Amp got da buzz

[Roosterman]

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?

[Roosterman]

Update: New tubes arrived (very nice they are too), but it hasnt solved the problem.

So, caps next. Good source of caps in the UK anyone? Juju, Im looking at you! The big red one you just swapped over in your vintage amp looks to be the same as the one in mine.

On another point, since Im still able to plug the amp in (and have been to test it), I assume the caps could have quite some kick to them. What is really the same way to remove them?

[juju]

yep it's looking like the Caps that have expired :-( - you can discharge the caps by unpluggin the power lead and switching the Amp on this should discharge any power in the caps - but just to be safe put your Marigolds on and short accross the Cap outputs - if you get a flash and Bang the Cap had charge in it but i doubt there will be anything in if you do the unplug power lead and switch the amp on !

well i found the caps i needed on Ebay - the big one i changed is a double output Cap - you will have to read what it says on the Cap for the value - mine is a 32+32uf @500v - a single output cap would read 32uf @500v - the double output Caps share the same neg (-) terminal and has two pos (+) terminals one for each half of the cap

juju.

Roosterman said:

Update: New tubes arrived (very nice they are too), but it hasnt solved the problem.

So, caps next. Good source of caps in the UK anyone? Juju, Im looking at you! The big red one you just swapped over in your vintage amp looks to be the same as the one in mine.

On another point, since Im still able to plug the amp in (and have been to test it), I assume the caps could have quite some kick to them. What is really the same way to remove them?

[Roosterman]

Okey dokey... marigolds and ear plugs then :o)

[smojo]

Ben - you can discharge the caps more safely by putting a resistor across the terminals. Make up a lead with a couple of croc clips and a resistor in series say 10k ohms with a high wattage. When replacing electrolytic caps make absolutely sure you have the polarity (+ve -ve) correct.

A couple of other possible causes are "dry joints" and faulty valve bases. Look at all the soldered joints. Do they look nice and shiny. Any dull grey ones should be reheated and a touch more solder added. Are there any "whiskers" of wire sticking out and possibly touching other components. A good test for dry joints is to use an insulated stick of some sort - plastic rod like an old biro. Do a "vibration test" by tapping each joint with the power on with the stick (one hand in your pocket to avoid shock and catch your balls when you blow them off). The crackling will be more evident when you hit the faulty one. Resolder if you find one.

The conacts in the valve bases might be oxidised. Try a little contact cleaner on them and ease the valves in and out carefully to clear them (use the old ones in case you bend the pins). Might need to replace the bases if you still haven't sorted it.

[Roosterman]

Thanks Smojo :o)

The solder joints arnt very accesible unfortunately, but they do all look good. Il definately be keeping a hand in my pocket anyway!! So funny :o)

I already tried cleaning the valve seats but again, no change. Im thinking the big red cap may be to blame... its old, and contacts to all pots (and they are all buzzing).

Trouble is finding one... Ebay has nothing, so Im off to Maplins.
50+50+50uf 350VDC, 50uf red outer? Certainly sir, just £2 each.... NOT.

[juju]

what it's rated at 3 x 50uf - never seen one of them before - how many terminals does it have ? and what are they 3 x (+) and 1 x (-) ?
the other way around it would be to buy three single 50uf x 350vdc caps connect all the (-) together and have three individual (+) job done ! :-)
juju.

Roosterman said:

Thanks Smojo :o)

The solder joints arnt very accesible unfortunately, but they do all look good. Il definately be keeping a hand in my pocket anyway!! So funny :o)

I already tried cleaning the valve seats but again, no change. Im thinking the big red cap may be to blame... its old, and contacts to all pots (and they are all buzzing).

Trouble is finding one... Ebay has nothing, so Im off to Maplins.
50+50+50uf 350VDC, 50uf red outer? Certainly sir, just £2 each.... NOT.

[Roosterman]

Shoulda known it wasnt going to be straight forward! lol

The full cap details (just for your entertainment) are thus;

Made by 'CCL'
50 + 50+ 50 μf
350 V.D.C WKG
Red 50 μf outer
Plain Neg
PP 77 S J72

As far as I can tell, Juju is right - 3 x + and 1 x -, so 4 terminals...

Cunning plan though - Im gonna speak to the chap who designed the amp (he still repairs them), and see where he gets his!

Failing that, Il be going the 3-stuck-together route :o)

[juju]

Well thats a new one on me , but i'm far from a Electrical expert lol! - just goes to show you can learn something new everyday :-)

well i'm sure if you talk to the guy who designed them and repairs them he will be able to tell you all the usual faults - if he dont know who will !
juju

Roosterman said:

Shoulda known it wasnt going to be straight forward! lol

The full cap details (just for your entertainment) are thus;

Made by 'CCL'
50 + 50+ 50 μf
350 V.D.C WKG
Red 50 μf outer
Plain Neg
PP 77 S J72

As far as I can tell, Juju is right - 3 x + and 1 x -, so 4 terminals...

Cunning plan though - Im gonna speak to the chap who designed the amp (he still repairs them), and see where he gets his!

Failing that, Il be going the 3-stuck-together route :o)

[Roosterman]

Nuther update:

I just had a very helpful chat with the 'tube wizard' - he reckons that the caps in these WEMs are good, and he has never yet had to change one.... which is a good job, because the 4 terminal ones ar not made anymore! He did tell me how to rebuild the caps if need be though!

Anyway, this is what Iv learnt (I only put it here so that you guys who also have an interest in playing with amps may glean a little info too).

First things first. There is no crackling when the volume is turned right down. Turning the vol up introduces a buzz, rather than creates the crackle (hope this makes sense!).

This indictaes that the problem lies before the vol pot.

Makes sense huh?

Anyway, turns out its probably the resistors that are at fault, as there is nothing else that side of the pot.
Im yet to identify exactly which ones need replacing, but Im reliably informed that I need

'...off the top of my head, HT83, anode resistors, about 220k, pins 1 & 6 I think....'

Um, right.... :o)

[smojo]

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.

[smojo]

wem diagram

I think this might be the little bugger

[smojo]

If you look at the diagram, you can see 4 x 50mfd caps but I'm guessing the two of the three you have are the two on the top rail where ones side is conected to chassis/earth. can't figure which is the other one because neither of the other two seem to be connected to chassis. If you can get a single 50mfd 350v cap you can try disconnecting the non-chassis side of each on in turn, then temporarily connect your new one between chassis and the point where you disconnected it. I doubt that all three parts of the cap are faulty.