Peavey TKO 65 distorted/low output

[scottsmith6]

I have a TKO 65 with the following symptoms:

•Sound is distorted, output lower than normal and “Comp” light on continuously when playing.
•+/-43VDC and +/-15VDC are all good (and very close in magnitude) with normal AC ripple (millivolts).
•Pre-Amp Out to a known good power amp and speaker work fine – all pots respond as they should (gain and tone controls).
•Injected a known good pre-amp signal at the PWR AMP IN jack with same symptoms as before.
•Removed U3 (DDT chip) with no improvement in sound.

My next step for trouble shooting will be to try one of the “known good” 4558 chips from the pre-amp as a replacement for U4 and then U5 to see if this repair can be solder free! Of course I run the risk of damaging what was a good 4558 in the process…

I found the attached schematic on here already and have started my own "layout" but the real one would help. I tried e-mailing Peavey at 'pvrepair@peavey.com' and got a delivery failure.

Any suggestions? THANKS in advance!

[scottsmith6]

Update

Had better luck at this address... repair@peavey.com


Hi Scott,
Remove Q2 , sounds like you have a bad muting FET
If that doesn't work then swap out the 4558 IC's
here's a copy of the complete schematic
hope this helps.
thanks
gene

Sounds like I was headed in the right direction. Thanks for the diagrams. I’ll try the Q2 tonight!
Thanks again,
Scott

[scottsmith6]

Results from tonight’s troubleshooting…

1) removed U4 and jumpered from Pre-Amp Out to U4A-1/R44/R47 junction (with one end of a spare cable plugged into PWR AMP IN to separate the signal from Q2) – no improvement in sound quality
2) swapped U2 and U4 – no improvement in sound quality
3) swapped U1 and U5 – no improvement in sound quality
4) removed Q2 – no improvement in sound quality

Now I’m stumped… at least for tonight!

[Enzo]

OK, leave U3 and Q2 off the board for now, and make sure neither clamp diode CR8,9 is shorted, sending 15v onto the input.

And just use the powr amp jack.

SCope the output. What does the distortion look like? Missing half the waveform? Clipping? Even clipping or more one sided?

More importantly, does connecting the load make a difference. In other words is the output low and distorted with a load and without, or does removing the load allow the full undistorted output to come out?

Right at the output is R81 in your zobel network. Make sure it is not open and the power amp is not oscillating at RF.

The power amp section is chock full of DC voltage references. Check them all. Probably all OK, but check anyway.

With steady test signal applied, what signal is at:
C37
the collector of Q5
the bases of the drivers and outputs.

And lift diodes CR17,22 from the circuit there at the bases of the drivers. That will disable those limiter transistors.

[scottsmith6]

Thanks for the help ENZO! I may not get a chance to pick back up on this through the weekend but I will post my results as I move forward. It was getting late last night after I tried the things I posted but I did start "poking around" with my meter. I didn't write down my measurements (yet) but I recall the voltages being significantly low at the base of the diffy pair and in and out of Q5. Again, this was just poking around in frustration!

[scottsmith6]

Progress made!

Finally got around to some more troubleshooting…
Removed Q2, U3, U5, CR17 and CR22. None of these are in the signal path and none had any effect on the quality of the output, as expected.
CR 8 & 9 verified good. R81 and C50 verified good. No scope at home so I checked DC everywhere and found this MAJOR anomaly – from the junction of R73/R77 to the base of Q8 the DC voltage is good (approx 1.13 volts) but to the base of Q9 the voltage is effectively zero. Found R84 OPEN! -43 volts on one side of R84, effectively zero on the other. Ohm reading confirmed R84 open, R72 good and R82 good.
So, now to solder in a new 2.7K 1W resistor and hopefully I’ll have good news!
I'll post my results...

[scottsmith6]

Success!!!

Replaced R84 and checked DC voltages - good. plugged in my bass and the distortion is GONE - clean, full output! Resoldered the limiter/diodes and Q2. Reinstalled U3 and U5. Buttoned everything back up and played throught the amp for about an hour. Sounds great - neat little practice amp!

Thanks to Gene at Peavey for the schematic/layout and a HUGE thanks to Enzo for your advice (in this and many other threads). Your troubleshooting advice is clear, concise and very methodical.

Another Peavey lives again!