built incredibly well, usually they only fail because of operator error and nothing more, if it was me, especially because the case looks nearly new, I would replace every transistor and fully recap it, check all solder joints, clean all the controls inputs outputs power switch out with deoxit d5 and possibly put a better and more quiet fan inside like a noctua or something similar, build a little power supply with it, a full wave rectifier, a nice cap and a voltage regulator coming off the fans ac line with a noctua 4pin pwm fan controller, put a light foam airfilter over the intake vents and use it as my man amp in my main system and get a good 50 more year's out of it till my future kids get it and I'm long gone. Qsc still does make reliable traditional amplifiers that still sound nearly as good, but everyone else, is pretty much junk with very few exceptions. Vintage qsc gear is the holy grail, on par with the old school crest CA stuff and the old crown macrotech, d series or k2 stuff....

[@soothcoder]

Might be interesting to measure the parameters of the eBay transistors using your checker tool. I wonder if they are damaged or just relabelled? Could dremel the lid of a blown one and have a peek?

[@1fertube]

Great explained! Where can I get a copy of the Service Manual? Thanks

[@pedromonardi]

Hello, let me tell you if you can guide me, please! I have one the same that I bought about two years ago and after a while (an hour + -) it starts to do like a scratch-type distortion (the volume is reduced and it sounds as if they graze the speakers). Do you have any idea what it can be? He does it solely with the guitar and the speakers are not. I will appreciate any guidance or suggestions. Many thanks!

[@ABDOU-cj3pp]

Good job

[@pizzablender]

Isn't there a class D board that will work with this power supply? IRS2092S based boards for example, they can match the power supply as those are single ended. Given the simplistic schematic of that amp, it is likely to be an improvement. And you can keep the input boards and power supply.

[@andymouse]

I wonder should it sound nice ? what where its audio specs ? if its any good ask yourself would I use it or could I sell it ? because its a lot of money...happy new year !

[@ralphyrocket5770]

Great Work! i used to use those amps...nice memory!

[@mattsnook1723]

I noticed in the schematic the original outputs were japanese 2S numbers... Perhaps it had outputs replaced before which explains the mismatched complimentary pairs. Assuming the originals are obsolete at this point and the MJs are the only choice. Either way, QSC are good amps, it would be worth repairing.

[@leandrofreiberger1649]

I leave a tip, I had a team of them and the problem that did not activate was the red LED that is on the board.

[@dlunsford1980]

Looks like these are for sale between $125-200 on reverb.com. I would get my money back from ebay and then buy the right parts to fix it as long as you don't end up losing too much money in the end. This has happened to me a few times and I try to look at it as I paid for the fun of solving the puzzle. You win some and lose some. One thing to consider before you head down that road though... your current draw is pretty high even though it is working. Hopefully those good transistors are not wounded. I'd cycle through the other pairs to see if the current draw is any different. If you have to replace them all then this unit might have to go to the boneyard, I don't mind losing $20 on a piece but not $100. Also look up what the power transformer sells for these. Often times parting a unit out will bring more than the unit whole. One last suggestion is what if you found an equivalent in another package such as the TO-264? Those seem to run cheaper. MJL3281A maybe? Of course you'd have to get creative with the wiring/mounting.

[@rasz]

Look at complete ebay auctions. They actually sell, as opposed to being listed and sitting without any interest, at $37-50 in the last two months. Transformer and the case are the only valuable parts here :(

[@dlunsford1980]

@@raszgood investigating. I hadn't taken the time to look. He could fix it and then donate it. I've gotten some business over the years doing such things.

[@dalemyrie597]

I have one of those amplifire but the USA900 version .But the clip light comes on and when I turn it up it works fine . I get to understand that the resistor are bad .they were change and it's good again . Then one day it happen again someone came to look at it and change some resistor . It never works since .But a next technician says that the technician uses 47k ohms resistor which are wrong. But I am yet to get it fix . My question to you are there any 47kohms resistors in that amps that you are fixing . However I would love to see you fix that amps .it is little and neat and play good . I can't afford a CVR .

[@laoluu]

Hi all, I have the USA900 (must be a close relative) and I find it to be a very reliable workhorse. I would repair this one. Find transistors with equivalent data, especially voltage, current and hFe, and you should be fine. Would be interesting to know why the amp originally got blown? Connecting the output to ground somewhere? Short circuit under full power? In my experience, these oldschool-amps can take a lot of beating and abuse, before they give up.

[@fayyazbahrami296]

l has some similar problem with fake 2n3055 used tip35 in to247 package , wiring was a bit hassle but worth it

[@lukejm106]

How do you test the bias?

[@ericleroux9873]

hello and happy new year. i don't understand the signal path in the output transistor as they are directly connected to the supply +76v or - 76v and the 0v. Where is the swing output that should go through the output capacitors??

[@feedback-loop]

The midpoint of the power supply is not connected to the ground. So the output stage pulls the rails towards the ground. And that midpoint is the output of the amp relative to the ground.

[@xtevesousa]

Fixed one of those once, took me a while to figure out the very unconventional output stage, the center tap of the push-pull is grounded, its the entire output stage that swings up-and down!

[@MadScientist267]

How the hell do you feed something like that? Knee jerk says isolation would be required but I didn't see anything like that with the limited view I got from my yard

[@jeanangi3233]

How can you help me please I have an amplifier it functions normally but the moment I increase the volume it turns off automatically and sometimes even blows the fuse 

[@CABohol]

Nice!

[@icesoft1]

Another option would be to load 8 of the good used transistors into one channel, and install a set of 8 new transistors into the other (perhaps at a later date when finances allow)... This would reduce any issues from non-matched transistors as the channels are independent...

[@SidneyCritic]

Is there a newer cheaper T0-220 substitute, because it could be screwed down with one of the original screws. I thought they were fakes because the blue bits were too bright and new looking. I bet the writing comes off with acetone.

[@H-77]

A TO-220 package would be a very poor substitute for these transistors from a power handling perspective.

[@PowerToneAudio]

Nice troubleshooting bro. Are you based here in UAE , because in this video I noticed that the transistors you ordered form Ebay was delivered from Emirates Post printed from the package. Thanks!

[@ultrasniper]

Hello, very interesting channel! Have you some document about Fluke 789 other than owner manual? Thank you very much.

[@feedback-loop]

No, sorry

[@SlinkyStoney]

I think those eBay output transistors are just fake rebranded 2N3055 and MJ2955.

[@rasz]

did you get ebay refund for the fakes? there are pictures on the net of those opened up, pretty much packaging whatever just so they pass transistor tester :/ As for the amp, imo its old junk, Sold ebay auctions for a fully working unit from just last month: Nov 25, 2020 , 6:20PM Winning bid:US $37.00 Throw out the pcb, reuse case and transformer to power two IRS2092 +2xIRF6645 $15 D class 500W modules.

[@feedback-loop]

Most probably I will get a refund for the fakes.

[@Citronitroify]

You need a transistor curve tracer.

[@halituzun7913]

hello i need circuit diagram of qsc gxd 4 amp can you help me?

[@NadeemKhan-vy1wu]

Sir I have Qsc Amplifier Model DCA 3022 Both Red Light blinking Kindly if you have possible then help me about solution

[@EmilioGarcia-fr5po]

Cool video learning allot, allways American baby

[@Chokechain1111]

you really need to find the N series and newer with balanced input and output so you can daisy chain a balanced signal from pre to multiple amps

[@thomasboos2470]

I would suggest to populate one channel with old ones and the other channel with new ones

[@ralphysound]

В схеме есть GND и есть (gnd)A. В чем разница? И как с этим быть?

[@ELECTROGYAN]

I watch total vdo Nothing seems boring

[@sw6188]

Spend the money and replace the transistors with genuine parts from DigiKey. The QSC amps are very good quality, worth the investment.

[@user-vl9ig5lj5z]

I need schematic diagrams please

[@y_x2]

Please open both bad transistors, the chineese and the original to see if they are the same. For the amp: other parts can fail that you did not checked, caps and pots can be bad also!

[@davidwoodbridge862]

I have one of this model QSC amplifiers, I purchased it in a known faulty state (from eBay as it happens) as I'm capable of repairing it. Same basic problem with blown transistors, both output and drivers and someone had "had a go" before me and made a bit of a mess. I managed to get a circuit diagram but not the whole manual and it does tend to get a little warmer on one channel than the other so I will follow your shown procedure for adjusting the bias. I seem to remember transistors being quite cheap though but I do get components at wholesale rates so maybe that's why, I actually replaced all 16 as I don't trust semis that have been stressed but test OK. I have this amplifier just sitting on a shelf in the workshop at the moment as I don't really need it for anything. Class D units are now the go to amplifiers in many cases and are much lighter anyway. Could you please point me in the direction of the complete manual by any chance? Enjoyed the video BTW.

[@xtevesousa]

you can find complete qsc_usa-400_900_1310.pdf just by googling, inside there are detailed troubleshoot and adjustment procedures.

[@ptronix]

I agree, really not worth spending any more money on, unless you can find a cheap source of genuine transistors

[@MerlinMerlinL]

All chinese transistors are 2N3055 remarked

[@transforminggravity15626]

🙌💯💯💯💯💯💯🙌

[@weedonman]

When the amp draws too much ‘currency’ it’s probably wise to call it a day on it.

[@newpath.newpath2010]

repair it completely, then either make it a regular duty horse, or keep it as a spare.

[@risingfromashes2239]

Fix it and keep it

[@MrDubje]

If it is not worth it now, just keep it for spares and repair for other devices that you may come across. Another option is to write it off as a bussiness expense and use some youtube money to get parts, sell it at a loss and just think of the investment as a learning fee to learn to never touch these again, haha.

[@andr27]

best option would be repair this PA, make it fully functional, then sell it on ebay or where-ever (ad on yt video) and get invested money back, and in the meantime have a good video content on yt - completed reapair. Everyboudy would be happy. Now he stuck with unfinished one and no idea what else to do.

[@rasz]

@@andr27 buy $40 of parts and sell whole thing for $37, brilliant! :)

[@MrDubje]

@@andr27 Yup, that was the second option I mentioned, if it wasn't clear. Fix it with youtube money and sell it at a loss. the money lost would be an "investment in himself" to learn you can't hit a homerun on every repair. There would be a video in the process and over time he would earn the money lost back trough the views on youtube.

[@Centar1964]

This amp has a weird output topology which besides having common collector connection and no separate output cap I can see no reason for it other than to be different. If it was such a great topology why doesn't everyone do it? It actually seems more expensive to implement as it requires a seperate power supply for each channel and the only thing that can be shared is the power transformer, albeit needing 2 separate secondary windings. Voltage balancing across the serial connected caps is not regulated in any way and differences between the serial connected power /output caps will render the output waveform asymmetrical under load. It depends on the caps being matched and a symmetrical A/C waveform driving the outputs. If the power supply caps are not matched (ie. one is open) the voltage will raise on the lesser capacitance side causing an offset and can possibly blow the output transistors. I was never a fan of capacitively coupled output amps and this design seems more of a trick than anything useful.

[@feedback-loop]

I can see at least a couple of advantages: 1. no DC offset even in case of failure (not killing speakers) 2. no need to insulate output transistors from heat sink (better heat transfer)

[@Centar1964]

@@feedback-loop I think the disadvantages outweigh the advantages and thus you don't see anyone else using this topology.

[@MadScientist267]

If goal is working amp: Replace them all and be done If goal is make money off of it: Cut your losses here If goal is happy customer: Charge part cost and eat the labor

[@jeanangi3233]

Hii

[@muctop17]

6:15 Who can read this today?? Why isnt there a FRITZING translation? 🤣😁🥴

[@mattmoreira210]

Oh god! That cooling system is pathetic, isn’t it? The fan blows a “breeze” through the fins of the heat sink. They should have spent a few more cents on a cowling around that fan. Would have improved the airflow quite a lot.

[@feedback-loop]

There is a plastic guide attached to the top cover. I did not show it.

[@James_Bowie]

You buy stuff from a random China seller, so what do you expect? Why did you not test them before using them ??? I would have just bought the Digikey transistors.

[@eduardomartone8120]

Se a qualcuno interessa vendo usa 900

[@wynton416]

For that price, of course they're fake.lol. To check if parts are original on ebay, just scroll down and look at the brand, fake one's will have unbranded or something like that, original parts will have there name there, like toshiba etc.

[@ralphysound]

привет! хотелось бы на Русском языке.

[@feedback-loop]

привет. я полагаю, что особого смысла для меня это не имеет. размер аудитории несопоставим.

[@ralphysound]

@@feedback-loop у меня QSC USA850 и я его пробую отремонтировать уже 15 лет... )))

[@canadianwolf-hybryd]

Русский акцент!

[@CliveTrezona]

Repair it and sell it.

[@nigelroake3185]

At least check and match the gain of the transistors and check the breakdown voltage - NEVER use chinesecomponents - the amp looks like rubbish -toy unshrouded fan and a tiny heatsink ,with no no thermal cutouts on the heatsink /no visible temperature tracking transistors on the heasink for bias control - in class B the max conversion effiency is about 70% -so at 800 watts output the heatsink is expected to dissipate 800/70*100 =1150watts less the 800 out =350 watts !!! the main transformer is too small as well

[@andr27]

knowing that chinese semiconductors most likely are fake u still couldnt resist to spent 20 USD for crap, but 40USD at DigiKey was way to expensive for quality goods? dont get the way u think. Now you stuck with no solution coz of 20 USD u didn't want to pay more

[@feedback-loop]

1. most probably I will get a refund for fakes 2. $40 at DigiKey is for 5 devices - mixing old and new - not ideal, $20 was an attempt to replace all 3. not stuck, not because of $20, just not sure which way to go: mix old and new for $40 or replace all, which is probably more expensive than the amp is worth

[@4067536]

Dont speek please