Carvin Legacy 100 head arcing issue

[WholeToneMusic]

Swapping componets in V3B didn't help. Since the problem affects both channels I have a feeling it may be closer to V1B since this location is before the switch.

[lowell]

Ok I finally got around to looking at a schematic. Disconnect one end of C66 right after the relay. Lets see if the noise is coming from before or after the transistor cathode follower for the "send." See if the popping is still there with one end of that cap lifted.

[WholeToneMusic]

With one leg of C66 lifted the problem still persists.

[lowell]

Ok great! So the issue is post C66, pre return jack. I'm suspicious of the cathode follower transistor. With power off check resistors R29, R82, R83, R65 to be sure they're correct value. If they are then I replace that transistor and coupling cap C67.

[WholeToneMusic]

That is good news. R29, R82, R83 & R85 measure within spec. I will go get the transistor and cap tomorrow.
Thanks Lowell.

[lowell]

Hopefully that'll do it. Let me know.

[lowell]

another suspect is the filter cap for the D supply. It is also for the return stage so most likely it's ok, but maybe check AC voltage on that cap while you're at it.

[WholeToneMusic]

Will do, Looks like our local supplier is out of that transistor. Going to have to get them mailed in.

[lowell]

MPSW42G ON Semiconductor Bipolar Small Signal

only .73 each

[WholeToneMusic]

thanks for the link. Have a good weekend.

[lowell]

Thanks U2.

[WholeToneMusic]

With new parts in I played the head for a few minutes with no issues and then slowly the dreaded hissing noises started resurfacing. I did not see any blue flickering within the power tubes. So we have cured the loud popping.

[lowell]

That's good that the popping's fixed. What hissing noise? I don't believe you've mentioned that yet. Any more specifics about it?

[WholeToneMusic]

The noise was a sputtering type noise with some popping mingled throughout sprinkled generously with hiss. I was monitoring the output tubes again and did notice the blue flickering again. I did test the D supply for AC and there was not any present. Moving down the row of filter caps all was good until the 3rd from the end. the 1st to 3rd filter caps have 4 volts AC across them. I checked the cathode side of the rectifier diodes and sure enough theres 4 volts AC across that point. Must be leaking through the rectifier diodes.

[WholeToneMusic]

I checked the rectifier diodes and they are good. There's a couple caps and a diode on the bias supply that may be suspect.