Hello all. New guy from Florida.

[Amp]

Hi everyone 

Seems like a great forum. Amps are my great weakness in the never-ending gear chase  :lmao:

I'm interested in learning more about how to do basic repairs and the like. I've arrived at the point where I can take stuff apart and clean contacts (which is a shockingly consistent way to bring a lot of discarded stuff right back to service) but would like to learn more about how to do more advanced stuff. I'm a pretty high level woodworker, have built a few acoustic and electric instruments, but the amp side of things is new to me, even though amps are really a passion.

Anyway, I've rambled enough. Thanks for the forum. Hope to learn a lot.

[Roly]

Welcome @Amp (...that could get confusing)

Shhhhh ...    cleaning stuff is our little secret.  I've been fixing stuff for going on 50 years now and cleaning is a large part of what I do.  This became even more so when the early PC's arrived with about a thousand sliding contacts, and I estimate that something like 80-90% of problems I encountered were basically due to sliding contacts that stopped contacting.  The number of machines I "fixed" simply by seating all the RAM down again, or pulling all the cards and giving the edge connectors a good scrub with an eraser ...   :duh

One of the most common faults that comes up here are the bridging contacts in the Fx Send and Return connectors; a drink of contact cleaner frequently brings them right back.

Possibly the second most common would be blown output transistors, but provided you take a methodical approach to faultfinding, and a careful approach to testing your repair (power-limiting lamp), quite a few newbies manage to get their amps going again just fine.

I take my hat off to you building guitars; much harder than building amps IMO.  Electronics is fairly approximate, but instrument building requires precision.  I've never built a guitar but I've done a lot of instrument repairs and I always find the required accuracy a bit of a challenge, but very rewarding when you pull it off.

[Enzo]

Hi.

I too have been working on this stuff since the 1950s, and running a pro audio repair shop the last 27 years.  What you describe are what I call the 90% repairs.  Roly said it first, but I maintain that about 90% of the repairs I do boil doen to maintenance.  Clean the noisy controls, clean the dirty shunt contacts on the jacks, touch up the solder on the jacks and controls, maybe resolder the occasional filter cap.  ANyone who knows which end of a spray can the stuff shoots out can likely do this work.

So that leaves the rest, which really will require some knowledge. 

Learn Ohm's Law, trust me, I use it every day.  Not in some philosophical sense, I really use it in a practical manner.  I have a small pocket calculator next to me for Ohm's Law calculations - voltage/resistance/current.

And what I preach in most of my posts - systematic troubleshooting.

[Roly]

To which I can only add that getting into the habit of doing "round number" calculations in your head can save a lot of button-stabbing.  One of my tricks is "round up, round down".  For example if you read 12 volts across an 8.2k resistor, round 12 to 10, and 8.2 to 10 (one number down, the other up so the estimation is balanced), then "one volt per milliamp in one kilohm" tells you you should have something like one milliamp flowing, but a bit more because the voltage is higher and the resistance is lower, so a bit more current.  {exactly; E/R=I  12 / 8.2 = 1.46mA}

Electronics in generally is pretty approximate, and with guitar amps you normally only need to know if a reading is in the ballpark or suspect.  Sometimes you need to get accurate, but normally 5-10% is fine; after all, most components and instruments are about that tolerance anyway.

Apart from internalising Ohm's Law (and the Power Law E*I=Watts) so it becomes intuitive, the resistor colour code is something you will certainly need, but pretty quickly you will find you can look at a resistor and know it's value straight away without having to resort to decoding the colours because you just recognise the look of the common values.

A mnemonic for Ohm's Law is "E is always on top".

At their most basic amplifiers are a chain of stages from input to output and the initial problem is finding which link in the chain isn't doing what it should, then within that stage finding out why; mostly from voltage measurements, but occasionally by signal tracing.  The simpler method being to work back from the output stage injecting a signal and seeing if it comes out, or a bit more techie injecting a signal in the input and following it through with an oscilloscope.

But as you have already observed, a fully equipped workshop is only occasionally required and careful observation and a bit of logical poking about can often get you there.

[Amp]

Boy, that's a heckuva informative welcome!  :dbtu: :dbtu:

Thanks so much guys!

I'm ordering the books suggested in the stickied thread. Still learning about the theoretical stuff.

My main objective is to be able to maintain my Roland JC120s in the event they ever have an issue. The new Chinese ones are very cheaply made and sound a good bit different compared to the old Japanese ones and I couldn't imagine having to try to find replacements after having these for so long.

Also, just knowing how to do basic stuff on amps, man, what a goldmine. This local craigslist ad:



Where I wound up buying a Marshall JTM 30



... for a whopping $35, just because it needed a contact cleaned

[Roly]

There is more than a couple of hundred years of experience with sundry stage gear on tap in this forum, and as you may have gathered we like helping and explaining.

First you find out what is wrong, then you worry about obtaining the parts needed.  Just about everything is substitutable or work-aroundable, and it's pretty rare, even with very old valve/tube amps, to get really stuck.

It's helpful that guitar amps tend to be pretty generic, and once you've worked on a few they tend to be pretty obvious and straight forward to repair.

As it happens I have a classic Jensen 50 valve amp, built in New Zealand, on the bench and I'm presented with a particularly curly problem - a worn pot in the tonestack which has a half span tapping, once fairly common, now practically "unobtainium"; so if I can't restore it by cleaning I may have to get creative and try and make one out of a standard pot, but this is a pretty extreme case.

More normally when it comes to old gear it's like the Jands 24 channel desk I acquired, and swopping all the old op-amps for LM833's literally made it better than new, quietened it right down.

just knowing how to do basic stuff on amps, man, what a goldmine.

As you have discovered with your amp, quite minor faults can totally demolish the price on second hand gear, and I always consider "faulty" gear to be well worth the gamble because it is so often something very repairable.

When you are starting out some things will present quite a challenge, may even defeat you at that time, but firstly these are always good learning experiences, and secondly if you just file it for a while on your back burner you may come back to it in a year or two with a much clearer idea of what is wrong, and how to fix it.

I date from when TV's came in black & white and were full of valves, and I wish I had had the resources that are available today via the interweb in terms of self-teaching sites, circuits, and skilled advice in forums like this.  I started doing on-line guitar amp help desk via e-mail about ten years ago, expecting to get wins in perhaps one out of ten.  Instead what I have found is closer to nine-and-a-half out of ten; school kids to old geezers like me normally only need a bit of a helping hand and a few pointers to get loud noises happening again.

Learning about stuff like fixing amps and stomps tends to be an unstable situation - the more you learn, the more you get thrown at you to fix, the more you learn.

[phatt]

Re JTM 30,
Don't wish to be negative but
a  Local guy (Nambour Qld Australia) has that exact Amp and has blown power transformers 3 times. Have not actually worked on that Amp but have read about some issues with Power Tr.

Otherwise when working it sounded good to my ears. I did note Power Transformer ran very hot.
lets hope they made better versions where you live. :tu:
Phil.

[Amp]

Roly, thanks so much for the wisdom and encouragement.
I'm really excited to be exploring this path... I have a bad habit of trying to get ahead of my personal learning curve and fly the plane before having learned how to properly sweep it, but I think that's natural for a lot of folks. Pacing is the key, as with everything else, I suppose.

@ Phatt: They have a weird design flaw where they run hot that's well known. I took off the valve cage and run an aux fan at all cimes. Tone wise, it's an absolute beast for a 112 but I'm aware of the overheating issues some units have. Per internet chatter, they seem to categorize themselves as either problem children or reliable on a unit to unit basis. We shall see how this one works. So far, it's a gem but as noted, I do run an aux fan just to be safe.

[Roly]

You'd think after all this time Jim would know how to design an amplifier, wouldn't you eh Phil?  With a few very notable bloopers in recent times they sure ain't what they used to be.

@Amp - Added cooling should help, but if it ever eats its power tranny just replace it with the biggest mother you can find.

I always find that paying attention helps.  There is a saying, "Never struggle with a computer problem from more than an hour without calling someone".  These days something similar applies to amp repairs, and although the turnaround on a forum like this, or e-mailing somebody is a bit slower, it not only is sensible to ask for assistance, the very act of explaining the problem to somebody else frequently knocks loose that stuck wrong idea.  I've found on many occasions simply going over a confusing fault, even to somebody who doesn't have a clue what I'm talking about, can provide that "ah ha!" moment.  Sleeping on problems works for me too.

Never be satisfied to say "something funny" was happening.  Whenever one of my techs said this to me they knew the coming lecture by heart - if you can't explain the fault, if you can't re-create the fault, then you don't actually know what it was, and can't be certain that you have actually repaired it.  Sometimes this is difficult or even impossible, but mostly it isn't.  Be suspicious.  Be curious.  If you don't understand why, then research it until you do.

Every failure happens for a reason.  You can't always nail down the reason, but it's very important that you try.  I had a stereo PA brought in with a blown output stage, but no obvious cause.  A couple of months later it came back.  Checking my records showed that this time it was the other amp that had failed.  "Bring in all your speaker leads" - and sure enough, an intermittent short inside one of the connectors. 

Your best service instrument are your eyes, and everything is at least worth a look inside and a bit of a poke.  Initially at least you will often be out of your depth, and it's important to know and recognise that, to call for help, or pass altogether, and there is no shame in that.  Tech must also live by the doctors credo - do no harm; at least try to not make matters worse, to muddy your own waters, or for the tech who will follow.

[J M Fahey]

You'd think after all this time Jim would know how to design an amplifier, wouldn't you eh Phil?

Hardly, Jim was an average drummer and a heck of a *Bussinessman* but doubt he could solder a plug to save his life.
Different from Leo Fender who had a long distinguished career as a Radio Repairman (Fender Radio Service, Fullerton, Califormia) before starting to make guitars and later amps.
Jim *always* hired external help, starting with his first Service Tech, Ken Bran, who started it all by cloning a Fender Tweed Bassman.
And along the way, some of those external designers, or Production Managers, those who actually specified the parts, suppliers and built them were, to say it politely, a mixed bag.
JTM30 and 60 were made in the middle of the commercial 90's tsunami caused by Valvestates and (SS preamp) JCM900, which sold 100X what previous Marshalls did, and turned them from Guitar God stuff into a garage band standard.
Which commercially is VERY good .
Since some purists objected to the SS stuff , Jim launched a small series (these amps) which "seem" (only he knows for sure) to be designed to be "all tube" but *not* compete with the mainstream line.
They had to meet budget prices, though.
One casualty was the transformers.
*Terrible* reliability, and that's an understatement.
What Phatt says about burning many in a row is common 
If they work, they do so scorching hot.

[Amp]

Andy Brauer did an article in the 2009 issue of Vintage Guitar called "Sleeper Amps, part 1" where he talked about this amp and the heating issues. It's by no means an unconquerable quirk.

For the tone and volume they produce relative to their size, they're definitely worth working with. Always a bummer when there's a factory design flaw but sometimes, that can be a real goldmine for tinkerer types who can devise clever ways to remedy them and wind up with a great product, as these amps appear to be (based on my casual examination of them so far)

If I have to get to the point where I need to swap a trans, I'll go that way but so far, I've blown the doors out with it and it's run fine. Clean tone wise, I'd stand it against pretty much any other amp in its class. Once the heating issue is resolved, whatever it may lack theoretically in the design department, it makes up for practically with beastly sound.

[Roly]

Since I have always tended to concentrate on Aussie "era" valve amps most of the Marshall's I've worked on have also been older and well built, but the occasional more modern one has suggested a downhill slide caused by getting big.

There are persistent stories about one model that was built under contract in Asia that has a "conductive PCB".  I remain sceptical until I can actually get a bit of this board to test, but there is no doubt that a couple of the resistors on that board suffered a decimal point error and were off by a factor of x10.  Given it was so obviously woggly it was not good to hear Marshall dodging responsibility and charging through the nose for corrected boards.

As I wrote the other day, reputation, both good and bad, often has no relationship to the reality.

@Amp - bolt a bloody big heatsink to the tranny?   

[J M Fahey]

Well, those rumors are true.
Will check later, but pulling it from memory, there was some batch of JCM2000 with that problem.
The problem was (is) so bad, that Marshall, without making much publicity about it, of course, is offering for-accredited-technicians-only a "good", fully populated PCB for a very low cost, don't remember now if it was $70 or $120 which is ridiculously cheap, although of course what they should *really* do is to recall *all* those amps and replace bad quality PCBs at no cost, same as Auto Industry does (Toyota, etc.)
Juan Manuel, keep dreaming  :loco   :duh   :lmao:

[Roly]

what they should *really* do is to recall *all* those amps and replace bad quality PCBs at no cost

My feelings exactly.   :dbtu:

But what I'm really dying to know is the backstory of how they managed to wind up with conductive PCB's.   :duh

[J M Fahey]

Will ask, not too sure will get an answer or even that *they* know what happened.

The problem was not immediately apparent (they are not dumb, saving $$$ is one thing, shooting your own foot something entirely different).

Epoxy-fiberglass by itself is not conductive, but ***maybe*** (lots of guessing here) it was something porous (so far it's possible) and catched some nasty salts from the etching bath which were poorly washed (regular washing would clean the immediate surface quick, but anything embedded much slower),
*or* if the PCB was double sided with plated through holes, the way to be able to plate lead inside the holes (epoxy fiberglass as said is not conductive)  the PCB gets a bath of powdered (colloidal)  graphite or a lead salt which covers with a layer of lead everything it touches.

Poor washing of any of these two baths would leave conductive deposits.

They noticed it only because after a couple months, power tube bias started to drift and tubes became red hot and melt.
The confusing symptom is that poor quality tubes do exactly the same, so they didn't realize it was the PCB until about a year later.