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Amp got da buzz

Need a little help here fellas...

Iv just taken an amp in to fix for a friend, but before I tear it apart, I just wanna check my thinking - and show yas some pics of this little beauty! lol
By the way, these arnt the pics of my mates, but its exactly the same... and nearly as clean!

Its a nice little WEM Westminster combo which he bought brand new in the late 60s / early 70s. Its in relatively good condition, but crackles and pops no end. As far as I can tell (from the stamp on the big red cap), it was last checked in 1973!!

Iv tried spraying switch cleaner into the pots and its no different, so Im thinking next step is replace the pots entirely. Trouble is, I cant tell what size the pots are... they dont seem to be marked.

Any tips on rebuilding the pots?

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Permalink Reply by Roosterman on January 15, 2010 at 10:57am

That IS the little bugger right there :o)

smojo said:

wem diagram

I think this might be the little bugger

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Permalink Reply by Roosterman on January 15, 2010 at 11:00am

Well, soon as its switched on, there is no crackle.

Soon as it warms up, its all three!

Turning any of the pots changes the nature of the buzz / crackle / pop, but when the volume is down, theres no noise. It isnt the kind of noise you get from a scrathy pot.

Maybe I should do a sound clip of it!!

Anyway, the vol pot seems to introduce the noise, rather than be making the noise itself.

I too am having difficulty correlating the diagram with the actual hardware... seems I got a few more bits on the chassis.

smojo said:

Ben, do you have a circuit diagram for it, would be very useful? I'm confused about the noise, is it hum, buzz or crackle? I'm guessing the big cap is part of the rectifier smoothing circuit. If so I doubt that's the problem too. If the noise is hum then it might be that but could also be mains hum induced from elsewhere. I agree with Naz, about the volume bit. Not necessarily beofre the pot.

Do you have access to an oscilloscope? Would make the job a lot easier to trace the fault.

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Permalink Reply by smojo on January 16, 2010 at 5:14am

Ben - OK so don't assume all noises are coming from one fault, you may have several issues. I would forget about the hum for now, you're gonna get some mains hum anyway, it's the crackles and buzzes you need to sort first. Crackles suggest intermittent faults, like a dodgy connection or a component breaking down.

When you switch on it will be quiet anyway till the heaters have warmed up. The noises will most likely change when you adjust the tone and vol anyway. Are the noises there when you have nothing plugged in? If so then leave it that way while fault-finding. Have you tried the vibration test on the joints and components, do you hear any changes?

Look carefully at the resistors and capacitors for visible signs of damage. Brown burnt out resistors, bulges on the end of electrolyic caps etc. Do you have a voltmeter? Test suspect resistors by disconnecting one end from the circuit (with power off) then putting meter set to ohms across it. No reading suggests it's duff. With a capacitor do the same. If good you'll get a short reading then tailing off. Reverse the leads and it will do the same. No reading says it's duff.

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Permalink Reply by Roosterman on January 19, 2010 at 5:54pm

Yes, noises still there when nothing is plugged in.
I agree that its a component gone sour, but havnt had the time to get any further with it sadly.

Visually though, all the parts look ok.

When I get chance Il go through the full board with the meter and hopefully that will be the end of it!! :o)