5150--crazy problem

[Valvehead]

I picked up a broken Peavey 5150 head for a good price . Took it apart and found it had a shorted disc cap across the 2amp fuse , a burnt ribbon connector (from main pcb to output tube pcb) and an open screen resistor,

so i fixed all that and fired it up with the current limiter and im getting some wild stuff.

very loud hum--i can silence it by pulling V3 or V4

also--when i turn either the lead or rhythm master volumes to 0 , i get louder hum accompanied with sparks and smoke !!! from right where each pot is

what the hell would cause that kind of chaos at the master volume pots ????

i will attach video so you can see and hear it .


http://vidmg.photobucket.com/albums/...ps3fkibsxu.mp4



<a href="http://smg.photobucket.com/user/solrizzo/media/5150-2_zps3fkibsxu.mp4.html" target="_blank"><img src="http://img.photobucket.com/albums/v225/solrizzo/th_5150-2_zps3fkibsxu.mp4" border="0" alt=" video 5150-2_zps3fkibsxu.mp4" style="width: 160px";></a>

[tedmich]

Yikes! Voltages all check ok without tubes? No PS caps shorted to ground?

[drewl]

Awesome!
Which pot was that?

It looks like you have voltage where it shouldn't be.
Pull the tubes, pull the fuses and check ac voltages from the power transformer.
Start there and report back.

[Valvehead]

it does it on BOTh masters..

i did find that 2 of the tall pins going to the power tube board are showing continuity---without the ribbon cable on . i will have to trace them but they go to 2 diff power tube pins

[Valvehead]

ok-- with the power trans un plugged from the board--all the voltages look ok on the secondary plugs of the trans---however, when testing the Filament connector, i have a short between the 2 wires . That shouldnt be shorted ??

and with the power trans completely un plugged from the board...i show a short between the 2 filament connections (on the pcb)..that cant be right, or am i missing something

[tedmich]

be thankful its not an XXX with metal knobs

[Enzo]

If you mean the heater winding of the transformer reads shorted, it is doubtful, the thing is a low turn count winding of very heavy wire. It has a DC resistance close to zero.

With tubes in their sockets, the board will measure a very low resistance across teh 6v too.

The volume controls should have no efect on anything burning and smoking. Please make VERY sure all teh ribbons are on their headers correctly, make sure none are off one pin or something like that.

[Tom Phillips]

...with the power trans completely un plugged from the board...i show a short between the 2 filament connections (on the pcb)..that cant be right, or am i missing something

Are the tubes pulled out when you make this measurement? The resistance of a cold tube heater is very low. All the tubes in the amp connected in parallel will appear to be a "short" well below 1 Ohm.

[g1]

Please post the schematic you are working from, so we can all refer to the same areas and component numbers.

found it had a shorted disc cap across the 2amp fuse

I hope this is just a typo, and the cap was not really across the fuse. I'm pretty sure having anything in parallel with a fuse would be illegal. What is the component designation of the cap that was shorted?

[drewl]

The ribbon cable that was burned, are any of the wires shorted?

If that's the master volume pots that could be the bias voltage or high voltage going to output board getting to the pots from shorted ribbon cable or shorted coupling caps.

[Valvehead]

Please post the schematic you are working from, so we can all refer to the same areas and component numbers.


I hope this is just a typo, and the cap was not really across the fuse. I'm pretty sure having anything in parallel with a fuse would be illegal. What is the component designation of the cap that was shorted?

sorry , not across... C32


http://www.blueguitar.org/new/schem/peavey/5150evh.pdf

[Valvehead]

Are the tubes pulled out when you make this measurement? The resistance of a cold tube heater is very low. All the tubes in the amp connected in parallel will appear to be a "short" well below 1 Ohm.

ok--so thats normal.

[Valvehead]

The ribbon cable that was burned, are any of the wires shorted?

If that's the master volume pots that could be the bias voltage or high voltage going to output board getting to the pots from shorted ribbon cable or shorted coupling caps.

no , and i clipped the burnt one on each end of the ribbon. the burnt section is a heater circuit

also, the amp still arcs at the masters with the ribbon cable OUT of the amp... with standby switch on or off

another thing -- when i move those masters and it sparks, the channel LED will flicker--on either channel

[drewl]

Do you have a voltmeter to measure what voltage is on those pots?
AC, DC, positive, negative?

Is the board grounded properly?

Input jack sleeves should be grounded to the chassis as well as most pot housings.

There is a high voltage filter cap on the far right side of the main PC board above the input jacks.
Make sure the negative end is grounded properly.

That caused weird voltage problems on one I repaired.
All those little PC board mounting screws come loose often.
Make sure they are all there and tight.

[Enzo]

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.