HH Electronic S500-D Power Amp Modification

Mark retrofits a speaker protection circuit to a vintage power amplifier. This S500-D amplifier has 2 channels, 340W each, and was built by HH Electronics in Cambridge on 6th August 1980. Hopefully Mark doesn't destroy it...

[@towerman75]

You remind me of a boss I once had. While in the Navy stationed on a Destroyer Escort, we were in what was known a Tropical Hours, and if you didn't have duty, you could leave the ship. Rather than go to my Navy housing home, I got a part time job at a repair shop that serviced everything just like you do. The owner was a man from a different planet. He was a gifted technician like you, and nothing could get past him. The first day there, he put me on a repair bench, and told me, if you have problems, ask me. He said this shop makes its money on time of repairs, and if you get stuck, you are costing me money. I thought about it, was brought my first piece of equipment to repair. While I was removing the covers, he came by and said, " are you having problems ", I said " no ". He left and as I was probing through some points with a scope, he came back by. He ask me, " have you found the problem ", I said, " not yet ". He told me to move over, he took the probe made 2 checks, told me, this pass transistor and two resistors are bad, change them, and grab another piece to repair. I thought he was being an ass, but come to find out, he was just that smart. I learned a lot from him.

[@WhatItBeLike123]

This world needs more people like our guy. Always leave a place better than you found it.

[@bobgrob4]

totally agree.

[@nandi123]

What a perfect way to express it. Mark is 10/10.

[@MickeyMishra]

You know how some people were just born to do something? Cable lacing is my FAVORITE thing to do! In fact its wonderful to watch it being done.

[@davidcrocombe1322]

@@tog8525Cable lacing was what we did with waxed string before cable ties really took over.

[@Ea-Nasir_Copper_Co]

Whoever first bought that amp certainly got their money's worth with respect to quality. 40 years old and still nearly as good as new.

[@jamescollins6085]

I think these designs have very little that can go wrong, so unless the user operates it outside of specifications, it'll run for a very long time.

[@phillcrossland6394]

I have watched with delight your video tear downs and repair of various consumer Hi Fi and the latest one I watched was on the HH Electronic S500D professional Power Amp. I was pleased you gave this 1970s power amp the credit it truly deserved as before the HH Mosfet range emerged in the 80’s this was the backbone of many if not all large touring bands and large club bands with a reliability todays far eastern amplifiers still dream of. As you underscored they were almost bomb proof but when they did go wrong, usually from abuse, they were a costly repair both in amplifier and unfortunately speaker systems. I found your mod to incorporate a speaker DC protection board very interesting and informative because this year I have rebuilt 5 of these beasts and contemplated incorporating such a device into them myself. I do have one or two observations to make though, as I have mused over why you opted to use only one DC protection board to protect both independent amplifiers which obviously if one fails they both go down I would have used 2 boards one for each channel. Another point I noticed was that the relay contact rating was only 5 amps this is well below the peak current that would be drawn if the output transistors went dead short as those big red 15,000uf computer grade capacitors which are capable of at least 12Amps each, so I guess this protection board would only work once as the contacts would be burnt away. This leads me on to another point and a problem with many of the Chinese DC Protection boards I have looked at including this one and others using the same IC, is that they reset after a short period of time after the DC has gone away, now because the S500D does not have any secondary voltage fuses to break there is the possibility that this could cycle one off to the speakers before maybe eventually blowing the primary 240v fuse rated at T10A a/s after destroying other parts within the amplifier. I congratulate your valiant effort to incorporate some form of DC Protection to this legendary professional power amplifier but I do feel it requires further thought. I was also surprised that you did not look inside the driver modules as they were not all potted when you had the modules out and check the test voltage of 2.4v between the test pins 2 and 10, this sets the bias level for the output devices. which operate in class AB+B mode however this is not adjustable, it is what it is but has to be correct. You should check out the Crown DC300A for comparison. The Mosfet versions of high power amplifiers HH Electronic followed on with used a couple of various simple DC Protection methods one being a fuse blower (crowbar) across the loudspeaker output designed to blow the DC supply voltage fuses that HH reluctantly incorporated, however due to the high currents possible I guess the Thyristor providing the crowbar only worked once as well. Another point from your video is that of the five HH S500D’s I refer to, 9 out of the 10 capacitors had developed due to age an electrolyte leak some much worse than others, I thought after 50 years in service you may have considered replacing them, I have replaced all 10, the correct value and size are difficult to find but Mouser Electronics UK can supply a Chemi Corp version of the exact fit. E36D101HPN153MC92M. I also observed that the driver transistors on the modules were connected straight down to the PCB copper plane, various versions of this module incorporated an alloy plate of varying thicknesses and shapes as a heatsink because the initial output in class AB these are the output transistors and do run hot. Adding a thick alloy plate does help these transistors run cooler and stops the PCB from becoming heat damaged. You rightly observed that the fan did rotate slowly once powered, this is a fault, the capacitor fitted across the thermal sensing switch is leaky and should be replaced as well as the 2 sensors noting different temp cutouts. I have also watched your repair of the Peavey CS4080HZ power monster what a load of rubbish in design compared to the HH S500D, I would not have taken on that repair job, credit where credit is due. I hope you don’t take this as criticism for the sake of but rather a sharing of ideas between fellow Engineers.

[@MendItMark]

Hi Phil, Thanks for taking the time to write this. I'm flattered that someone of your expertise has paid so much attention to my videos. You make a lot of valid points, and I agree with most of what you say. I'm not an expert in any make of equipment, and often work on things I have never seen before (including the S550D). Keep watching, I'm still learning every day!

[@ArlenMoulton2]

@@maxfactor4209 For 6 months, before it goes bang, not 42 years!

[@alexmarshall4331]

Ahh Crown DC300A... beautiful 👉🇬🇧👈👉💎👈❗❗

[@vishaalgargi]

@@maxfactor4209 imo class d is good for powering sub. I will take class ab or pure A for speakers

[@jtsotherone]

@@alexmarshall4331 named DC for the DC they love to put out when they die and toast your tweeters!

[@ctbcubed]

I really liked how you integrated this mod into the amp. When I was building power amps with direct coupled output, I learned pretty fast that losing either the positive or negative supply voltage would dump the remaining full rail voltage into the load. This was before that neat little chip was available, so I designed a circuit to monitor the supply rails and if either one went away, the speakers were disconnected immediately. It also produced a delay on and instant off function to eliminate annoying thumps when powering on/ off. One other learning experience was that I put fuses on the rails but they only fed the output transistors, not the driver board. If the fuses blew, the driver circuit tried to to supply power to the load which resulted in a cascading failure of every transistor in the non-fused driver board. Luckily these lessons were learned during testing on a dummy load, so no speakers were sacrificed!

[@AstrosElectronicsLab]

Yeah. Unfortunately, the design of that circuit is pretty bad and a poor implementation, Chinese "let's just laugh and follow what was in the NEC datasheet". Sigh. A, it's a 24V relay (with a DC coil resistance of around 1.2k), B, it's running on a 12VAC (16.9VDC) supply, C, the turn on delay is FAR too long (look at both points A and B as to why) and D, the drop-out time when detecting 1.5VDC is also WAY too long. Finally, the 100uF that is used in the input DC detection circuit should NOT be electrolytic. It should be bipolar, and probably a lower value such as 22uF (which will decrease the detect time). If the amp fails with a negative DC fault, that cap will eventually be destroyed as it's being reverse charged.

[@davidquick3535]

Mark I find my self wondering why you haven't got more subs , Ive been going down the mark rabbit hole and find that you are a proper craftsman entitled the recognition of your devotion to your expertise !! Well I want you to know I appreciate your content !! Awesome!!!

[@yt4krist0f]

I loved the cable lacing: attention to detail..! Respect!

[@Gez492]

What a pleasure to see such a nice well thought out design and layout, with properly dressed cabling. We used HH power amps in our PA systems because they did have such a clean but muscular powerful sound. I guess that huge Transforner and filter caps help a fair amount in this respect. Also had Had a HH combo guitar amp with dual concentric speakers. It was the loudest bloody combi I have ever heard completely crazy power! HH were a Great British Amplifier design with superb stylish products. That Green glow backlighting looked the "muts nuts" on stage, I think Naim electronics copied that for their HIFI products.I don't believe any class D would stand up to road use and still sound good for 5 years let's alone 42. Great Video loving your work Sir.

[@SauvikRoy]

I loved it when you praised the loomed cables which showed good service/build history. In the end, you took time to organise your wiring with the exact same type loom. Good workmanship never cuts corners. I wish I could apprentice for you. Regards.

[@burner8959]

I was yelling at the monitor.."Nope it's upside down. Nope still upside down...Oh Mark figured it out." Loving your content man. As others have said, your coolness, and ability to chuckle at adversity is commendable. Let's just say if I were able to make these, mine would mostly be beeped out. Haha.

[@johnsampson1096]

Amazing modification, Mark! Can't get over how that board and transformer look like they came from the factory when installed. I think the cable lacing was the icing on the cake. I think I'll go crack a beer on this video and watch it again..............

[@eddiewillers1]

Nice! The cable lacing puts me in mind of military-grade gear (such as by Redifon, Thomson, Racal, or Marconi).

[@magikjoe3789]

The S500D is a beautifully built amp. Much like the amplifiers I built at my first job at Derritron Electronics, but without the water cooling!

[@rupertprice5508]

Hi mark an old electronics guy from the 80's and before . very envivous of your shack load of test equipment. Mine a lot more basic but still able to have fun. recognised assembley jig which I used in 80.s in SA.. love your video cheers Rupert

[@phillippatterson512]

That amp is truly a thing of beauty! You did an excellent job on it. Thank you for sharing. I wish we had more people like you in the world who take pride in their work as you do.

[@Andy-sh9eq]

You said very 70's design, you got to agree it's better made than a 1990's design amp, well done Mark another great informative video 👍

[@thanhhuynh272]

This takes me back! Way back when I was a “wet behind the ears” teenager I worked at a local hi-fi place here in Hobart. They did P.A. Stuff too and had this amp that was a direct clone of the bigger brother of this amp you have here. Someone had dug the front end out of the potting and literally copied it. They were quasi-complimentary I recall as the semiconductor manufacturers had not quite yet worked out how to make the really solid PNPs yet. (c1978). We used 2N3773,s, three in parallel each side with 220 m Ohm emitter resistors and had added an eight band graphic int the case too….it was built around the LM3900 Norton mode op amp I recall. Much later, in my mid ,20,s, I recall fixing one if these H and H,s, probably not this exact model, but it sure looks familliar…I got a hell of a wollop from the supply to the electroluminiscent front pannel which basically was the MAINS directly in parallel with the transformer primary….I learned “the fast way” to treat it with respect after that, but I have never forgotten it. I have built and modified dozens of ams in my lifetime and added so many speaker protectors I cant count anymore. Have used many different designs of speaker protectors. In the early days we just copird circuits from wherever we saw them, Dynaco, Haffler, Japanese Amps etc. Much later we settled on the same circuit you have here, just out of sheer convience and low component count, thanks uPC1273! One thing I would have done though…..and we learned it from some Japanese amp speaker protectors….make those two summing resistors at the inputs to the circuit from the speaker lines, 2 x 56K here, a little different, say 47K for one channel and 68K for the other. This guards against the rare case where both channels fail ne slammed to the positive rail and the other slammed to the negative rail…..which can happen of the amp is being full bridged.

[@BigCar2]

Another great video. Thanks!

[@fretlessfender]

Nice to see another big channel supporting Mark! Thanks Big Car!

[@RWL2012]

Didn't expect to see you here!

[@donlunn792]

When I was leaving School at 15. I had a choice,car mechanic apprentice, or radio tv apprentice. My dad wanted me to be a radio tv engineer! Which in later life would have given me a much better wage,especially as electronics became so much of our life.. Guess which I chose, I was Car, Motorbike mad. Radio,Tv not so much. Boy do I regret my decision. Now retired,so nothing I can do about it now. Electronics is now just a hobby. I have to say as a Mechanic (Blowing my own trumpet) I was a natural. Some people are born to do a certain job. Me? I was born to be a mechanic. Mark! You are a natural electronics engineer. Terrific channel. Hey! Horses for course’s! 😀😀 Mark as an electronics engineer you could hold any job.But being your own boss,and enjoying what you do means so much.

[@subculture-records]

This channel is so addictive.

[@user-gx3te1kb2w]

Those 500W modules had super efficient thermal transfer to the fins with forced air cooling. Carlsbro bought HH around this time, I bought these modules from Carlsbro integrated into large PAs (100V line) for use on several North Sea Oil Rigs.

[@LZDEN]

i really do like a 'mend it mark' video, excellent work Mark,i could watch you working all day..all the best to you Mark.

[@markusbFPV]

Hey Mark! Found this channel yesterday from a YT recomendation. Love these old gear repairs as I used to repair similar stuff many years ago as a hobby. Also your smile and happy energy is contagious 🙏

[@Kopter6]

Mark I'm addicted to your videos.

[@Boards-dont-hit-back]

Whenever the world is getting me down I come here to watch Mark.

[@pickholder6189]

Worked on a few of those over the years. I did my original electronic training in 1985. Sure not many people trained later did cable lacing.

[@barrydevonshire9749]

What you do is interesting. Being in the industry if we did lay on joints we would be shot by QA. I did so much lacing as an apprentice. I found it therapeutic

[@wirdy1]

Must be a few of us old apprentices who remember the professional pride of doing some neat, even, lacing :). Nowadays it's just plastic twists or ty-raps everywhere!

[@briansimonds]

Perfect job great attention to detail even on parts no one will see lovely to you can still make a living doing repairs and servicing.

[@user-yy4mf8js5q]

С удовольствием смотрю как вы ремонтируете технику ! У вас хорошие знания и хорошее оборудование ! Мне тоже нравится ремонтировать технику , которая достойна ремонта ! Вы молодец ! 👍👍👍

[@murraywebster1228]

One of the most notorious amps ever for going DC, I learnt to be good at speaker re-coning because of them🤣🤣🤣

[@justicelut]

Thanks Mark, that amp is nearly as big as your screwdriver!!

[@wackowacko8931]

I have a feeling you could use some Clecos (instead of screwdrivers) to keep sheet metal holes in alignment. They were made for the aircraft industry, to (oddly enough) keep sheet metal panels/holes in alignment when fabricating aluminum cladding on wings and engine cowlings. You don't use them often, but they are extremely handy when you do need them. You will probably need one of the smallest diameters, along with the pliers to install them. Summit Racing carries them, but there might be some places in the UK (automotive and aircraft sheet metal fabrication supply stores) that might have them. If you have a contact in the aircraft manufacturing world, they can probably tell you where to get them.

[@GraemeSPa]

I remember when HH first came out. I was eye shopping in Hamiltons in Middlesbrough and these things were sat lurking - sleek electroluminescent, glass clean , light as a feather (compared to a Selmer TB50) - looking like something out of Star Trek. Way out of my price range - I had just bought a 2nd/3rd/4th hand WEM ER40 and 2x12 cab for an eye watering 45 quid. HH were nobbut a dream for me.

[@thetruthchannel7073]

A true professional here attention to detail fantastic how he explains everything and a good sense of humour worth the weight in gold you are to know no someone have there are still repairing electronics I've got a little job that I need doing so I will come over ASAP

[@Parivesh_S]

You seems like the worlds most happiest person…and I love watching your videos…love from india❤

[@THEDTSMAN]

Your attention to detail and pride of workmanship is what is seriously lacking for some people these days. Truly inspiring and fantastic to watch.

[@panther105]

You are so good at what you do I hesitate to make a suggestion, but drilling into confined spaces where you don't want to puncture something on the other side, just put a piece of tubing or wood that the drill bit will drive into and go no further. Leaned the lesson many times.....the hard way

[@stephensomersify]

I remember the HH electronic amps all too well - I used them for P.A. systems and they did a mixer/amp with full sliders and equalisation array

[@mik310s]

Thats one sweet setup you have there, one of the best electronics benches I have ever seen.

[@jimmy_jamesjams_a_lot4171]

Electronics Engineering is my favorite proficiency, and it’s great that today there’s plenty of instruction available online. You convey very interesting dialect in your language! I don’t know the accent from Cardiff any more than that of Northumberland. Maybe you are Irish? Geographic location isn’t important, but you have developed a very cool sounding lecture style! I can truly appreciate everything that you say. I find it equally captivating when I don’t follow your words at all! I love that PCB holding jig, that gives me a great idea to copy it!!! Why didn’t I think of that on my own?!? Foam rubber being just elastic enough to squish in and flex about and hold the components in place, NEVER a need to bend the leads!! I know you HATE bent leads!! THANKS Mr. Mark FOR ALL OF YOUR ENDEAVORS, REALIZED OR NOT!!

[@duroxkilo]

spring loaded center punch is the key to success. next time you see one, get it. you pop it 2-3 times on the same spot, the drill bit doesn't go anywhere :)

[@WilliamPatu]

That solder board holder jig - Wow coolest.

[@bobt2414]

Let's be pedantic. Are they really GAIN controls and not VOLUME? A gain control should alter the gain of the input stage (usually via a virtual earth op-amp) thus controlling sensitivity. Volume controls are simple potentiometers offering a variable input voltage to a fixed gain pre-amp. Dear oh dear! I just loved that cable lacing. THAT is professional!

[@outfield1988]

Would trust Mark with anything I own. Mark is just amazing. This channel is awesome.

[@johnward3874]

What a neat and clean PROFESSIONAL work bench! I need to take note. Great approach and well done! These HH products are really well made.

[@wirdy1]

I was nervous when you were drilling close to that lovely toroidal transformer! & thank you for lacing those wires; brought back memories of doing lacing cord & the big fat lump of beeswax that always sat next to the reels, for lubricating the cord.

[@snakeyengel]

Mark, you're a treasure, mate. I have learned a LOT watching you work. My only constructive criticism is to get a camera with the ability to turn off autofocus. Since a LOT of your work is close up and detail oriented, it makes sense to have something in place for those close ups that isn't always trying to autofocus on the hair on your arms, or the whatever has a lot of contrast in the fore or background. Other than that, just WOW! Great job and a real renaissance man, you are. Cheers!

[@bobrose7900]

Beautifully made unit...

[@nutsnproud6932]

Thanks for the video. I remember doing cable wrapping before cable ties were popular.

[@hydorah]

There's always a 'wow' moment in all your vids. When you busted out the cable lacing skillz that was amazing. I love H|H stuff I have three V800 amps doing the driving for my PA

[@markallen1782]

Hi Mark I was working on a similar model HH amp from the late 90's, that had been used in a TV studio to run foldback audio from the control room. The neon inside the power switch had died, so no one noticed that it was turned on all the time, until it developed a crackle in both channels. When I checked the main board, the components for the low voltage power supply had burnt the fibreglass on the PCB to the point that the copper tracks were lifting off the PCB. I am guessing that it had been left on for about 20 years or more... The hard decision was made to junk it, and buy a new amplifier, one that did have a speaker protection system, and with working power lights, so that you could see if it was turned on....

[@maneeshsharma9860]

I am spellbound whenever I watch your videos…thank you Mark for spreading the Happiness 😊🙏🏻

[@freednighthawk]

I do love it when a board has the component values in the silk screen. Makes it much less likely to make a mistake.

[@wagsman9999]

Hope this gentleman gets a million followers!

[@marktubeie07]

I'm lovin' that PCB jig, I would like one of those Mark! Enjoy your vids mate.

[@RanTausi]

Great workmanship 👍, I have bought an assembled SP protection board with 220volts input so no need for 12 volts ac power supply, I have bought a soft start board too , but I am lazy for the project at the moment 😭. Thanks for the vid, cheers

[@Extrikit]

Good day out for Mr. Makita.

[@playpaulbee]

Excellent, these amps back in the day had such a poor reputation, it's easy to forget how well build they were, it was HH's step up into the pro market, a mate had a very good one, which unlike most of the others I came across worked well, it was the forerunner of their superb V Series, I still own a V800, which is an amazing amplifier, built like a tank and very powerful but at 25kg doesn't get to go and play much these days, thanks Mark.

[@AkkerKid]

I came across these amps in college in 2006ish. We used them in our robotics lab to run servo motors. We actually liked them BECAUSE of the lack of DC filtering. I personally rewired their transformer taps to run on our 120V AC in the US. I remember being struck by how well they were assembled.

[@ErnieLampe]

You are the Bob Ross of electronics! Stressless magic work at the bench.

[@Setsuna_Kyoura]

OMG yes, truly like Bob Ross. So damn relaxing watching Mark work on some nice electronic gear, while learning so much about problem locating and solving.

[@Mr.Rude.]

100%

[@rajvindermanku825]

Hi Mark great videos i would try to make a good mechanical connection when soldering the wires to the transformer eg feed the wire into the holes on the tags then bend slightly and solder, look forward to more videos.

[@jeromestevenfaigin6059]

My dad would of love to you work with him repairing TVs and electronics of all kinds from 1957 to 89. Cheers!

[@simonlunt353]

Not long l have found your videos and I am enjoying them very much so I am going through your videos you can get automatic center puch for your holes l like your thought to detail well done sir for taking pride in your work 👍👏👏

[@BobBasshead]

I really liked that PCB holding jig, SNAZZY! Looks old school.

[@thefilthygringo9228]

Your cable lacing is on point! Love it

[@andreasrose5709]

Pardon for my bad English, I am German. I am pastor and practise electronics only as hobby. I had the idea to modify the speaker safety board: I connected the non used nc contact of each relay to ground via 27 ohms resistors, so that the output capacitor of the power amp is charged quickly when amp is started. Without that it really took very long time until the safety circuit didn‘t detect dc offset on output level anymore.

[@tomaskorenek1259]

handy kit, presume it can be installed into old 70s hifi amps as well. Thank you Mark for another nice video.

[@user-vp3ch9vi6v]

Спасибо! Приятно смотреть на работу мастера. Заряжает своим позитивом.

[@StephenCashmore]

That takes me back to doing cable lacing, it must be 30 years or more since, but the H&H amplifier was my favourite amp ever we had a 500 on each channel which you had the option to Bridge, and we ran it bridged into a 26 inch super bass speaker that originated from the Abc cinema Leicester. It was originally used for the film earthquake along with another 11 of them I believe.

[@tomthompson7400]

wow ,,, I had a rack full of them driving an H and H concert system ,,, you had to unplug your horns before turning those on , or off , or you could loose them very quickly , and it was advised not to stack them ontop of each other in a rack , you had to leave an air space ,, now the V800 that followed them was simply a thing of great beauty ,,, ah , the good old days ,, I did a tout with Adamski who ran a Mayer Ms8 system from about 40 V800s all in parallel mono , and boys did it sound sweet.

[@markharrisllb]

Only found your content this morning. I’m the complete imbecile and know nothing but you keep your videos simple and easy to follow. Brilliant, thank you.

[@barrybradfordlocksport]

Breath of fresh air watching you chnnel mark your always so happy a joy to watch !

[@madmatt8946]

I don't know anything about electronics, but it's all very fascinating. I have been watching your vids lately and I appreciate your personality, attitude and apparent high level of knowledge. I would love to build my own audio amp someday! Best wishes from across the pond! 👍✌️🤟

[@joseabrahaoabrahao7937]

Exatamente trabalho. Essa dica para instalar sensor de velocidade para o ventilador é importante para deixar o ambiente com nível de ruído baixo.

[@DocBain1]

I always enjoy your sense of humor. On the video where you forgot to discharge the new capacitor, you simply laugh it off. Great job on all your projects!

[@stephenyoud6125]

Excellent work Mark including the bundling of the wires. Unclue Doug on KZbin repairs vintage Guitar valve amps and always keeps the wires separated and brings them in from high then directly down (or up) into the valve base pins.

[@BenState]

heater wiring is different, its done that way to keep ac noise of the tube

[@fretlessfender]

Mark, again! Great video, love your diction and laughter... stay at it! You'll be great!

[@gibbogle]

Besides your exceptional skills, I really like your unfailing good humour, Mark.

[@FVEBlog]

That support PCB for soldering components is so cool. Verry nice!

[@kthwkr]

At the college radio station students took care of all the equipment. One night an architectural engineering student during his Friday night show called me to report that the audio power amplifier in the studio had failed again. It was a Marantz which had output transistors with emitters to the rails and collectors together to the output. Completely the backwards implementation of complimentary output. And was very unstable. When it failed it was usually an output transistor shorting to the rail producing rail voltage on the speaker output. And it had cooked some speakers in the past. It came equipped with DC protection relay but a failure even further in the past had welded the contacts so that level of protection was non-functional. He told me he had hooked up a substitute "amplifier" but it did not work. Turned out he had pulled out of the engineering storage closet a +/-24V DC power supply and hooked it directly to the speakers because he didn't understand the difference in an amplifier and a power supply. He had no idea. Architects! The speakers were Bose 901s. I hated them. I got there and thought that surely he had destroyed those speakers. I was kinda hoping. But nope. He had blown the power supply. Those darn Bose were happy to sit there taking that 24V DC until the output transistors in the power supply blew. So now I had to fix that Marantz again plus a power supply to boot.

[@marklimbrick]

Bose had a quartz lightbulb for protection. Non linear resistance increase as power goes up. It works.

[@RickMahoney2013]

I just started watching your videos and you sure do make audio repair fun to watch and understand.

[@billsmith5166]

I really like the build quality on that one. Nice clean work on the mod. Well done.

[@outfield1988]

Cable work reminds me of the wiring bundles on the F16 SO cool.

[@fillo1971]

Sto leggendo molti dei vostri commenti e concordo con ciò che scrivete, c'è chi chiede perché Mark non ha più abbonati, chi scrive che il canale o i video di Mark creano dipendenza, o perché questo e quell'altro. TUTTO VERO SOGNORI MIEI Mark è veramente un artigiano di altri tempi, un vero appassionato di elettronica e di cui può vantarsi di conoscere veramente la materia. Sono un tecnico elettronico anch'io, sono cresciuto in laboratori di riparazione radio e tv, ma sono rimasto fedele a ciò che mi è stato insegnato, ossia l'apprendimento dagli altri più bravi di me e non avere invidia del lavoro altrui, anzi rubare con gli occhi per migliorarsi. Quindi se Mark non ha tanti abbonati è anche perché qualcuno prova invidia del suo risolvere determinati guasti, che a me personalmente potrebbero sembrare molto tosti, allora che faccio? guardo i video di Mark e i commenti di ognuno di voi, per farne tesoro personale. Perché così dovrebbe essere, si deve condividere con gli altri per raggiungere l'obbiettivo finale ossia la risoluzione del guasto, in questo caso qui si parla di elettronica, ma in tutti i campi dovrebbe funzionare così. Il rispetto del lavoro altrui. Un'altro scrive in un commento che mentre era in Marina il proprio datore di lavoro gli sembrava che era uno stronzo, ma non era così, era soltanto un tecnico capace, alla fine era diventato e lo è ancora anche colui che ha scritto quel commento, questo si chiama crescere e capire. Quindi io vi ringrazio ad ognuno di voi per apprendere da ogni video qualcosa di importante. Questo si ché un bel canale di riparazione elettronica. Grazie. 👍🏻💪🏻💯

[@shaunmorrissey7313]

If "the devil is in the detail" that makes you one hell of a demon 😀

[@user-ge8wy8so3q]

Based on the Amcron 450 that Slade bought back after their US tour. Great piece of kit and ran 3 of these back in the day. Early units had a fully encapsulated driver stage but later models werent potted. Note that this would produce 500 watts per channel into 4 ohms. Not that any HF distortion may be due to poor connections on the power transistors, so make sure they are tight.

[@simonbaxter8001]

Always a pleasure to watch your videos Mark. Just wondering why you didn't loop the cable through the transformer tabs before soldering ... that's what they are there for!

[@ramhajare8844]

Good smile bro, You're working style is positive,god given you grace.God bless you

[@mikem9403]

Amazing work. I always enjoy your videos.

[@kaktusadas622]

Hello, I envy you this workshop and the fact that you are able to make something out of nothing, I greet you warmly.

[@pascalhibon7928]

With DC protectors who use a relay it is good practice to use double throw switches in the relay. In the "off" position, the switch should connect the speaker to ground. Whenever there is a DC fault in the amp, the relay will make sure that the arc dies when the relay switch reaches the off position. Otherwise, the arc will still potentially cause the speaker to die. I must say that these HH amplifier is nicely build and is very service friendly.

[@hubbsllc]

For a time Stanley Clarke was using HH power amps in his bass rig - probably running a set of full-range EV cabs as I recall. I seem to have it in my head that Van Halen used them too but I don't recall for what.

[@bornagain2641]

Absolutely like this. Cant believe all the equipment u have.

[@prillewitz]

Great to watch your videos Mark. The electronics remind me of my school time when a lot of guys chose to be an electronics specialist. Unfortunately this education ceased at my school after a couple of years. Love to see you busy with the stuff. 😊

[@Prometheus574]

The Black Watch is an electronic wristwatch launched in September 1975 by Sinclair Radionics. It cost £24.95 ready-built, but was also available for £17.95, as a kit.[1] These prices are equivalent to around £190 and £140 respectively in 2021, when adjusted for inflation. The Black Watch was supplied with a plastic band as standard, with a black stainless steel bracelet available as an extra at £2.00

[@hancecrawford]

Great job Mark nice protection mod well done

[@TheBudgie29]

Should always turn the Volume knobs down before switching off the amp. Because when you switch on the next time, you are not going to get huge Thump from the speakers. I had a H/H amp on my disco years ago, and it was the best sounding one at the time. Nice mod to.

[@Si1983h]

The knobs are connected in line with the input, not the output, they have no effect on switch on thump, the output stage is directly coupled to the speakers. That’s part of the function of the module that Mark is installing here, it will provide a start up delay and will physically disconnect the speakers if any DC is detected on the outputs.

[@nakfan]

This video had everything. Thanks 😊

[@paulchasteauneuf2098]

Nice work Mark, great work, excellent viewing 👍

[@reallyniceaudio]

glad to see other people having to do metal work on the bench! nice mod!