Fender Pro rvb...wild oscillation with NFB attached. Tips?

[Tone_Terrific]

Old Pro, likely an AA1069, has been out of service for a long time.
I did some physical refurb and have plugged it in.
Voltages are stable but it oscillates as soon as it comes off sby.
If I disconnect the nfb line, it is usable but a bit distorted, no osc.
If I disconnect the blocking cap to the PI, nfb connected, it is also stable, but can't pass signal, of course.
It is acting like the nfb is positive feedback but not if the coupling cap is disconnected. There must be a component failure or feedback path that I am missing. Any suggestions?
Thanks.

[Old Tele man]

...did somebody either (a) replace the OT, or (b) work on the speaker output jack?

...sounds kinda like the FB lines are "crossed", ie: the OT leads got reversed somehow.

[stoo]

...did somebody either (a) replace the OT, or (b) work on the speaker output jack?

...sounds kinda like the FB lines are "crossed", ie: the OT leads got reversed somehow.

:agree

It's like you have +fb instead of -fb.
Stew

[Tone_Terrific]

...did somebody either (a) replace the OT, or (b) work on the speaker output jack?

...sounds kinda like the FB lines are "crossed", ie: the OT leads got reversed somehow.


Yep. But the OT is old and the wiring looks old. It would have never worked that way....although I don't actually know if it ever worked.
There is a green wire as the spkr hot and black as ground.
Does that sound Fenderish?

[WaltC]

That sounds fenderish, but...<G> if the primarys are crossed you get the same result. Which tube does the brown wire go to and which the blue? (and you can just switch them and see what happens just to check it out, provided they reach of course...)

[Tone_Terrific]

if the primarys are crossed you get the same result.

Inspecting the OT-
Looks like the leads HAD been cut, repaired and reconnected, so I tested to see the center tap was right, then swapped them....IT WORKS....sorta...lacks power and distorts too easily.
Will carry on, thanks.

[WaltC]

Hey, cool. One step at a time and sooner or later all will be well!