Scratchy pot how to hunt down DC voltage?

[distorto]

Hello,

I'm working on an Acoustic 150. It sounds great but I have a scratchy volume pot on the guitar when plugged in to the amp. I suspect DC voltage leak (the guitar's pot is fine in other amps) but am not real sure how to go about tracking it down. Do I merely measure for DC voltage in the signal path with one probe to ground and look for caps passing DC? Is there a specific place to look judging by the schematic? The schematic is of the 140 amp but the boards are the same. All the amps pots are fine with no scratchiness.

I have replaced the 1000uf 50v power cap. and added a three prong cord.

I really appreciate any insight.

Thanks,

Chris

[52 Bill]

Plug a cord into the input and read the dc voltage from tip to ground. What do you get?
The schematic shows the preamp input cap as C102, 1uF. That would be where I'd start looking.

[J M Fahey]

Agree and add: C102 (1uF electrolytic) is unnecessarily large, it's passing down to a couple Hertz frequencies, for no useful purpose.
I always substitute them with a .047 or .022 Polyster cap, which by the way solves the leak problem forever.
jm2c

[distorto]

Thanks for the replies! Yeah it was that C102 cap. I just now replaced it and no more DC on the input. I used a 1.0uf tantalum. I do like the idea of solving it forever though. Should the other tantalums be replaced even though they aren't leaky? The amp sounds great for bass although the guitar channel's board is a mess (cut traces, changed pots, missing components etc.) I was going to rebuild it but decided since that channel wasn't going to be used I'd just disconnect it and keep an eye out for another broken 150 with a un-bunked board.

Thanks again for the replies.

Chris

[greekie]

Tantalum's have their own problems (including civil war in Africa, but that's a whole other thread).

My advice would be to use a good film cap with one of the values listed, and live happy ever after :-)

Jake

[distorto]

I hear you greekie. It originally used a tantalum and being a hobby/novice guy I didn't know if I could use a film there or not so I erred on the side of what was there.I will toss in a film cap though. Happily ever after rules! Thanks for your reply.

[Gtr_tech]

Just to chime in here....if that cap was bad, then the amp is prolly *full* of bad caps. I'd replace every damn one if you want it to sound the way it should.....

[distorto]

That's a really good idea. I shall replace them all.

[trobbins]

With respect to the unit possibly being full of bad tants - just remember that the leaky cap was the input cap and so was exposed to much more risk of failure from externally applied stress - and so is just as likely to have failed from that independant mechanism.

Ciao, Tim

[Gtr_tech]

He replaced it with a tantalum. I'm sure the amp just has garden variety lytics in it.

Myself I don't much care for those tantalum time bombs. They get real unpredictable when they age. Not too long ago I had one go all supernova as soon as I applied power. Woke me up inna hurry...

[distorto]

Yeah, I used a tantalum because that's what was in there. I didn't know any better. I do now so I'll probably swap it out when I'm in there again.

[gbono]

You must derate a tantalum capacitor by at least 50% (i.e. 10V application needs a 20V part). REVERSE bias will kill a tantalum cap so be aware that you can' t put more than 5% (10% max) of the WVDC across a Ta cap without cutting its life short - I wouldn't use it on the input of an ACC 150. Temperature is another issue. These devices can provide good capacitance/size, very low ESR and leakage if properly derated for your application. Ta capacitors can also "heal" themselves (like film caps).

[distorto]

So what would you stick in there gbono? I believe there are 5 tantalums total on that board. Is there a reason they chose a polarized cap? I have to say these things (Acoustic amps) are a joy to work on compared to this stinkin' Sunn Beta bass head I just recapped. The power amp board is easy but that preamp board is nuts. I replaced 19 NP electrolytics. They mount the pots over the solder side of those little suckers too.

[gbono]

See Senior Fahey's comments for a replacement capacitor. They picked Ta capacitors because they were and are a quality component for many applications.

Yup Sunn Betas are fun to work on and at least one of the logic ICs are almost unobtainium now. Try working on a Acoustic 470's preamp boards for fun. I also find that the Sunn Concert series PA board is a pain to work on if all the external wiring points have been soldered to the board - POA (previous owner abuse).

[distorto]

Thanks once again gbono? Yeah I'll use the polyester jobies. So I can use them anywhere there is a tantalum?

Not to derail my own thread but while I have your ear on the Sunn Beta...As I said, I replaced all electrolytics including the filter caps. It cleaned up a bunch of the hiss, pops and other assorted crust and sounds like a different amp altogether. Except for one annoying problem I'm trying to track down - Intermittently, there is a slight click or pop and the overall level drops off for a moment and there is some static behind the notes. It doesn't appear to be thermal cause it occurs when being switched on as often as after it's warmed up. I'm hoping it's on the power amp board. I tried running a guitar through a external preamp into the power amp in and it appeared to have the same problem.

"POA" I have to admit I am guilty of this a little on thpreamp board. I accidently took out a few traces trying to work under those dang pots and had to rebuild them. I need to invest in a better desoldering tool. I can move this to another thread if it would be more helpful to others. I've been reading through the other Sunn Beta threads and there is helpful insight there.

This forum rocks btw!