Peavey TNT-100-79

[adamscr]

I have a Peavey TNT-100-79 with distorted output. I was unable to locate a schematic so I was troubleshooting blind. I did find that the monitor port output was clean so my first attempt at reapir was to replace Q6-Q9. This had no affect so I called Peavey directly and they sent me a schematic and board layout diagram. I have an oscope so I dug in and found the input to base Q1 clean but the collector was noisy and clipped. The same noise and clipping was evident throught to the speaker output. To keep from having to remove the board over and over I just ordered and replaced the remaining transistors knowing it had to fix the problem. No luck. I did check the voltages while I had the scope out and they seemed to be in the ballpark. I did notice about .1V sawtooth ripple on the +38V rail. I didn't feel this to be an issue since the supply is not highly regulated but perhaps someone knows better. My next replacement would be C31 and C32 but I wanted to get your thoughts on my logic. Any assistance would be much appreciated!

Thanks,
Canie

[Jazz P Bass]

You have all of us at a slight disadvantage.
You have the schematic.
Can you post it?

[adamscr]

Attached is the schematic.

[adamscr]

Attached are scope images taken at the request of Enzo. The first is the input (base) of Q1 which remains constant. The next is the collector of Q1 with the speaker attached. The last two are the output to the speaker with the speaker attached and disconnected. I did check R43 and 44 as suggested and they are good. I have not yet tried lifting the leads of CR10 andd 11.

[Jazz P Bass]

Have you verified that you have the + & - 16 volt rails & that they stay steady?

[Enzo]

So the last image is unloaded output? Looks like a nice clean sine wave from here. And the third image is WITH a load, and I see the top half of the waveform missing.

That means the output stage can swing the voltage but cannot provide the current for a load. My suspects then are Q8 and R51. One of them open? or a broken connection to one?

[adamscr]

R51 appears to be open. A .33 ohm resistor should read almost like a short. Q8 has already been replaced. I did lift lhe leads on CR10 and 11 and the output did clean up. How could that be with the resistor open?

[adamscr]

They were reading +-15.7. It appears that R51 is open. When I get it replaced I'll let everyone know the result. Thanks to all.

[adamscr]

Its fixed!! R51 was definitely open. I found one locally and replaced it yesterday. Crystal clear sound now. Thanks to everyone for the help and support.

Thanks again,
Canie