Peavey 6505 (5150) Distortion

[DPM309]

I am working on a Peavey 6505 (5150) that is experiencing low volume and distortion. The pre pots do change the volume level somewhat but the post pots do nothing and I still get the weak, distorted signal even with the post pots at zero. All of the tubes have been replaced and the power supply voltages are good (measured with the tubes pulled). Checked all of the cap and resistor values and they seem to be good except for R125 and C75. I read a previous post on this issue (http://music-electronics-forum.com/t3993/) that mentions K2b as the possible culprit. Since the post pots do not affect the signal, I am suspecting that the problem might in the area of K2. This is not an easy amp to work on as it is nearly impossible to measure voltages with the tubes in place due to the C shape of the circuit board. This also makes it extremely hard to trace the signal with a scope or listening amp. I also noticed that V2 is measuring only 500K instead of 1M but since I am getting the same problem on both channels, I don't think this could be an issue. Using a listening amp, the signal is clear before this point but distorts after it. Should I try changing out R125 and C75 before I look at K2? I have attached the schematic for reference.

Thanks,

Dan

[Enzo]

Hi.

First PLEASE please please, always mention you have the 5150COMBO, not the 5150, when asking about this amp. Only clue to that is the schematic you posted.

By V2, I assume you mean VR2. Look at the schematic. VR2 is a 1 meg pot. Right next to it is R43, a 1 meg resistor. They are in parallel. What do we expect two 1 meg resistances in parallel to read?

Look upper right in the schematic, see Q1? Remove it. Did the amp wake up?

[DPM309]

Enzo,

Thanks for the quick response and advice. Sorry for the confusion on the model of the amp. Also, I should have seen the 1 meg resistor iparallelel with VR2. The only reason I suspected this was that I recently worked on a PV Classic 20 that was experiencing distortion and low power and it turned out to be the 1 Meg pot. Anyway, I removed Q1 and the amp is alive, although I am getting some hiss. Also, I noticed that both sides of V4's (PI) heaters are now lighting up. When I first looked at this amp, I could hardly see a faint glow from 1/2 of V4. Now that Q1 is pulled, do I replace it or should I be looking further?

Again, thanks Enzo.

Dan

[tedmich]

I'd replace it!
J174_D74Z Fairchild Semiconductor | Mouser
before they're gone, although they're probably good subs available...

[Enzo]

Q1 is a mute during channel switching. I'd replace it, but if you can switch channels back and forth without loud pops or squawks, you could do without.

However, Q1 has a friend Q4. Look left of Q1 to the middle of the page. It is a mute at the reverb input. The two are wired to gether at their gates. So either one can affect both. If I replace one, I always just replace both. For example Q1 may be causing the drop in sound, but it could be a failed Q4 making Q1 sstick on.

So, with Q1 gone, the amp now works, but does the reverb work? If not, then Q4 is a suspect.

[DPM309]

The amp does make some noise when I switch channel so I will go ahead and replace both Q1 and 4. The reverb is working fine but I will replace Q4 as long as I am in there. Will order a few of these from Mouser as per Tedmich's suggestion.

Thanks Enzo for your excellent help and suggestions.

Dan

[Vin]

Hi Enzo, been reading your posts on the post volumes not doing anything on 5150 combo seems my amp needs these q1 q4 replacing maybe ? i bought my amp second hand the reverb never worked but i didnt need it but all else was fine. been gigging with it for months no problem until now. im good at replacing parts soldering wiring etc but cant read schematic charts. could you visually direct me to q1 and q4 if i am looking at the board on the bench ?? Hope you can help, anyone else's input much apprieciated too - Thanks Vincent

[Enzo]

Oh sorry. I suggested in your PM that you start a thread, and here I see you already have posted in one.

What I wrote back was that those transistors ar on the center board. ON that board are two relays, one mounted lengthwise and one mounted crosswise. Your transistors are next to the lengthwise relay.