Peavey Bass 400 m3 DC on Output

[stuartO)))]

I have a Peavey Bass 400 mark3, or a 400BH. I bought it "not-working" blows the mains fuses. I have identified a shorted output transistor and replaced the whole octet. I powered up and no blown fuses but found about +48VDC on the speaker output. (An overvoltage crowbar would have been a good feature for this amp apparently.)

I have obtained the schematics, (THANKS PEAVEY!)

I found a very similar thread from last year. This thread identifies a shorted 5W resistor R68 which acts as a voltage divider to make -15VDC from the -52VDC bus. This is NOT the scenario in my case. My voltages are good +/- 52VDC is good and +/-15VDC* is good. I have removed the cables from the power transistors to eliminate them as a possibility, but left CR13 in circuit, and still have the same +48VDC on the Output.

I have also made the following observations: I have also observed +48VDC on the collector of Q9. C15 IS blocking the DC from the collector of Q8. If I pull up R20 which is a voltage divider which makes +26VDC I only have +14.85VDC on the output.

I have pulled Q2, Q12, Q9, Q8, Q7, Q10, and Q1, R26, C20, R19, R58, C18 and ALL of the electrolytics and verified them to be good with my Huntron 2000 tracker. I have also pulled the op-amp chip but that changes nothing

This one is quite the puzzle! If anyone who has a comprehensive understanding of this circuit and can help me solve it, I would appreciate your collaboration!


(*for those of you who are curious or loooking into a similar issue, this is not the same circuit as coming off the voltage regulators which has its own separate circuit originating from the ORANGE wires coming from the power supply transformers)

[Jazz P Bass]

Peavey 400BH

Are you getting 0.87 Vdc on U1 D pin 14?

[stuartO)))]

I measure .92VDC on the output of 1Ud opamp

[Jazz P Bass]

Peavey 400

Are you getting 0.87 Vdc on U1 D pin 14?

Sorry about that, Chief.
U1 D pin 14 should read -0.87Vdc

[stuartO)))]

My bad, it is -0.92VDC.

What next?

[Enzo]

Right now, you can sit there with 48v on the output while you look for the problem. If PV had added a crowbar, you would have a dead short across the output when it detected that 48v and it would blow its fuse. How on earth would that be better?

Until the DC issue is solved, do not connect a load.

It is not a good idea to disconnect the output transistors from the circuits. If you do, then the predrivers have no place to send their current, and they have to try to drive the output bus alone. This can damage parts.

You do know that that 400BH power amp board will run alone, so you can unplug the umbilical cord to the preamp for convenience.

You already replaced the outputs, so check all the cement power resistors for opens.

The predrivers are the two small TO126 parts on the small finned heatsinks, the 5331 and 5332 types. Check them. There is a 47 ohm resistor with each, and a pair of 4.7 ohm resistors between them. ANy of those open?

R19,20 are 2k 5w cement types, is either open? (top center of drawing)

And check Q9 for opens and Q10 for shorts. And right below tham is 10 ohm R48 - is that open?

There have been more than one thread on this forum troubleshooting blown 400BH boards. You might search them.

[Jazz P Bass]

Peavey 400

It appears that Q9 is not conducting the B- rail voltage.
Measure the voltages around Q10 & Q9.
Compare them to the schematic.
Double check that R 45-R48 are intact & connect how they are shown on the schematic.
Check that you have 0.96 volts across R46. This turns Q9 on.

[stuartO)))]

SUCCESS!!!

HAH!

Thanks "Jazz P Bass", you were right! R46 was not in circuit somehow. The resistance reading for it was all floaty, like it had capacitance in parallel with it. Once I took it out of circuit and back in it read 1K and I observed that the collector voltage on Q9 was down to spec. No more DC on the Output! I put in a new 1K resistor just to be on the safe side.

I would like to burn this thing in on a dummy load for a few hours. If I get a 4ohm Ohmite or Multicomp 250W ceramic power resistor, would that do the trick? I have a simple HP function generator, will that do or do I need one of those Pink Noise generators? All of the electrolytic caps are good. Any other advice?

Thanks again for your help. WHAT FUN! This is the first amp that I have ever repaired! I am an industrial controls technician by trade and my first job out of tech school was repairing motor drives and servo amplifiers and the like, but it has been years since I have done board level repair and this is first audio circuit I have worked on. I thought that it was going to be just a simple shorted power transistor but I am glad I had the opportunity to learn something. Talk about going back to school, I even dug out my "The Art of Electronics" textbook by Horowitz & Hill!

[Jazz P Bass]

Peavey 400

Glad you got it working.
You picked a good one to work on. Peavey 400 amps are a treat.
That dummy load will be fine, along with the HP generator.
Monitor the Vac output voltage.
There is really no need to blast the amp.
I "bake" them at 30% power rating.

I'd like to thank all the posters at this forum for helping everyone at learning correct troubleshooting technics.

[Enzo]

A sine wave is actualy harder on amps than music. I usually burn them in on something from a CD player or the shop stereo.