Thanks Phil,
I didn't notice any trim pots inside, but I wasn't looking for one either. I'll take another look.
Pete

Thank you for responding, "Loudthud".
Yes to the first question, but I think it's a 300 series amp. I also made an additional discovery upon further visual inspection last night. During original factory assembly, it appears that a trimmed off piece of transistor wire accidentally found it's way to the back of one of the power transistors, between the transistor and the aluminum bar on the edge of the pc board, (which bolts up to the heat sink). I could see that the transistor was not making good contact with the aluminum. I de-soldered the trans' and it was plain (from the footprint of the thermal grease), that the contact patch was only across maybe 1/4 to 1/3 of full contact. Seems like this might have been causing the transistor to heat up and maybe distort? So my follow-up question would be, once the transistor has seen enough heat to make it distort, is it junk? This is just my hypothetical so far. I haven't had opportunity to test the amp yet.

I can't tell you the model name of the speakers, but they are Peavey also. One 12" and a horn. I use two of them, most of the time. I believe these are the ones that were marketed with this amp.

Again, thanks in advance for any suggestions.
Pete

Hello all,
Well, last time I was here I got some great advice. I'm hoping for 2 for 2.
So, I have a XR500C Peavey powered mixer that I've been using for several years in an acoustic duo. The sound has always been excellent, in spite of the age of the unit.
My problem: Everything sounds great at sound-check. One and-a-half, to two hours into the gig, I start getting distortion. Sometimes it's worse than others. Had a local guy look at it. He found nothing after running it for several hours. I suspect it was not being run at the sound level that we perform at. He said he just listened to music from his I-pad in it. In my mind, it won't start distorting unless you're really pushing some air. The distortion happens across all channels, suggesting (to me) it's not the pre-amp section. Upon the distortion starting, I have tried omitting everything in the input signal chain except for a single guitar. Same distortion. Try another guitar, same results. Just to be sure, change out cord, same results still. Upon inspection nothing visual is jumping out at me. Everything looks clean. Nothing looks like it's experienced any overheating. No apparent physical damage. I'm getting quite frustrated. Every gig I go to, I just hope it's not going to happen again. I'd just go out and buy new, but to be honest, I have a hard time finding anything that really measures up to this relic Peavey.
Anybody care to weigh in?
Thanks and appreciation to any suggestion,
Pete
p.s.
All I have for diagnostic equipment is a Borbede (BD-168A) multimeter. In light of this I would be willing to try replacing a few components if you think they might be the likely suspects.

YOU GUYS ARE AWESOME!!!
Hooked up to my Peavey Bandit speaker.
Noise GONE!
The benefit of experience.

Well, I'm glad it turned out to be something so simple. And as luck would have it, I just happen to have a 15" speaker that I got for FREE some time ago, that I repaired and is now working fine. (broken voice coil wire) So now I have a back-burner project of repairing the original speaker. Thank you both for sharing with me, (stated previously), the benefit of your experience. May you and your families both, enjoy a happy holiday season.

Pete

Hello, and thank you in advance for reading my post, and/or trying to help out a newbe.
I have a Peavey, KB100 keyboard amp that I'm having a problem with. When I got the amp, it had a burned out transistor, which I have replaced, along with a few obviously overheated resistors. I replaced the filter capacitors for good measure as well. Well, overall the outcome was quite successful except for . . .
As the input signal dies out and becomes very quiet, (in my case, this would be a guitar and/or a keyboard) the signal seem to break up and sound distorted. You wouldn't notice it at normal listening levels, but if you let the note sustain, as the volume decreases the distortion sets in. Maybe better described as "breaking up", more than to call it distortion. I don't know if that's enough information to go on, but can anyone tell me what I might be looking for to remedy the situation? What might be causing it?
Thank you,
Pete

(link to schematic, or see attachment that I tried to include)
https://www.electronica-pt.com/esquema/audio/amplificadores-audio/peavey/peavey-kb100-amp-28103/

First of all, I want to say thank you all again for all your support. I've been ill for a while, so all my projects have been on hold for several months. Sorry if you've been waiting on hearing back for a progress report.

I finally did replace all the caps and the not-so-few resistors that didn't quite make the cut, spec-wise. Replaced the 6L6's with 6L6GTs. Turned out one of the tubes had an internal short. I know the schematic says GT's, but the chassis says 6L6s. I did this mostly to save cost. This did require quite a change in the bias resistor to get things to a reasonable plate dissipation, but I'm pretty sure I've got it right. I went over all the spec's several times. I'm not writing back to burden anyone with further analysis or information. I just wanted you to know that I did eventually follow through and got it done, thanks in great part to everyone's help and generosity. The amp does sound pretty good, although it does seem to break up a little early. Bear in mind, this amp is sought after by harp players BECAUSE it breaks up nicely. Gives the sound they want. So I'm marking the whole thing up as a win.

I'll be reading the forum regularly to see what I can learn just by eavesdropping. If I need help on something again, I'm hoping you'll all have a little bit more patience left for me.

Pete

Well guys, I will admit that sounds somewhat intimidating. I wouldn't say I have three thumbs, but it is far from essential for me to alter the amp in the first place. Simply changing out a couple components . . . not so scary. Maybe I should just leave well enough alone. Personally, I have no interest in the "drive" channel. I simply thought it might be cool to see what could be accomplished. For now, Phatt, I'll just wait and see if you can get any results on the simulator, that seem like they may be worth pursuing.

Mr. Fahey, I think your idea is a good one, but I might be in a little over my head in it's execution (according to Phatt's description).

Thanks to you both for you help, and moral support too, I should mention. You have both provided encouragement for me to continue trying to learn. Dabbling in electronics is something I've been fascinated with all my life. I finally have the time to do it!

Thank you both again,
Pete

Hello again Mr. Fahey,
I find the idea VERY intriguing. If it not a lot of work for you to draw up, I'd love to see it. If I think I can handle it, it would be so fun to give it a try!
Pete

Yes!!!!, Thank you Phatt, for taking the time, and probably saving me a bunch of time. Mr. Fahey, I was hoping you'd weigh in on the simulation. After I read the post from Phatt, I got to thinking exactly what you describe. The absence of responsiveness is probably no accident. Probably for the best. So all this leads me to being one happy camper. Although I do have to say, I'd love to squeak a little more responsiveness from those two pots. If it was just a matter of changing out a couple caps, and/or a couple resistors, I'd be game for that. What do you think Phatt? Would you be willing to plug it a couple different numbers and see about configuring a MODEST improvement? I think anything more than that might be asking for trouble. I'd be delighted if I could at least be able to tell which way each knob was rotated. If not, thanks just the same. Once again, this forum has been very helpful. I hope I can pay it forward somehow, someday, some-way.

Thank you,
Pete

Thank you Mr. Fahey,
More than I can handle tonight, but I'll be back to you as soon as I can. I do have to occasionally earn some $$$$. I usually do this by repairing antique clocks. Electronics may not be my strong suit, but mechanically, I stand undefeated! It's the one gift God chose to bless me with. I don't know what I'd do without it. So as soon as I can follow through with that download, and figure out a way to feed it through the amp, I'll be back to you again. I can't thank you enough for your generosity, and your tenacity.
I'll be in touch soon,
Pete

Sometimes I'll take a repair of stringed instruments too. This is my latest endeavor. It's almost done now. Some may find it interesting.
http://s1085.photobucket.com/user/peterblair1/media/A1%20as%20found_zps5ymcxy7h.jpg.html?sort=9&o=1

Time for an update. Parts came in a few days ago, but I've been quite busy until tonight. I replaced the two obviously burnt out resistors, R75/76 and C49 based on sheer proximity. I apologize if that violates approved procedure. I couldn't help myself.

I know that I made no changes in the very poorly functioning tone stack for the "drive" channel. (The original reason for all this investigation.) Not surprisingly, the symptoms have made no change either. Gain and Volume both work fine. Contour behaves like a bass pot (somewhat). Tone pot and Bass pot do almost nothing, or nothing at all. It's a pretty tough call. If either one IS doing anything, it almost defies detection. To review, I've checked values on every resistor in the tone stack. Just to report exact numbers, this is what I got. I know these numbers warrant no concern, but I thought I'd post them just so you KNOW I did the work.

Resistor       value      as read value
R39             47k          46.1k
R41             47k          46.1k
R44             47k          46.7k
R45             22k          21.6k
R47             47k          46.7k

R40  Contour Pot - Working fine, I think. Behaves like a bass pot, more-or-less.
R42  Volume Pot - Working fine.
R43  Treble Pot - I did not write down the value, but I know it was close. Nice smooth sweep, yet unresponsive
R46  Bass pot - Right on the money, nice smooth sweep from 0 to 250k, but again, unresponsive.

Just as a point of comparison, the tone stack on the normal channel works great! Very responsive to any adjustment, so I don't think this is just a characteristic of the amp. EVERYTHING else on the amp is working as it should. If anyone has any idea on checking anything else, I sure would appreciate hearing from you.

Thank you,
Pete

phatt ,
Thank you for your expression of support. I've always been very comfortable with humility. My aspiration is complete absence of ego. There is one thing a lot worse than making a mistake, and that's sticking to it. Ego encourages the latter. (My "words of wisdom" for the day, acquired through life experience.)

Enzo,
No, I did not interpret your outline as criticism at all. It sound very much like the words of an experience instructor. Nothing in there sounds like criticism. Just good information.

OK, one comment on topic. Now I don't feel so much like I'm replacing parts for no reason, (thanks to your comment Enzo). Excuse me for repeating myself, but; I will report on function again, once everything is back in one piece and I can fire this puppy up again. (To put you at ease, I will be using my test lamp.)
I'm hopping the Solid-State gods will be smiling on me.

Thank you all for your display of compassion and kindness. Both attributes indicate a good human being.
(not to get too philosophical)

Pete

Thank you Mr. Fahey, and you as well, DrGonz78.
I'll report back with how everything is functioning after I receive and replace the resistors that I guess I could get away with leaving in, and get everything back in one piece and hopefully working again.
Thank you for your generosity.
Pete

Just as a point of amusement, here's a picture of the damage to the PCB directly under R75/76, with the suspect resistors in the foreground. You have to admit, this at least, doesn't LOOK good. Maybe this will help explain my apparently misguided concern for this area/these components.

Ooops! Excuse me while I wipe this egg off my face. I totally mixed up the AC, DC, capacitor thing. I must have been tired or something. I know I wasn't drunk, 'cause I don't drink! Please excuse the pun, but I got my wires crossed!!!!!!
You are right about R75, 76. (I think you might have mis-read or typo-ed on the value. They are 47ohm, not 4.7ohm. One of them is reading 23.8 out-of-circuit. The other one reads OK (also out-of-circuit). It just looks really bad. I had already looked up literature on "zobel network". . . WAY over MY head! I'm just a hobbyist, mostly whistling in the dark. I just took this up recently. So far I occasionally have success. I don't have the benefit of decades of education, training, and field experience. I'm just trying to see if I can get lucky and maybe with a little help, find a solution. Again, I apologize if my lack of knowledge is tedious to you. I'm not here to try to offend anyone. I'm more than happy to admit what I don't know. Perhaps I should re-evaluate whether or not I have the right to be on this forum.
My apologies to anyone who feels that they were wasting their time trying to help me out. I did try my best to follow everyone's instructions. Sorry if I let you down, or if you feel insulted by my ignorance.
The best to you all.
God bless,
Pete

phatt

Thanks for reading the post, and thank you for all the time it takes to look everything over. Everyone on this forum continues to amaze me with their generosity.

When I first got the amp (see first post) I did check the function of everything. EVERYTHING was working EXCEPT for the tone and bass controls for the "overdrive" channel. This is what prompted me to investigate in the first place.

* The channel switching DOES work.
* Reverb does work.

  "Normal" channel -
* Everything works A-OK,

   "Overdrive" channel -
* The Gain, Volume, and the Contour work, though the Contour maybe seems not quite as effective as I 
   would expect, but I could be wrong about that. It DOES impact the tone to some degree.
* Tone and Bass controls - very little, or no response.


Going in, I thought my ONLY problem was somewhere in that tone stack. I have come to understand that the two burnt resistors (R75/ R76) are totally unrelated, and are a separate issue. Looking at the schematic, I do see they couldn't possibly have any affect on the tone stack. How could it impact one channel and not the other? It's all so downstream from the pre-amp and amp. I have to admit, I am somewhat puzzled that those two resistor, completely shorted out, did NOT seem to interfere with the functioning of the amp.
And as if I haven't caused enough trouble . . . 
I do have another question about this, and I don't want anyone to get into a real lengthy explanation, because I probably won't understand it anyway, but . . . isn't the output signal to the speakers AC? If so, what's that capacitor (C49) doing there, directly linked to the + speaker output? Is it actually allowing some -DC from ground to pass to the +AC output? Is it sending stray +DC in the output signal to ground? I don't get it.
(Please excuse my ignorance. I'll mention again, I'm trying to learn.)

So back to the tone stack. I thought likely, just a bad pot or something. All the resistors that I've checked so far are all very close to, or right on spec. I don't know how to test the caps, but they all LOOK OK. And I know they can look OK and still be bad. To tell the truth, where they are so inexpensive, I wish I had ordered replacements just in case I did somehow find one to be bad. I know that the preferred approach is NOT to just start replacing parts willy-nilly, without knowing that they're bad, so I resisted the temptation to do so. I don't even know what the symptoms would be, of a bad cap in the tone stack. Maybe someone could answer that question for me. And/or instruct me on how to test said caps. I'm assuming there would be some really obvious symptom that I haven't mentioned, as so far, nobody has brought it up. I understand that the general consensus seems to be that caps in the tone stack rarely fail.

So thanks again for reading. If anyone has any more ideas, please chime in. Please correct me on any erroneous statement I may have made. I AM very much a novice (as if you couldn't tell), and trying my best to learn. Thank you all for your help. In the meantime, I'm waiting for replacements for those two burnt resistors so I can reassemble and re-check functions.

Pete