Ampeg SVT-II (non-pro) craps out after a few minutes

[vercingetorix]

I love this amp and it sounds fantastic, for about 3 minutes and then it dies (sound gets squashed to nothing, amp keeps running). If I flip the Standby on and off it will keep working for a couple of minutes and then crap out again. I can plug into the Power Amp In input and it runs fine, so the problem must be in the preamp somewhere.

As a side note that probably has nothing to do with the main problem, the best I can do with the bias controls for voltage between TP1 and TP2 is -0.035V, when the instructions say to get +/- 0.01V.

Any ideas?

edit: schematics found here: https://www.dropbox.com/sh/wdcfkjkly...RxNlrUfRa?dl=0

[Jazz P Bass]

Post a schematic?

[vercingetorix]

Post a schematic?

https://www.dropbox.com/sh/wdcfkjkly...RxNlrUfRa?dl=0

[nevetslab]

I'd take a look at C16...1uF electrolytic cap, from the cathode follower of the last preamp tube stage that goes out the ribbon cable header, passed thru the preamp output/power amp input jack. All the other coupling caps are film or maybe paper dielectric. If you have a scope, you could follow the signal thru the preamp and see where you begin to lose it. Could also be at the Graphic EQ, which follows this cap...but you do have an EQ in/out switch.


C16.....10uF/16V. My SVT II schematic was a bit blurred, Jazz P Bass' s schematics cleared that up.

[g1]

That link is for the owners manual, not a schematic.

[Jazz P Bass]

Attached SVT II schematics.

combined ac term schematic.pdf
combined early power amp.pdf
combined eq schematic.pdf
combined later power amp.pdf
combined preamp schematic.pdf