Vintage Fender AB763 Molded Coupling Cap Bad - Replacement Suggestion?

[Jecarroll7]

Working on a vintage '66 Fender Tremolux and believe one of the original molded caps are bad. its a .1uf 200v in the inverter circuit (AB763).

I was thinking about using an orange drop but thought I would check in to see if someone has a suggestion?

Thx.

[Jazz P Bass]

Orange drops are good.
Go for it.

[Bruce / Mission Amps]

Any plastic polyester cap is fine there. But, why do you think it is bad? I've restored gazillions of vintage Fender amps and have never seen a bad cap there....

[jonamojo]

same here, I've yet to come across a blue molded cap that was bad

[Jecarroll7]

Well, truth be told its a first for me as well. It measures fine out of circuit with a cap tester, but I've had bad caps test fine before. If anyone knows how to test one other than substitution please let me know.

Getting sharp spike static mixed with the audio signal, spikes increase with the volume and bass. I can see it at the coupling cap to the inverter and flows through the power stage as you might expect. Channel independent (normal or tremo doesn't matter, and the trem is off and the trem tube is pulled. Effectivelly I have eliminated the preamp. Have also swapped all tubes and re-flowed the board. Voltages all look clean, zero hum, resistors all check out but I haven't replaced any yet. I've read the posts and hoping its not the board itself conducting.

History is this amp was a barn find, looks to be all original. He bought it in '68 and it was parked in a closet since 1980 after the guy's kids lost the fuse holder. Had to take an air compressor to it to get the spiders out before bringing it into the shop. A bunch of time cleaning everything including jacks, contacts, etc., but the original tubes checked fine on the Hickok and it fired right up. Played okay for a bit, check it a few days later and got the static. Hasn't cleaned up since. I did a complete tube swap just to make sure. Sockets and tube pins cleaned/retensioned, etc.

Ideas welcome, otherwise will let the group know what I've found. Thx.

[guitician]

The coupling cap is blocking 200 VDC and you say it is rated at 200 VDC? They should have gone with a 400V one. Carbon comp resistors can give these symptoms too. Ones on the grids would be my first guess, then cathodes and then plates.

[Gtr_tech]

If the pwr supply caps haven't been replaced, I'd start there. Failing caps can cause strange things to happen....