Peavey Classic 50 lead channel volume drop

[Kindly Killer]

First of all, here is the schematic to the amp:

http://www.blueguitar.org/new/schem/peavey/c50schem.pdf

Next about my problem:

• "Normal" channel sounds great - no issues.
• Lead Channel has a drastic volume drop
• Conditions: All knobs under the "Volume" control section are dimed, low "Master" volume setting, switching from "Normal" to "Lead" causes ca. 9dB drop. No other properly working Classic 50 does this. I have gigged a few and tried many.
• Problem persists with any change in V2, which is what I think overdrives the "Lead" channel

Finally about me:

• I can generally breadboard simple stuff from schematics
• i'm pretty weak on theory tho. i get how tone stack works but could never design something from scratch
• i am scared of high voltage - kinda shy about troubleshooting with amp plugged in
• i do know how to discharge caps and i am very careful
• my best ideas are: "Post" volume pot, switch, or a board trace, but...
• I don't know how to figure out what my problem is

Halp! I want to gig with this amp b/c is sounds great on both sides but the volume drop makes this amp unusable for what I want to do with it.

[lowell]

You tried a new tube for v2. I think you may need to check voltages on v2's socket. Clip black meter to ground, 1 hand behind your back. Then power up and report back w voltages on pins 1-3, and 6-9 of v2. BE VERY CAREFUL.

[Kindly Killer]

Okay I did this with a tube still in - hope that was right. The pin voltages are as follows:
1) 158V
2) 0
3) 1.14 V
4) [DIDN'T READ]
5) [DIDN'T READ]
6) 168 V
7) -27 mV
8) 1.16 V
9) -18.5 V

BTW forgot to mention that the "Lead" channel does overdrive nicely - it sounds like it is supposed to as compared with other Classic 50's, just not loud enough. It's like the "Post" knob will only go halfway up or something.

[lowell]

ahhh just wrote whole post and it got deleted. anyway voltages look ok. fyi note the DC filament supply, hence -18.5v on pin 9. Voltages seem low to me, but could be correct as far as Peavey's design goes. Check the resistance of the Dirty Post pot from top to ground. Check for DC voltage on the top of that pot too.

[Kindly Killer]

Lowell THANKS! Amp is fixed and ready for service. I measured the "Post" pot and it seemed close enough to spec (8.9k) and on a lark I reflowed the solder for the pot itself and everything it connected directly to. Problem solved. Thanks so much for your help.

[lowell]

That's awesome! Nice work.

[Kindly Killer]

Okay Mo Amp Mo Problems. I have been happily using my Classic 50 at home and decided that it was time to start using tube amps on the job again. I am a pro player and I work about 4x weekly. I have been using an Acoustic bass amp with a Rat for dirt and RC Booster for girth, but I figured with the Classic 50 being my third amp suitable for gigs, I could give the tube amp thing a try again.

Last nite before I left for my first show I put in earplugs cranked the master volume and compared it to my other amps to make sure it had the volume for the stage. It sounded okay at first but was kind of flubbing on the bass. Then it stopped switching between channels and I could smell something burning - like full on electrical fire burning - nasty. So I chose another amp and did my shows.

This morning I opened the amp back up and tried to recreate the problem. I literally saw smoke rising out of the circled area in the following image. My sense is that the cap with axial leads lying down on the board is the one that is smoking.




Here is the detail. You can see that the goo around the cap is all melty. Hard to see in this pic but the glue or whatever around the cap has melted and run across the resistors between the two vertically oriented radial caps (center of the image). So it's something right in that area that is getting hot.



Question: how do I figure out what's going on? Seems like it's power related eh? Where to probe? Would it be a good idea to recap it once I get it working right (FINALLY)? What are some high quality, reasonably priced caps for this amp that won't change the stock sound?

[nashvillebill]

The axial cap is C40 I believe, on my print and it's on the screen supply. Recapping would be good (IMHO) but I suspect you've got more going on. Check fuses F3 and F4, if you lost the -27 volt supply you would lose the channel switching as well as bias. Check all the connectors to the power tube board, is there a short or bad connection causing you to lose bias. Were the tubes glowing red? Any more bad solder joints, particularly go all the way through the -27 volt supply if you don't have open fuses. Enzo would be the expert on these--even though I've got one, I'm just a hack.

[Pedro Vecino]

They look like resistors of 22 Ohms 1/4 watt (R61/62) . If so, they must be connected in parallel (you can check easily.) They belong to the -27 volts line. See in the schematic



If you must remove PCB, a good advice is:
if the reverb IC is welded directly to PCB, remove and install a socket and make the first test without it connected (no reverb).
If originally have a socket, simply remove the IC for testing the amp without it.
Sometimes they break down. In the process also can damage the line

[lowell]

Of note is the 2 resistors b/t the caps in your first picture. They look burnt to a crisp. Not sure what they are but my guess would be that they're the 220k's from cap + to ground.

[Kindly Killer]

Okay when I take the board out and look at it, the resistors definitely are burned. On the trace side of the board, they measure 11 Ohms (2x22 in parallel) but by chance I switched my meter to Continuity while still connected and it beeps like there is no resistance - WTF? I have resistors on hand to replace these, but I imagine the new ones will burn up, too, no? I thought resistors were the last thing to go bad? I really don't know what to check for - there aren't a lot of points on the schematic that list the voltage like on some schemos, so I really don't know where to go probing. Y'all are saying that the symptoms all point to the -27v line, but how do I pinpoint the problem?

All fuses have continuity.

I remembered something else about this amp. When I first got it (on trade, used) the pilot light didn't work. Then it stopped powering on and my simplistic, chimpanzee-with-a-stick troubleshooting approach was actually up to that particular task: it was the fuse holder. I replaced that and it worked okay until this recent rash of problems. Or rather I didn't notice any problems.

BTW looking at the underside of the board the soldering is real... not good. Must have been someone's first day at the factory and the supervisor was on lunch when this one was assembled. I'm going to reflow all the joints on this board while I wait for more help.

...Oh I just saw something that I may have messed up. I didn't pay any attention to the orientation of the jumpers in the picture - the ones at the far right of the board in question. I guess I thought that if they fit on, they were oriented right. But they do go on both ways. When I measure where it says -27v in the schematic by the 2x22Ohm resistors, it reads about -12v. Would putting one or both of the jumpers on backwards cause the problems I'm having? How to check orientation/correct it? Is there a source for wire codes somewhere?

[Billy R aka DynaFreak]

gotta love those Peaveys. At one point I almost told the stores I work for "no more Peaveys", but I changed my mind and decided to just be The Guy That Will Fix Anything. They do have a good service dept. Anyway, if I remember correctly those connectors you are talking about can be oriented either way, but they have to be installed on the correct set of pins. The HV secondary connector feeds a FWB and does not use a center tap so you can flip it either way and it will work. The LoW v connector DOES use a center tap, but it is in the middle, so again, the connector could be oriented either way cause the center tap will always go to the middle. Not sure if it is physically possible to put the HV on the LV connector.

First thing I would do is pull the pwr tubes, then pull R61 and R62 out then test and see if the rectifier and first filter cap are working OK - in other words are you getting -30 or so VDC @ C42? If so I would start checking components downstream. there arent that many.

[Kindly Killer]

Thanks for all info. I am going to try to make time to tinker some more with this tomorrow. One more stupid question: when you say checking components downstream, are we talking about lifting one lead out of the PCB and testing its actual value, or what? I am eager to get this amp working but I'm a little light on theory. I'm not sure if I'm looking for voltage, audio, resistance/capacitance values or what.

THANKS!

[Kindly Killer]

Okay a lot has happened since I first starting poking this amp with a stick. I became sufficiently frustrated that I have [just] started to study electronics from books, videos, and knowledgeable friends. I am two chapters into The All New Electronics Self-Teaching Guide. I have developed some unrelated skills (e.g. PCB etching, Arduino tinkering, etc). I was able to borrow a friend's tube amp repair DVD with Gerald Weber and I just started watching it. I get the real high level concepts that Weber is talking about but I'm still pretty clueless on the specifics on my problem. In general I think I am on the path to having a clue but still pretty far from my destination.

Reading over this thread again, I see that someone asked about a glowing tube and I never did respond. Yes one of the four was glowing at one time, after which I replaced all the power tubes.

Anyway, I just followed the last bit of instructions and pulled the burned resistors and power tubes. I get DCV -28.9 at C42, so I guess I'm ready to hunt for the problem. In this image, is the small axial leaded cap on the left border - is that C49? If so I should see -14v on the negative side of that, right? What does it mean if there is no voltage there?



I'm still not clear on what I'm doing, theory-wise or practically.

[Kindly Killer]

Okay I get why there is no voltage on C49 - because I removed the resistors LOL!

So "downstream" is the other way? I don't see in the schematic where J70, 71, and 72 show up anywhere else? Trying figure out where they go in the schematic.