Servicing my Amp - old caps and Safety

[zpyder]

"Never" having worked inside an amp before, I figured I would consult the experts and the experienced before opening up my amp.

I have an old Peavey Bandit solid-state amplifier (by all hifi standards a p.o.s., but I think it sounds pretty good when I make it bleed  ).  I'm experiencing buzz & hum, and I've decided I will try to tackle, or at least identify, the problem(s) with my setup tonight.  This thing is years old and I wouldn't be surprised if some of the electrolytic caps have gone bad.  I'm thinking that if I have a bad power filtering cap in there that it may be causing some of the ill effects.

So some questions before I open the thing up:

1) Is 5 minutes or an hour enough time to wait, and where do I drain from?  I don't want to die (yet) and I understand there will be some residual voltage inside those caps in there.  The amp is unplugged, and will be for the next hour or two before I even get a chance to open it.  I read the safety sections R.G.'s tube amp faq (http://www.geofex.com/tubeampfaq/taffram.htm) which says to first unplug it for 5 minutes, and then drain any remaining voltage off of a tube into ground.  This is a solid-state amp, so there are no tubes - where should I drain the voltage from??  It also seems odd to me that I would "drain" the voltage to a ground that is not earthed - do I need to have the plug in the wall w/ power off in order to get rid of the voltage  

2) How do I test the filter caps, how do I test any cap?  I have a DMM, but it does not do capacitance.  I read R.G.'s article on PS filter caps that mesaures resistance.  He indicates that a PS filter cap with a resistance of less than 15k across it is probably bad.  What about smaller capacitors elsewhere in the circuit??  Is there a formula for this?

3) This is an old amp that's seen a lot of use... I'm gonna open it up... Might I investigate other caps/components while I'm in there?  Anyone have any suggestions as to what to look at?

thanks in advance everybody!!,
zpyder

P.S. On my honor, I will do my best, to do my duty, to dog and my roommates, to keep my left hand in my pocket at all times.

[Tubebass]

Solid state amps operate at much lower voltages than tube amps and so are safer to work on. Still it couldn't hurt to check the filter caps for voltage with your DMM before beginning work. Chances are there will be only a volt or two present. Electrolytic caps have a rubber seal at one end. If this seal is bulging or shows residue of leakage, the cap is probably bad. If the amp is old, it's probably a good idea to change every electrolytic in there.

[R.G.]

Good advice, Tubebass.

1) Is 5 minutes or an hour enough time to wait, and where do I drain from?

SS amps are much less of an issue than tubes in this respect because the voltages are much lower. Find the biggest capacitors in there. They should be on the order of inches, not fractions of an inch in size. Probe the two terminals of the cap with your multimeter. If there's less than 20V or so, you're fine. I doubt that you'll have over a volt.

You drain the caps to the signal ground, or drain them direction from one end of the cap to the other. A cap with a charge on it does not need a "ground" just like a flashlight battery does not need a ground. Generally one terminal or the other is connected to signal or chassis ground. But putting a resistor across the two capacitor terminals directly will drain it. You do not need and should not plug it into the AC line to do the draining.

2) How do I test the filter caps, how do I test any cap?  I have a DMM, but it does not do capacitance.  I read R.G.'s article on PS filter caps that mesaures resistance.  He indicates that a PS filter cap with a resistance of less than 15k across it is probably bad.  What about smaller capacitors elsewhere in the circuit??  Is there a formula for this?

What you want to know is whether the cap is acting like a good electrical reservoir. Unfortunately, your DMM will not do this directly. Measure ohms across the cap. If it measures less than 1M, be very suspicious. If it measures a short, but the resistance steadily increases, that is your ohmmeter charging it up. You really need to remove the cap to do that test, and that's 90% of the work of just replacing it, right?  On solid state amps over 10-20 years old, I usually just replace with new if I can get physically fitting replacements. There is an instrument that tells you what you want to know - an ESR meter. I have one, but these are rare birds. I spent three decades working without one.

Look at all of the components. Look for evidence of liquid leaks, crusts of oxidation, burn marks, things like that. Use a vacuum cleaner to remove dust and bar funk (i.e. smoke and stale grease/dust residue from being used in bars) so you can see. Clean all contacts, and tighten all the nuts, screws, etc. that connect wires. Clean input/output and loop jacks with contact cleaner. Tighten the nuts on all jacks and pots; some manufacturers use these for grounding the signal ground to the chassis and a loose one can cause hum. Ask about anything that looks funny.

If it were mine and I wanted to keep it, I would make up a list of all electros in the amp and just order replacements. I've done this repeatedly over the years, and it can make a huge difference.

Be careful. Don't die.

[zpyder]

Thankyou to both of you...!

I will be careful - and I'll run from death with a fury.

cheers,
zpyder

[zpyder]

I'm holding my peavey's PS filter caps in my hand... there are two of them both rated at 2200uF 50v.

I'm confused though... R.G., you said to check the resistance across the caps.  When I did they started off as a short, and gradually rose - just as you said might happen.  So I pulled the thing apart and pulled the filter caps off... but then measuring them after that, they did the same thing...

Physically they're pretty solid looking, with the exception that I had to slice them off of the board because the wrapping had melted onto it... no bulging or discoloration...

Idea?

zpyder

[R.G.]

Explanation of the results:
An ohmmeter works by putting a fixed current out through the probes. Then it measure the voltage. When you do this to a cap, the voltage that the DMM is telling is a resistance reading starts at 0V, and ramps up as the cap charges. That just shows they're not shorted. It does not tell you the cap value nor the "goodness" inside the cap.

That just means that they are acting like a cap both before and after you pulled them off the board. Since they are not shorted, they MAY be OK. But in my own wierd view of the world, you could put those caps back and take a chance, or you could order new caps to replace them and have known good caps. It'll cost you a few bucks to be certain; how much is that worth to you?

Do you see any area with burn/scorch marks, liquid leakage, oxidation fuzzies, stains on the PCB, etc?

[zpyder]

Yea... doesn't it suck when stuff works that was supposed to crap out?   

Well I share your wierd world view R.G., I've got my amp all apart and I already pulled the big ol' caps off - I might as well spend a few bucks and solder in some brand new ones.  Just hope I can get em tonight before practice!

The PCB and the components on it actually surprised me as far as how good of shape they were in.  With the exception of some larger voltage solder joints (pretty much just the filter caps and where the juice came in), I couldn't spot any burnt or singed areas.  Clearly the filter caps had gotten warm, as their wrappers had melted onto the PCB, requiring a leatherman to slice them off - but then again this could have been glue.  I kinda doubt that since there was hot glue dabbed all over the top of these things, but whatever's holding that piece of wrapper to the PCB isn't hot glue.  The PCB was actually impressively immaculate, I gotta say.  I removed the mains ground bolt, cleaned and scuffed it with steel wool, and clamped it back down.  I did the same for the input jack.  I resoldered both of these locations as well for good measure.  I assessed all bolts and jacks, they were all tight

The ugliest part of the whole thing were the cables coming from the reverb tank going into the PCB.  The enter via a HD-power-style (don't know what they're called) plug.  The plug for that, as well as the headers it goes too are pretty freakin dirty and ugly, and I may replace those.  Those headers were actually showing some of the brown solder grunge around them.  The plug is half-burnt and melted.  That cable may have carried something other than just the reverb, I'll have to take a look and see.

No obvious cap bulges, oxidation residue, very few stains - all of them around what appear to be the higher voltage solder joints, no liquids, burns, or scorch marks.

One thing that I noticed was that the power transformer is clamped onto the bottom of the amp chassis.  Seems like that thing's gonna spew EM fields all over something... I could move it more away from the chassis, closer the the speaker and reverb tank...

Side-note: TI 4558's - 4 of 'em   That's what makes that thing tick.  Maybe I should toss a couple TL072's in there while I'm in there.  Maybe I could stack 'em http://www.diystompboxes.com/smfforum/index.php?topic=16784.0 

cheers,
zpyder

[britelite]

I had a Bandit 65 many moons ago. Not a bad sounding lil amp. Mine sounded good dirty but clean left something to be desired.

I would follow RG's advice and change those old electros. Chances are that will fix buzz/hum. Ya might wanna look at changing out tone   
caps. I like Orange Drops.

Peace

[zpyder]

britelite:
Did your bandit ever hum or buzz on you?  Curious as to other people's experience with this thing...

I'm thinking while I've got it open I'll make a list, as R.G. suggested, of all the caps and put em on order.  I'll get the power filter caps ASAP (since they're already out and I think I can get em at Norvac) and put the thing back together to use, and then once my caps show up I'll open it back and and solder em in.

The thing is pleasantly easy to get into by the way ... No hidden screws or things you swear must have been bent to get around the corner/between those whatevers..

cheers,
zpyder

[britelite]

zpyder: Never had a problem with hum/buzz. It just did not sound good with single coils. Sounded great with an LP or other humbucker. But 95% of the time I played/play Tele's and Strats. I had it for 10 years and had bought it used so it was very durable.

I bet the PS caps will cure most if not all of your unwanted noise. Depending on your Bandit's age and how hard you drove it, you might wanta think speaker(after the cap change).

Peace

[zpyder]

Does anyone have a clue as to where I could find a schematic for the Peavey Bandit 45 ?  I want to go ahead and do a cap job on the thing, but most of the electros on the circuit are axial and happen to be soldered in so that the ratings are hidden- facing down.  I could just go in with a soldering iron and take them out, but I don't want my amp to be down for 4-6 days while I wait for the parts to arrive..

Anybody?

zpyder

[britelite]

zpyder: Any luck with the schematic?

Peace

[zpyder]

britelite:

yes...!  As it turns out my Bandit was a 65 (I kinda thought that anyways) - I didn't have my amp near me when I posted that.  I don't think Peavey ever made a Bandit 45...

But yes, I called Peavey's repair department, told them I needed a schematic, and they emailed it to me promptly and for free.   Prettttty sweet.  Schematic and layout... If you're interested, send me your email address and I can send you a copy.

cheers,
zpyder