How to Fix Marshall JCM800 Lead Intermittent Sound

Sad old girl this amp. Needed a fair bit of TLC.

[@John_Bauers]

Dear Suart, Thank you for all your hard work my JCM 800. I am using it again, gigging and at is as good as ever. I had no idea of the snags you encountered. I promise I won't leave it so long before it's next service.

[@stuartukguitarampguy5830]

I'm pleased it's still out there and being used!

[@workshoptazz7893]

I can identify with situations just like that for sure. sometimes you just have to walk away, rethink, come back and dig in. good job!! thanks for walk threw on the schematic. I did my first amp repair in 1980, on a Fender Vibrolux non reverb. always learning something! thanks again.

[@stuartukguitarampguy5830]

Thanks for the feedback!

[@charlesshaw9090]

Best lesson I was ever taught was to test every single component before installing, always. Thanks for the entertaining vid.

[@stuartukguitarampguy5830]

Ha ha. Yes good idea!

[@goodun2974]

The biggest mistake a technician can make during troubleshooting, with the exception of your probes slipping and blowing something up 💥, is making assumptions. Never assume the schematic is accurate; and always be willing to consider if a part you have replaced might be defective (or was perhaps installed incorrectly by the factory, with wrong polarity polarity perhaps, and therefore you might have repeated somebody else's mistake). Been there, done all that!

[@montygore]

Hi Stuart. YOu sure know what your doing. In 2007 I bought a JCM 800 2210 likr you are working on. Mine was an early 80s model in perfect condition. It proved to be one of the best amps I ever owned. Goode eye on the repair.

[@stuartukguitarampguy5830]

Glad you enjoyed the vid!

[@rightlurker4674]

Love it! Especially your written-on-the-screen comments. Your videos are not only immensely informative, but entertaining as well. "You can imagine me tarting around here for three hours. . . ."

[@stuartukguitarampguy5830]

Thanks!

[@andyreevesarchive528]

"It doesn't matter how many decades of experience you've got...you can waste A LOT of time chasing a fault." My favourite quote of this video!!! I've chased a few faults in the 100+ pedals and 4 amps I've built, and it usually ends with me shaking my head with an embarrassed grin on my face.

[@tjsogmc]

Me too. I spend hours on a complex problem only to find out it was something stupid simple that I should have caught right away.

[@stuartukguitarampguy5830]

Ha ha yes indeed. But of course fault finding just means coming up with a hypothesis and following that trail until it goes cold. Then coming up with an new hypothesis and repeating. All experience does is allow you to choose better hypotheses (?) more often than bad ones!

[@davidshertzer900]

Thanks so much for sharing! I love the schematic and working through the signal path. Nice to see that we all scratch our heads every now and again.

[@stuartukguitarampguy5830]

Thanks David

[@alfaman4113]

Love the sound of old Marshall amps

[@stuartukguitarampguy5830]

Yes they are hard to beat!

[@edwardhannigan6324]

Nice fix, and love the walk thru and info with regard to the schematic...Very informative..Keep them coming, Ed..UK..😁

[@stuartukguitarampguy5830]

Thanks Ed.

[@rickycompton2610]

Agreed Julian Stuart is the man we love your videos here in the USA glad too see you still making them how I wish you were here in the USA

[@stuartukguitarampguy5830]

Thanks Ricky. I'm sure you have some excellent techs out there!

[@clownhands]

Great video Stuart. The fx loop is instrument level (for guitar pedals), hence the 50:1 voltage divider on the send.

[@stuartukguitarampguy5830]

Ok thanks.

[@zbaby82]

Thank you for sharing Stuart.

[@fredrikzachrisson4135]

Regardless of the imbalance you are talking about regarding the impedance etc and why they built it that way......the fact is (I guess)....the amp worked properly before. So the question here is not why they built it this way. Food for thought 👍😀 And like u mentioned, I'm just a guitar player with a JCM 800 Lead 50w 87 having these problems. Thanks for the video, always nice to watch brilliant people, we need more of that in this world! Up the Irons 🤘

[@jesse75]

Get the amp fixed ?

[@tjsogmc]

I got a Marshall MG50DFX last week for $40 because it was cutting out. The first thing I did was jump the fx loop and sure enough it worked perfectly. I cleaned the jacks and the pots and it's going to be used as my new practice/testing amp. Bummer about the faulty new components. I've been caught more than once by that sort of thing, so I feel your pain...

[@stuartukguitarampguy5830]

Nice one. That's my 'go to' solutio for intermittent sound!

[@tjsogmc]

@@stuartukguitarampguy5830 I think you should have said "if you have 45 years of experience like me, then you will also have encountered red herrings due to faulty new components ". If anybody says it never happened to them, then they are either inexperienced or lying.

[@stuartukguitarampguy5830]

@@tjsogmc Yes that would have been a better way to say it!

[@alexdeleon7135]

I too walk away from an instrument when it doesn't respond to proven modalities of repair. Once I return, I work it out in every case. It happens to the best of us. Cheers!

[@stuartukguitarampguy5830]

Yes it's odd isn;t it? Came to me quite soon after I walked away!

[@stuartukguitarampguy5830]

Yes it's strange how that happens!

[@el_micha]

@@stuartukguitarampguy5830 I heard the theory once that your brain is processing problems subconsciously when you're resting or doing sth. else. It needs some time but in the background it is working on finding a solution. That´s also why you sometimes sleep a night over a problem or a decision and it comes to you the next day. Also sometimes one can't see the forest for the trees. Been there me self many times as well…

[@nicksregor4208]

The effect send has to be at the same level as a guitar signal more-or-less, so pedals in the loop work properly. That's why they dump so much volume at the FX send jack.

[@stuartukguitarampguy5830]

Yes that's correct.

[@suso_alonso]

I always used alcohol before there were all these potentiometer cleaning products, I also used it to make life a little more bearable. Good job Stuart. I'm here again to learn from your videos.

[@stuartukguitarampguy5830]

I also use alcohol... :) Seriously though I find alcohol is ok sometimes, but others it doesn;t seem to clean a scratchy pot. A squirt of DeOxit and job done. I am unsure what the difference is and why alcohol works sometimes and not others.

[@qua7771]

I use alcohol before cleaning things as well.

[@jernejkurincic9050]

Regarding the resistor divider network in reverb circuit: dry signal comes through the resistor above (R32), wet (reverberated) signal comes in through the pot and the middle resistor (R31), so those three are actually a mixing network (calculated in a way that it mixes both signals and bleeds wet to ground simultaneously); because it is a passive mixer, you can expect some losses; 100pF cap is here to prevent some highs losses in dry signal. This is my guess.

[@stuartukguitarampguy5830]

Yes that's right.

[@fiddlix]

I love the videos like this. I too have done the same chasing faults. As you say, no substitute for experience which means seat time on the bench. Thank you for the excellent content.

[@stuartukguitarampguy5830]

Thanks for watching David.

[@stuartukguitarampguy5830]

Thanks David

[@robnic52]

Really enjoyable, informative and honest video Stuart, thank you. It occurrs to me that effects pedals are designed for guitar input level signals. 5v of already pre-amped signal would swamp most pedals. Also the effects loop is preferred to happen after the built-in reverb. So you get signal plus reverb then around the effects loop and finally into the power amp section. I imagine moving the effects loop with a whole chain of overdriven pedals to near the beginning of the preamp chain then amplifying their combined output would introduce much more noise than the circuit as designed. As you say there must be a better way than reducing the pre-amped signal back to near guitar output levels then boosting it back up again but I can't quite see one, ant suggestions?

[@stuartukguitarampguy5830]

You are correct in what you say but I don't have any suggestions.

[@dennismasterton3834]

Great work Stuart!!

[@stuartukguitarampguy5830]

Thanks Dennis!

[@montygore]

Air you are a genious. Thank you for the education.

[@stuartukguitarampguy5830]

Cheers Monty!

[@jackiefarmer4892]

Best pot cleaner I have found so far is Dupont Teflon Silicon lubricant (part no. DS6109901) Highly recommended! Unfortunately I can no longer find this in New Zealand, I still have several large spray cans to hand luckily. BTW At Age 68 I also have obscure fault finding experience. Alan. Anvil Amplifiers.

[@stuartukguitarampguy5830]

Ok thanks for the tip. Bet you can;t get it in UK!

[@pedrolourenco2707]

I Think the very low signal on the loop is to allow the use of pedals! the 120 pf maybe compensate from some highs lost in the reverb tank, not sure!

[@stuartukguitarampguy5830]

Ok thanks Pedro.

[@Frankentoane]

Lighter fluid naphta works too for pots

[@goodun2974]

Frank, I'd use Naptha for cleaning pot's before I used alcohol. Many contact cleaners already contain Naptha anyway, including DeOxit if I remember correctly.

[@joolsgrudgings5776]

maybe the 100pf cap brightens up the reverb?

[@stuartukguitarampguy5830]

No,. it's after the reverb so it would brighten both that, and the direct signal. I think it's for the latter.

[@infectionsman]

Those big blue LCR brand filter caps have a 1985 date code. Surely time for a re-cap.

[@stuartukguitarampguy5830]

True but it's all down to whether the customer wants to pay for it or not. Often if the amp is not gig critical, and the caps are still working, they elect not to pay to have them replaced.

[@jjsant3250]

Signal within an effects loop cannot be "strong" since that would overdrive the effects units. I use a 44:1 reduction ratio on an amp I built. Then "recover" with a 60:1 gain stage.

[@stuartukguitarampguy5830]

Yes that's pretty much exactly what they have done here. Great recipe for noise though!

[@el_micha]

But why amping the signal so high before if you have to reduce it down that much. Can't the FX loop be earlier in the chain and therefore save 1 reduction/amplification cycle? Seems so redundant… I don't know much about amp design - just curious to understand it as it was quite a popular amp in the market.

[@JulianMerghart]

This was a special 800 with the higher gain - much better for heavy metal

[@stuartukguitarampguy5830]

Ah interesting thanks.

[@wesleymorris1]

I have one of these amps, I had a conductive board, I wired it on a turret board, it's so loud, I don't use it for practice.

[@pda49184]

With the current price of Deoxit, I'm not surprised you looked at a more economical way of pot cleaning ! Quality control at JJ failed you on V4 then 😂... I've always found I'm a schematic dyslexic and have to trawl through them several times before the mist clears. Was you three hour break from it at the pub by any chance ? Mine would have been ! Cheers Stuart.. See you on the next adventure.

[@stuartukguitarampguy5830]

I actually fly a light plane so I took the old kite up for a whizz around. Had the breakthrough whilst flyng!

[@pda49184]

@@stuartukguitarampguy5830 Brilliant .. What a great way to clear your mind.. You literally had your head in the clouds 😇 .

[@goodun2974]

@@stuartukguitarampguy5830 , I do a lot of electronic problem-solving while I am driving.

[@kengevers8738]

Stuart, the alcohol into the pots may not be recommended. There is a lube on the resistive wafer that can dry it out. Deoxit has the lube in it which revitalize the pots. You likely know that, but in case you don't, a friendly reminder. Cheers 😀

[@goodun2974]

Potentiometers often use a different lubricant for the shaft than they do for the carbon track on the wafer. Sometimes the pot's original lubricant (or residue from whatever cleaner a previous tech used) reacts with the new contact cleaner, and the pot instantly seizes up. This is why I use a syringe to put just a couple of drops of contact cleaner on the wafer. Spraying the switch directly from the can is just asking for trouble. Also note that while Deoxit and other contact cleaners often take great pains to declare their product "safe for plastics", they don't make any claims as to compatibility with other lubricants and other contact cleaner residues.

[@fiddlix]

@@goodun2974 nothing wrong with using alcohol for cleaning pots as long as you don’t over do it. I am sure Stuart is well aware of this as long as he been doing this.

[@goodun2974]

@@fiddlix , I don't know about squirting alcohol directly into the pots, but I do occasionally disassemble particularly intermittent pots and (especially) slide faders and clean them with alcohol and Q-tips, and lube them before reassembling.

[@kengevers8738]

@@fiddlix Agreed, but if Stuart hadn't known the issue for as long as he has been doing this, I was only pointing it out as I have been doing this for a while as well. Just saying. We can all learn something from time to time no matter how long we have been doing things. Cheers 😀

[@fiddlix]

@@goodun2974 I get your concerns, but I think they are largely unfounded, and created by the the advertising team of Deoxit. What do you think techs did before Deoxit came along..? They damn sure weren’t taking pots apart to clean them when when they became the source of noise in the circuit. Bottom line is you clean pots the way you see fit and other techs will do as to what works for them. But, do respect the methods of others. If you want to question something a KZbin creator does, it is always more respectful to email them privately about it than to cause doubt in there KZbin subscribers. Just saying.

[@JulianMerghart]

This man is a legend - keep up the great work Stuart! Hoping to send you some amps all the way from Texas in the near future...

[@stuartukguitarampguy5830]

Cheers Julian!

[@pjf7943]

This man is a hack.....

[@pjf7943]

Send them to a real tech, AMPMEDIC AUDIO in Henderson, NV

[@daffyduxxx]

Thank you for sharing your troubleshooting method. I am in awe of your knowledge, Stuart. Fascinating video. Did you get a refund on the faulty new valve ? 😊

[@stuartukguitarampguy5830]

Thanks for the feedback. No I don't think I ever did get that refund!

[@jseco4544]

It appears that the 100pf cap is in parallel with R32 (470K)

[@stuartukguitarampguy5830]

Yes that's right. Not sure what it's doing though. Maybe a bit of very top end boost?

[@montygore]

I dont know much about guitar amps but in a hf tranciever that would be to reduce or stop outside noise from that circuit. Big question mark from me.

[@mwhcherokee1]

When you power up these old treasures do you use a current limiter, especially on one that has been sitting in garage for a long time?

[@stuartukguitarampguy5830]

I should do but to be honest, I don't!

[@MrDeticus]

Hello Sir. I'm working on a Marshall amp that should be simple, but it's confusing me. Do you ever help people by answering their questions?

[@stuartukguitarampguy5830]

Yes happy pto have a go but there's a strict limit to what's possible via this medium of course.

[@MrDeticus]

@@stuartukguitarampguy5830 Thank you so much! Is there a medium in which would be less restrictive or that you would prefer?

[@stuartukguitarampguy5830]

@@MrDeticus No I just meant that without the amp in front of me on the bench, there's a limit to what I can do.

[@MrDeticus]

@@stuartukguitarampguy5830 Understood. I am working on a Marshall Origin 50. I turn it on, then after about 2 seconds the main fuse blows. I disconnected the transfer and checked resistance at output of bridge rectifier and resistance was increasing, but read around 2.8 Kohms. My lead slipped off and I put it back on and the resistance was then decreasing, also from around 2.8 Kohms. Pulled my lead off, cause i thought that was odd, put my lead back on and the resistance was around 1 Mohm and was decreasing. Checked connections and I don't have bad solder connections. Unfortunately, I don't have, nor can I find, any schematics. I am unsure how to troubleshoot this issue. Do I need a variac to keep power low to troubleshoot the current draw? In my attempts at troubleshooting, I disconnected the various transformer outputs one at a time, then all at once. Only when I disconnected everything, did I not blow the main fuse. Unfortunately, I believe I have now damaged the transformer. The Origin uses two small secondaries connected together to supply rectifier inputs. Those two coils do not measure the same now. I have another transformer on order. I apologize for the lengthy description, but I'm trying to as descriptive as possible. Thank you for any help you can send me!

[@tomfoolery2082]

Does this one have master volume ? Cool amp .

[@stuartukguitarampguy5830]

I can't recall I'm afraid!

[@TheCyberMantis]

Wow, a 50-watt JCM800. Only two el34's inside.

[@TheCyberMantis]

An amp this old, wouldn't you re-cap it?

[@stuartukguitarampguy5830]

Ah, I often get asked that. The thing is it's all money. My customers are not very well off in general so I try to do minimum work to get the amp going.

[@TheCyberMantis]

@@stuartukguitarampguy5830 I understand. If I was low on money, I sure as hell would not be buying a used tube amp. I would be buying a used Boss Katana.

[@goodun2974]

When the tubes are marked Made in India, they're likely not original.

[@stuartukguitarampguy5830]

Yes I wondered that. Quite old though.

[@pjf7943]

Let me see if I have this correct? You determined the preamp tubes needed to be changed because they were dusty? I'll bet they are better than this chinese crap they are installing today... Then you powered up this amp w/out removing it from the case to make a visual inspection of the circuit, on full vac power, and w/out bias meters and a Variac???? LOL...!