Problem with Peavey XXX 212 combo

[ncjoyota]

I have a problem with my XXX 212 combo, It went quiet sometimes, and now it has lost all input volume all together.. I thought it might be the tubes, so I got eurotubes to send me a set of JJ KT77's an EC83's well it wasnt the tubes either.. I biased the power tubes to 37.2mA and still no sound.. then I noticed the middle preamp tube wasnt glowing.. so i took the chassis out and the socket has no voltage running to it... while the other preamp sockets have about 340v running thru them.. I cant see any cold solder joints nor any burnt resistors.. I am stumped.. Im not a electronic guru, so I dont want to get to fooling around too much inside this amp. any suggestions?

[tedmich]

checked all the fuses?

thats 90% of PV problems, often caused by a old or bad tube.

Look for obviously burnt components and bleed filter caps and check for shorts.

[ncjoyota]

I checked all 4 fuses, and they were all pulling .7ish resistance so I believe them to be ok.. I havent taken the board holding the preamp tubes off the chassis yet, honestly because I dont really know what I would be looking for, or which one of those little pieces are sensitive to multimeter probing. I am considering taking it to a peavey repair shop 90 miles away and dropping that $150 to fix it.. but it is probly just something super simple and I will get charged $100 to re solder a little piece..

[ncjoyota]

from the top of the circuit board, I dont see anything burnt, and the bleed filter caps all look clean and non burnt.. I cant even smell any burning smell or anything... by the way what do the bleed filter caps look like when they go bad? thats the round black things.. that discharge the white square resisitors right?

[tedmich]

You can probably fix this BUT you must get a handle on the parts and their safety!

SafetyTips
http://www.geofex.com/tubeampfaq/tube_amp.htm


get the schematic here
Peavey xxx Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
and slowly begin to trace the circuit on the board; bad resistors are often burned, bad caps maybe just shorted internally or bulgy/leaky, One common problem is a short in the wire connectors between the boards, pull them one at a time and look at them under a bright light to see if one is burned.
the power supply caps are C42/43 C47/48 and they have the "bleeder" resistors R69/70 73/74 across them; they should bleed their charge safely to ground in a few minutes after being OFF, PS caps C45-6 have no such bleeder resistors across them and you will have to carefully add a ~1w 220k resistor across them for ~15min to make them safe to measure!


go slow and be safe if you want to learn and don't mind the SLOW pace, otherwise pay a tech!

[ncjoyota]

I didnt see any bulging caps or leaking.. And didnt notice any burned resistors... But the wires i have not checked yet... They are really hard to unplug.. Almost to the point where im scared it might break the board. But i am going to exami.e them much closer when i get home from work tonight. As for ttesting the resistors and such.. I have never had a.y formal training on how to check them using the correct measuring scale. It could be wrong and i wouldent have a clue what the numbers were supposed to read

[tedmich]

some decent trouble shooting threads here:
• View topic - Peavey XXX Help
Enzo can walk you through the fix but he's only one guy AND has done this ~100 times so you can see his previous pearls of wisdom.

[g1]

then I noticed the middle preamp tube wasnt glowing.. so i took the chassis out and the socket has no voltage running to it... while the other preamp sockets have about 340v running thru them..

Could be the unlit preamp tube is bad. Try swapping it around with one of the others and see if it lights up in a different spot.
The heater filaments are what makes the tubes glow. They should glow even with the amp on standby. Sometimes different brands of tubes are harder to see whether they are glowing. For the preamp tubes, the heater voltage is from pin 4 to pin 5. The 340V you mentioned is not for the heaters. From the schematic it looks like there should be 12 volts DC between pins 4 and 5 at each preamp tube (except the phase splitter which uses AC volts on the heaters like the power tubes).
Check if you have heater voltage on the tube that is not lighting up.

[ncjoyota]

great thread on the power tube screen resistors... but my pre amp tubes are the ones messing up.. also I tried to swap around the tubes, and see if there was any defernce.. but all had the same results.. its the amp for sure not the tubes.. if there were a definant reason that 1 whole pre amp tube and half of the one next to it werent lighting.. or getting any voltage... I would test it, and attept to replace that said part.. but this seems like a hairy problem .. and that I might should take it to a tech