Randall KH75 sound like wet farts HELP

[PowoFR]

Hi,
I'm new to this forum and I come here to try to find the solution to fix my Randall KH75
Bought it for cheap at a garage sale, of course it doesn't work when at home.

It sounds like wet farts at every volume and even in clean (much less though). I first thought that the Celestion was completely blown but when I plugged my 4x12 Marshall it sounded the same... (yes that combo also has a speaker output and that's amazing).
I then thought that something was popped inside and I would instantly see it and replace it easily... nope, nothing obvious enough.

To me everything looks good. I only see the Q8 small transistor that has a small black shit stuck on his ass. For real that could explain the wet fart noise.
I'm a maintenance technician, my only skills in electronics are: see the burnt component, replace it. Or see the cracked solder, fix it. Nothing more.

Heh now that I see it in close up through the zoomed picture it looks a lot like molten plastic..... but no trace of heating around.
Could this be it?
I don't wanna waste too much time and money on this small combo.

[Jon Snell]

With no schematic to hand, I would check the main smoothing capacitor pair.

[PowoFR]

Yeah I didn't find any schematic.
The smoothing capacitors would be the two big ones I presume? They look fine and they don't move. Could they make that kind of sound?

[g1]

I'd be more inclined to think that black bit came from somewhere else and just stuck there because of the thermal goop. Any other places you can see any thing burnt?

[g1]

The board says RG75-B on it, I wonder is this schematic is close.

[PowoFR]

I'd be more inclined to think that black bit came from somewhere else and just stuck there because of the thermal goop. Any other places you can see any thing burnt?

No, that's the problem. I'd like to look underneath the board but I think I can see there is only solder on the back side.
I looked closely with a flashlight and I can't see anything wrong anywhere.
The black bit was hard plastic that could have been shattered or molten, hard to say, it's really small.

[g1]

It could be just the angle of the picture, but Q15 (on heatsink) looks funny from the top. Does it look normal?
Is that schematic in post #5 a match?

[glebert]

I would say first to try to find out if the problem is preamp or power amp. How does it sound through the headphone out? If you take the FX loop send and put it to a mixer or another amp does that sound OK? If you plug a guitar directly into the FX return does that sound clean?

[PowoFR]

You don't see the top of Q15 on the picture, it's hidden behind the metal frame. You can only see the screw. It looks fine to me.
I'm checking the schematic of the RG2-75-B

It's too advanced for me but I'm comparing resistors values between my board and the written values. So far they all match. I checked R21,11,2,4

[PowoFR]

I would say first to try to find out if the problem is preamp or power amp. How does it sound through the headphone out? If you take the FX loop send and put it to a mixer or another amp does that sound OK? If you plug a guitar directly into the FX return does that sound clean?

That's a really good idea, I feel stupid for not checking if this amp had an effect loop. Woaw it has a RCA in too, that's great.

So................ can't use my headphones because I can't find the 3.5 to 6.3 adapter that goes with it. I've looked everywhere and can't find it.
BUT now I know for sure that the preamp works. I used the send of the Randall and plugged it into the return of my Bugera 6262, added one of the four 6.3 adapters I've just found in my house to make the Randall shut up (because it went through even without anything in the return).
Sound is fine(ish) when it's not going through the power amp. It's the sound I expect from a solid state amp trying to emulate Kirk Hammet's tone.

Is Q8 part of the power amp? Can we eliminate that transistor?

[glebert]

Is Q8 part of the power amp? Can we eliminate that transistor?

Since Q8 is mounted to the heatsink even though it is a smaller transistor I'm guessing it is there to prevent thermal runaway. Basically as the output transistors get hotter it will compensate and bias them colder.

[g1]

I think you can ignore Q8 if the amp is basically functional.
It looks like the headphone jack has cut-out contacts that disconnect the speaker outs. It could be as simple as a dirty headphone jack (or even dirty FX return jack).
Give them both a shot of cleaner if you have some, or if not, exercise them a bit by plugging in and out of them several times. Sometimes that will clean them enough to restore the iffy contacts.

[PowoFR]

You think it could create that sound?
Damned if only I had some contact pray left. Going to buy some would cost me 5€ in fuel nowadays >.<
I'm gonna clean the contacts tomorrow with a q-tip and alcohol and it should be OK.

I've had a really bad experience with contact spray on a marshall amp in 2016.
I used contact spray to clean the potentiometers that were noisy and dirty, they looked fixed but I didn't pay attention to the condensation caused by the evaporating spray. A day later ALL THE POTENTIOMETERS were solid blocks of rust, impossible to turn.
I had to replace them all.

[Helmholtz]

I've had a really bad experience with contact spray on a marshall amp in 2016.
I used contact spray to clean the potentiometers that were noisy and dirty, they looked fixed but I didn't pay attention to the condensation caused by the evaporating spray. A day later ALL THE POTENTIOMETERS were solid blocks of rust, impossible to turn.
I had to replace them all.

There are many so-called contact sprays. Often not suitable for electronic components.
While there might be other good ones, I only rely on Deoxit D5. Never had any problem in over 30 years.

[The Dude]

Agree. I'll add, it doesn't take much and you should try to spray wafer and wiper only if possible. If you over spray the wrong cleaner, it can dissolve all of the grease that allows the shaft to move. A little goes a long way.