VOX AC15 Heritage... TB Channel bleeds into the EF86 channel.

[Cooper1962]

Hello to everyone! This is my first post on Listen Live.

I have a beautiful 2007 AC15 Heritage amplifier that was working fine. I replaced the JJ EZ81 with a NOS RCA EZ81 rectifier tube. The amp began emitting strange static, almost "frying egg" noises, so I shut it down, and put the JJ EZ81 back in. I also replaced the stock tubes with NOS tubes, which all test good. Here's what's happening...

- I can hear volume when the TB channel is set to zero.
- The TB channel is bleeding into the EF86 channel, with nothing plugged into the TB channel. If I plug into the EF86 channel only, I can turn the TB channel volume pot up and down, and hear it through the EF86 channel.
- The tone controls do effect the bleed-over signal, so I assume that this "leak" is making it past the tone stack.
- The EF86 channel operates normally, and does not bleed signal into the TB channel.
- If I remove V2, there is no bleed-over, as there's no sound from the TB channel at all.
- EF86 channel operates normally.

I've tried swapping tubes, clipped the bright cap, retouched a few iffy solder points... so far, nothing. Issue remains. I'm no electronics expert by any stretch, but I can effects repairs with direction. Could the NOS EZ81 tube have damaged some capacitors? If the problem is a failed TB channel volume pot, I would not expect the bleed-over signal to increase through the EF86 channel, as the volume pot is turned up. Could the TB channel be "polluting" the EF86 through the B+ somewhere? If yes, would the EF86 channel not also bleed into the TB channel? I have a full set of electrolytic capacitors, if needed.

[nosaj]

Service manual link http://dealers.korgusa.com/svcfiles/...&L_SManual.pdf

Did you test the RCA ez81 before you put it in? NOS can also mean old dead tube stuck in a nice box. Things like rectifier tubes I like to always test for shorts before using them just because it can be so catasstropic.....
nosaj

[Cooper1962]

I sure did. I checked it on a Hickok 6000A tester. Tested good.

[nosaj]

I sure did. I checked it on a Hickok 6000A tester. Tested good.

And it still tests good for no shorts?
Put all stock tubes back in please and start at the beginning. How does amp sound with all stock tubes?


nosaj

[Cooper1962]

I just re-checked the 6CA4 (EZ81). Tests good. I've tried several sets of preamp tubes, including the originals... Issue remains. I'm tempted to redress every solder point on the tube sockets.

[nosaj]

I just re-checked the 6CA4 (EZ81). Tests good. I've tried several sets of preamp tubes, including the originals... Issue remains. I'm tempted to redress every solder point on the tube sockets.

please take some pictures of the work you've done.

thanks,
nosaj

[Leo_Gnardo]

Could the TB channel be "polluting" the EF86 through the B+ somewhere? If yes, would the EF86 channel not also bleed into the TB channel? I have a full set of electrolytic capacitors, if needed.

I'm thinking along those lines. A hi voltage cap could have been damaged by a dodgy rectifier. Or just went bad anyway. Given the "quality" of components put in MI amps these days, we sometimes see electrolytic caps going bad in as little as 7 years. Try clipping-in a filter cap parallel to the others, one at a time, see if the problem doesn't resolve. If the crosstalk does diminish when you apply this trial to any of the filter caps, then that's your bad cap. Prime candidates are C1, C2, C3, C17, C18. Should take just a couple minutes. Good luck!

[Cooper1962]

I haven't done any work, yet.

[nosaj]

I haven't done any work, yet.

clipped the bright cap, retouched a few iffy solder points... so far, nothing.

That counts as work to me.
nosaj

[Cooper1962]

I have a full set of Nichicon VX series and Mallory 150 caps for the amp. Should I just replace the caps?

The wires going from the pots to the board are 3 into 1 bundled type. I've ever seen this before in a tube amplifier. Should I rewire the pots with individual wires to each connection point?

Pics of the 3 into 1 leads from the pots...

[nosaj]

I have a full set of Nichicon VX series and Mallory 150 caps for the amp. Should I just replace the caps?

The wires going from the pots to the board are 3 into 1 bundled type. I've ever seen this before in a tube amplifier. Should I rewire the pots with individual wires to each connection point?

Pics of the 3 into 1 leads from the pots...

i wouldn't rewire the pots, but how about this recreate your problem then use a chopstick and move the wires from the volume pots and see if there's any change.
nosaj

[Cooper1962]

Double post.

[Cooper1962]

clipped the bright cap, retouched a few iffy solder points... so far, nothing.

That counts as work to me.
nosaj

The bright cap leg was soldered back to its original position.

[Cooper1962]

I'm thinking along those lines. A hi voltage cap could have been damaged by a dodgy rectifier. Or just went bad anyway. Given the "quality" of components put in MI amps these days, we sometimes see electrolytic caps going bad in as little as 7 years. Try clipping-in a filter cap parallel to the others, one at a time, see if the problem doesn't resolve. If the crosstalk does diminish when you apply this trial to any of the filter caps, then that's your bad cap. Prime candidates are C1, C2, C3, C17, C18. Should take just a couple minutes. Good luck!

Should I use an equal value or higher cap for the testing?

[nevetslab]

Should I use an equal value or higher cap for the testing?

YES, Equal or higher voltage.