5150--crazy problem

[Valvehead]

There is just no way for high voltage to get on those master volumes unless the thing is mis-wired. The only place B+ goes in standby is the OT center tap. A 24v supply getting on there? I see no way for that either.

The masters arc with "the" ribbon cable out of the amp. The one to the powr tube board from the main board? there are half a dozen ribbons, four to the preamp tube board, one main to power tube, and one to the jack board.

I have to admit I don't even see any ribbon related paths. Any chance there is a loose nut or some other hardware rolling under the board?

Isolate the problem. Right behind VR6 and between VR5 and 6, are a couple wire jumpers, they connect to the master pot wipers. What voltage is sittin gon them? that will at least tell us what is getting shorted to the pots.

Pull the high voltage fuse, that removes ALL B+, so we see if it is involved.

If that changes nothing, leave it out, and pull the two 24v fuses. Does that stop the arcing?


OK, here is my suspect: Look for K2 relay. It is in front of the longest preamp tube ribbon. A little closer to that ribbon is a group of small parts: a cap, two diodes, a transistor, a couple resistors. Power off. Is one of those diodes shorted?

If I am right, you owe me a beer.

both master pots have +25 vdc at them when turned to 10 .

the ribbon thats out of the amp is the 7 wire from main pcb to output tube pcb
the grounds are good screws tight--all the large electolytics that are on the right side / preamp of the main pcb are grounded

i will check the rest you mentioned enzo--give me a few minutes

[Valvehead]

ok...pulled the hv fuse---still arcs

pulled the 24v fuses---NO arcing

not sure which relay is K2 , but i measured the 4 diodes next to the relays with my ohm meter---here is what i have=

1. 400k
2. 0 ohms
3.116 ohms
4. 1.2k

so #2 shorted?

[Enzo]

K2 is on the layout drawing.
http://bmamps.com/Schematics/Peavey/...Schematics.pdf

It is as I described, right in front of the longest ribbon connector to the preamp board. Longest in number of pins, not how long the wires are. Third ribbon from the end of the chassis. Straight back from VR2. Third relay from the end, the one most isolated from everything else.

I am looking for the two side by side diodes I described, right in front of that same ribbon connector. CR21 and CR22. I don't know which parts are your 1,2,3,4, but none should measure shorted.

[Valvehead]

cr21 - 500k

cr22 - 0 ohms

[Enzo]

And there you go, CR22 is putting 24v on the relay, which connects to the controls. Remove it and the problem probably stops, but it should be replaced. All the arcing and sparking may have damaged those two controls, so you might want to call Peavey and order those up.

Was your earlier #2 that one or a different one? It should not be shorted, wherever it was.

[Valvehead]

And there you go, CR22 is putting 24v on the relay, which connects to the controls. Remove it and the problem probably stops, but it should be replaced. All the arcing and sparking may have damaged those two controls, so you might want to call Peavey and order those up.

Was your earlier #2 that one or a different one? It should not be shorted, wherever it was.

yes #2.

ill order some pots---thanks--- i will post back when i get it fired up again

[tedmich]

"fired up..."

heheheheheheh

[Valvehead]

well, the amp is up and running , and now on to a new problem..........

when switching channels theres a loud static pop...ill try cleaning the switch ...any ideas on that one ?

other than that its working great...has a low freq hum...but i suppose that could be the old 90"s filter caps

[Enzo]

Yes, now that it works, consider the damage you already found.

Look on page 2 of the schematic. right above the bias supply. See CR21,22? And CR22 was shorted? That put 24v not only on your controls, but also on that poor little JFET Q7. The purpose of Q7 and its friends is a mute. Whenever you switch channels, that little circuit briefly mutes the signal so you don't hear loud noises as the relays switch over. I'd bet your Q7 was damaged by the shorted diode. The stuff to the left of Q7 is probably OK, but could have been damaged as well.

[Valvehead]

replaced Q7 today with no change ,, should i try c23 and the 2 caps?

[52 Bill]

replaced Q7 today with no change ,, should i try c23 and the 2 caps?

Was Q7 shorted?

Normally there is a positive on the gate of Q7 that keeps it from grounding out the signal at the wiper contact of relay K2B. When the channel switch is activated, the triac CR23 fires and kills the gate voltage for a short time, muting the audio signal from the master volumes.

Monitor the voltage at the gate of Q7 and see that there is a steady positive voltage there, that drops out when the channel is switched. If the gate voltage does not drop out, then check CR23.

[Valvehead]

Was Q7 shorted?

Normally there is a positive on the gate of Q7 that keeps it from grounding out the signal at the wiper contact of relay K2B. When the channel switch is activated, the triac CR23 fires and kills the gate voltage for a short time, muting the audio signal from the master volumes.

Monitor the voltage at the gate of Q7 and see that there is a steady positive voltage there, that drops out when the channel is switched. If the gate voltage does not drop out, then check CR23.

not sure if it was shorted--i tossed it.
at Q7 , one outer leg drops about 1mv , the middle leg which had 21 or so volts seems to drop around 5v very briefly, the other outer drops down to 0 and stays

cr23 does the same as q7 when switching
i do have a trans to replace cr23 if needed

[52 Bill]

not sure if it was shorted--i tossed it.
at Q7 , one outer leg drops about 1mv , the middle leg which had 21 or so volts seems to drop around 5v very briefly, the other outer drops down to 0 and stays

cr23 does the same as q7 when switching
i do have a trans to replace cr23 if needed

The two outer legs should have little or no dc voltage on them as they are part of the audio path. In fact one of them is connected to ground. Do you get a dc voltage reading on the outer legs?

The center leg is the gate and if the voltage is dropping when the channel is switched, then the triac, CR23 is working at least to some extent.

[Valvehead]

i just measured again.....i might have screwed up last time --SO,,,

Q7 on each outer leg , has 1mv to 0mv depending on channel switch

CR23 has 1mv -0mv on one leg , but always has 21 VDC on the other outer leg

[52 Bill]

One leg of Q7 is grounded, there should be no dc voltage reading there ever, unless the ground connection for the board is not making connection.

CR23 should have three legs. One is connected to the positive power supply (+22). another to ground and the third to the channel switch. So one leg should be at +22 volts until the channel is changed. Then it should drop and then come back up to +22 volts. This is what controls the gate of Q7.