OT protection diodes SR-2873?

[Jazz P Bass]

Something is amiss with the PI plate voltages as the schematic shows identical 68K resistors.
Here is the schematic, I called Peavey.

[gui_tarzan]

I went through and re-checked every resistor on the power tube/PI board by reading values first, then finding them on the BOM and the schematic. They all check ok. Voltages are all listed above. There is a ribbon cable going from the main board to this one, I will test those voltages tonight and see what I get. And Jazz, I'm not ignoring you, I'll try your suggestion as well.

[g1]

Both sides PI heaters lit, yes?

[gui_tarzan]

Yes, different tubes including new tried in all sockets.

[gui_tarzan]

^^^^^ I've been hanging on since post #15 .

Can someone post this XXX40 schematic, I've been looking at XXX which is not correct?

Your PI plate voltages are bad, you said the PI plate resistors are good, but how about heaters and all connections to the socket (and resistors)?

I had to take a few days break, so I thought I'd re-measure the voltages again.

Heaters are weird on this, they're in series on the pre- tubes and parallel on the PI and power tubes.

V1 - 6.8
V2 - 7.7
V3 - 6.4

V4 - 6.6
V5 - 6.7
V6 - 6.5

I went through the tubes again and this is what I got this time:

V1
1 - 146
3 - 1.1
6 - 172
8 - 1.3

V2
1 - 143.6
3 - 1.1
6 - 205
8 - 1.6

V3
1 - 219
3 - 1.4
6 - 214
8 - 1.5

V4
1 - 282
2 - 66.7
3 - 74
6 - 289
7 - 48
8 - 66.7

No, I haven't put a signal through yet, I must not have had a good connection on the probe when I checked the voltages earlier.

[Jazz P Bass]

Send it through.

[gui_tarzan]

I just got off the phone with Peavey tech support. They said to check Q9 on the send/return board as well as R125. Apparently Q9 is used as a switch so per the tech's suggestion I lifted the ground lead and measured the voltage. I have 14.2v on the clean channel and 17.7v on the dirty channel but still no sound. I have 11.5k resistance across R125 so that's not open either.

[gui_tarzan]

Here's the board.

[gui_tarzan]

The schem section to make it easier to read.

[g1]

Here's part of the reason we're waiting for signal level on the power tube grids (from post 15). Normally the OT is one of the last things we suspect. However, your OT protection diodes got fried. Hopefully they gave their lives and saved the OT, but maybe not.
Checking signal on the power tube grids will tell us pretty quick. If you have good drive at the grids, and proper DC voltages at plate and screen, then the only thing that would cause low output would be a bad OT, or some output jack problem.

[Enzo]

I don't follow what you did.

Look at R125. The left end of it goes to the FX return jack, the right end to Q9 and the power amp. WHILE IT IS RUNNING, first check for DC voltage on the right end of R125. There ought not to be any. Your reading earlier just said voltage on the transistor. I expect no DC on the R125 side, the gate lead of the transistor may or may not have voltage. We want to see about +15v on the gate to keep the transistor OFF. But we do NOT want that DC on the source or drain leads.

If we determine ther is no DC on R125, THEN turn the meter to ohms and check if the right end of R125 has a low resistance to ground or high. We do not want to see low resistance. If you make the resistance reqading with the amp OFF, you should see the low resistance of the transistor. In normal operation, that transistor is held OFF - high resistance.

[gui_tarzan]

Another question - there are two pins (P900, P901) on the send/receive board on the schematic that look like they should be cabled to pins P1000 and P1001 on V3, P1000 going to the grid (pin 7) on V3. Should there be a cable between those two boards? There isn't one on this but it seems like there would be signal loss without one connecting them.

Sorry for the delay, I broke a pin off Q9 putting it back in and had to scramble to find another one.

[gui_tarzan]

1K signal piped in through the input from my phone. No change on V4 (PI) or the power tube grids, very slight increase in V1 & V2 grids, nothing on V3 pin 2 and I get a buzz on V3 pin 7 through the speaker when I put the meter lead on it. The volume of the tone at the speaker with the volume and drive all the way up is very quiet. Almost like you bumped the volume just off zero.

[g1]

Sounds like you are measuring DC. With signal tracing you want to be measuring AC volts.
Try putting the signal in at the FX return jack. Try to set it up so you measure .5VAC at the FX return jack.
Then measure AC at V3 pin6, V4 pins 1&6, and pin5 of V5 and V6.

[gui_tarzan]

Duh... my mistake. I was looking at DC volts...

Ok, with my meter on AC the voltage jumps all over the place on each of those test points.