JCM 2000 problem

[shay]

Hi All
I have a jcm 2000 head for repair thats driving me crazy, the amp has no output , Power amp is working fine (put signal thru return jack) , Signal seems to be getting to V3 and then nothing, I've changed the tube, checked resistors around it, checked heaters and I'm stuck!!!

[Enzo]

Which JCM2000 please?

Follow the signal. Apply a steady signal to the input and trace it through the amp.

[Amp Kat]

^^^ no doubt what Enzo said. Had one a few weeks back like it and was a broken coupling cap that somehow clipped maybe from vibration however that amp is PCB so it could be a solder joint anywhere in the signal chain.

[shay]

Its a dsl 100 head, followed signal as far as pin 7 v3 then nothing will re check all components and solder joints around that area. Thanks

[Jazz P Bass]

Do you have a signal AT V3 pin#7?
Or is it dropping out before that point?
If you do have signal at pin #7, then check pin #3.
Also test pin #3 for Vdc to make sure the circuit is conducting. (no signal)
If pin #3 is producing signal, check Conn 11 pin #1.
I would also test this point for Vdc (no signal) to check the health of C27.
C27 is a coupling cap. A healthy cap should not pass Vdc.

[shay]

Yes there is signal at that point ,then its gone

[Jazz P Bass]

Gone where.
Test V3 pin #3.

[shay]

I did and there is nothing there !!!

[Jazz P Bass]

Please reread post , 2 above.
Pin #3 V3 : static Vdc Pin #1 & Pin #3.
C27. Conn 11 Pin #1. Should have no Vdc.
Hang in there

[Amp Kat]

May also want to take a sharpie and tap lightly around V3 on the components to see if you can get some tone or sound. Sure V3 is good ?

[shay]

V3 is good ,changed it out. will get back to it tomorrow

[cagamp1]

This one is a little interesting. If you have AC signal at pin 7 than you need to tell us what DC voltage you have at pins 6 & 8. You may have a problem at R1 or R31 depending on the voltages. This will tell us if that V3b stage is even working.

[shay]

Hi , sorry about the delay getting back to this, life took over.
VDC at pin 6 is 240v and at pin 8 is 1.8

[Jazz P Bass]

Please reread post , 2 above.
Pin #3 V3 : static Vdc Pin #1 & Pin #3.
C27. Conn 11 Pin #1. Should have no Vdc.
Hang in there

Check these dc voltages next.

[shay]

AHHHH how dumb could i be!!!! ( don't answer that) . The second half of V3 wasn't working because it didn't have a heater voltage ,
was measuring fine on pin 4 on pcb side of socket but not at the pin, didn't spot it on the tube because one side was lit and my worklight was in my face.
Thanks for your help guys , sometimes when your out all on your own in the wilds it helps having you guys on the other end of the interweb thingy