Vintage Ampeg V4 Rice Crispies

[Rciancia]

****Moving thread to correct link*****
Thanks for a great forum. I am working on restoration of a 1971 Magnavox Ampeg V4 ( 1971) and have done quite a bit ( as listed below). The amp powers on correct and is playable but there is a SNAP CRACKLE AND POP coming from the amp when taken off standby. Volume control does NOT make it louder/lower however the tone controls do affect it as when turned down the noise reduces dramatically. I have the reverb down to Zero while this happens. To date here is what I have done and looking or other ideas. ( THANK YOU )

Replaced all Electrolytic Caps
Replaced several resistors around the plates of the tubes
Replaced all tubes - problem happens with both sets
Removed and cleaned the Ohm selector switch with Deoxit
Unsoldered and resoldered all 9 pin tube sockets
Tightened all tube sockets
Spot checked solder joints on Power and PreAmp card and dressed up suspect solder joints
Removed one tube at a time - noise stops when the 12DW7 tube is removed. Replaced with a different 12DW7 same issue. 12DW7 appears to be slightly microphonic ( both tubes)


Any other ideas? I really appreciate any help that can be suggested.

Thanks
Ron

[g1]

Deoxit the tube sockets.

schematic attached

[Rciancia]

Thanks G1.
Thanks for the schematic. I have REV E. which is included on my unit.

I have already done that and just did it again with the same result ( no improvement).
I also rocked the tubes to help break away any debris.

Any other suggestions ?
Thanks much
Ron

[The Dude]

I might try running out of one of the external amp jacks into another amp and see if the noise is there. It might tell us if the noise is at the 12DW7 stage or before.

[g1]

Run a jumper from the C8/R16 junction (12DW circuit) to ground.
Is noise gone or still there?

Edit: simulpost, try what Dude mentioned first, it can be done with amp assembled.

[Rciancia]

I ran the jumper from C8/R16 to ground and a HUM was introduced along with the crackle. Thanks for the suggestion.

[g1]

The hum is expected, but the crackle is still there, so we can eliminate what's ahead of the DW.

[Rciancia]

I've just tested this and the crackle is present on both the External AMP side and the Speaker Out side. Thanks for the suggestion.

[Rciancia]

So basically this isolates it to the PreAmp section if I understand this, correct ?

[The Dude]

It would isolate it to something in or before the first half of the DW. I would pull the 6K11 next and see if the noise is still there.

[g1]

No, the noise is introduced after the C8/R16 junction.

[Rciancia]

I am guessing C9 (.47) may be the issue. Its an old cap as pictured.
Checking ( in line it is 3.4 MF) but I know this test is not valid unless I remove it from the circuit.
C209 is the same cap ( .47) and when testing in the circuit I indeed do get .47

Sound Plausible ?

[The Dude]

It looks cracked, or is that just the picture?

[Rciancia]

upon inspection it does have a crack. Unfortunately I do not have a .47 handy now... but I have some .47u on order.

[Rciancia]

I removed it (C9) from the circuit and it comes in a .48 (impressive for its age) so thats not the culprit but will be replaced anyway.