Peavey 200H Help Please!

[TigerAmps]

Hello - I have a Peavey 200H (A Series) Power module (from a XR400) with a high DC offset at the output ( around .37 volts).
The power transistors dated to 1980 and it looks like an A series power board

History: R20 was open and roasted R14 and R15 making a new path to ground; in the process two of the output transistors blew.

What was done so far:
Q7and Q10 are original transistors (checked out with meter)
Q8 and Q11 were replaced with MG15015 transistors.
R20 was replaced (.33 ohm/5 watt)
R14 (100 ohms) and R15 (120 ohms) were replaced
C13 and C14 were replaced due to age
C15 and C16 were replaced due to age
Q4 and Q5 were replaced (but original were not bad - no change)
CR7 was replaced (not blown - but as a precaution)
I swapped Q9 and Q3 with no changes
The transistors check out to be good (no opens or shorts)

Powers up; no smoke - just high DC on output

Note: I did not connect a speaker nor did I connect the front board.

Voltages:
Rails +/- 40.5 V on both sides
.31 Volts across R9
+.8 Volts at the anode of CR4
-1.46 Volts at the cathode of CR8
-1.3 volts at the anode of CR7

Any thoughts where to go from here? Could the 2 original output transistors (Q7 and Q10) be bad even thought they meter out as no opens and no shorts?

Thanks ....

[Jazz P Bass]

I do not see any voltage references.

The schematic is loaded with them.

If everything checks out & the amp functions normally but you still have a high offset, I would recommend changing BOTH Q1 & Q2.

[Enzo]

As a general rule it is a bad idea to swap out a lot of parts and move parts around unless we know them to be bad. Every time we change or move a part it is just that many more opportunities to contribute more problems to the circuit. The old blue main filters might be needing a refresh, but they were not likely causing the blown outputs or your offset. And poor little CR7, it is maybe the safest diode in there with that 22 ohm resistor in parallel. Once the amp is fixed, then is the time to upgrade.

I'd lift CR4 and CR8 just to disable the limiter circuits Q4,5.

The 5331/5332 are your drivers, are all those resistors in their column OK? 47 ohm, 10 ohm.

The voltages on the left half matter too, check them.

370mv is a lot higher than I'd like, but...

Run a sine through it and scope the output, is it clean? Runs up to clip evenly? Now back off and put on a load. Does the offset stay or disappear? A 370mv offset on an 8 ohm speaker is only 17milliwatts of dissipation. It will hardly hurt anything. Once a proper load is on the system, it MIGHT even correct itself. But if there is a problem it might also help reveal it. If the amp is clean unloaded, does it behave any different loaded?

[audiobrad]

Have you checked Q3+Q6.

Does this model use a crowbar protection circuit.If it does dis-connect it so you do not have anything leaking onto the output rail .

[Enzo]

This model pre-dates the Peavey crowbar strategy, but always good advice.

[loudthud]

The Base Voltage of Q2 should be nearly equal to the offset at the output. Make sure that the Base of Q1 is close to the -0.1V listed on the schematic. Check that R5 has not changed value and that the -16V is at the lower end. Check that R4 has not increased in value. If these things are OK, look around the Base of Q3 and check R3.

[TigerAmps]

Hi - sorry for the delay in getting back on this; been busy ...

My DC offset is still high (475 mv). I did check the other side of the power board.

1) R3 is currently a 1K ohm resistor (both as measured and per it's color code).
Not sure if this was a Peavey production change or someone else changed it for some reason.
I plan on changing it back to a 1.5K value tonight as the schematic indicates.
2) R5 was tested in circuit and gave me some strange readings(15.5K). I will disconnect it and test it out of the circuit tonight as well.
3) I did hit the resistors and small diodes with freeze spray and it had little effect on the DC offset.
4) I also realized that I need to connect the front board to the back board as that is where the 16 volt diodes are.
If not your voltage feeding R5 will be incorrect.

After I do the above, I will re-check the voltage readings and let you know what I find.

Any other thoughts, please let me know ...

And thanks for the help!

[Enzo]

22k resistors don't turn into 15k resistors, you either have a trace of residual voltage in the circuit or there are parallel circuit paths affecting the reading. Here is a trick: if a resistor measures low, reverse your leads. if it now reads high, then there is voltage present. If the reading is the same, we have parallel paths.

I ask again though, while half a volt is a lot more than I like, is there an actual performance problem?

[TigerAmps]

Update:

I replaced R3 with the correct value resistor (1.5K ohm).
Also, I checked all the resistors and they are all (yes all of them) to spec.
R5 was at 22.6 ohms after all the residual voltage drained off.
I fired it up last night with a speaker connected; it still measured around 450 mv on the output.

I played it a while but at low volume because it was late in the evening - it sounded good.
The amp was surprisingly quiet (sitting 3 feet away from a pair of Eminance Ledgend V128's;
all 4 channels work and the reverb works.
I will have to clean the pots and jacks on the front board as they are obnoxiously scratchy.

I will also play it at a higher volume to assess if there is noticeable distortion.

I am also considering changing Q1 and Q2. I have them and they are inexpensive transistors.

[Jazz P Bass]

"I am also considering changing Q1 and Q2. I have them and they are inexpensive transistors. "
Do it.

[TigerAmps]

Will do it over the weekend and will post the results ...

Thanks!

[TigerAmps]

And the winner is "Jazz P Bass"!

I changed both Q1 and Q2 as well as C7 and C9 (they measured good, but replaced them due to age and I had them on hand)
and the DC offset went to less than 10mv!

Thank you to all who responded!

[52 Bill]

And the winner is "Jazz P Bass"!

Well, Loudthud was the first to point you in the direction of the differential pair. Jazz just told you to change them.

Glad to hear that it is working.

[g1]

Well, Loudthud was the first to point you in the direction of the differential pair. Jazz just told you to change them.

Glad to hear that it is working.

See post #2 .

[52 Bill]

OOPS! Sorry about that. I stand corrected.