Sansui AU-7900 - Big POP when Powered off ONLY with Tone Control ON

My power point must be too old- I've never seen those cool little markup speech bubble/balloon things.

edit: I found them! Insert shapes, callouts. So cool. Thanks.
 
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Zeke my only thought is the relay is being held on to long either mechanical or the coil hold voltage is being held high to long.
The other thing that is disturbing in you see a flash when the relay opens...
IS your scope dual trace, might be interesting to note the time between the pop and the coil voltage.

The other thought is is it a teeny pop or are you really pushing the woofers ?
I assume this is just 2 speakers hooked up to a stereo, no external sub stuff or any external processor ?
( yes reaching for straws here )

This is a bit of a stumper....
 
I have a Tek 465B

It's quite a pop and it moves the woofers. I'll take a video and post it to YouTube.
 
Replace the snubber diode across the relay coil? How about the VD1212's?
 
The VD1212's have all been replaced. I'll check the diode I think I may have replaced it...().
 
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All diodes replaced in the protection circuit...still no change.
 
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Tonight I am going to transplant a known-good tone and switch circuit board assembly into this unit to further troubleshoot...

...Result...!!! POP!!! The problem is NOT on the tone / switch board. So to recap...I have swapped out both the main driver motherboard AND the tone / switch board and I still have the problem. So the issue must be on the Secondary power supply/ phono equalizer board F-2599....this board was pretty filthy with much crud. It's been cleaned but could use more work. ...not sure if that may be a contributing factor. I have, by the way, installed new transistors in this secondary power supply already. Resistors have not been verified. Nonetheless...any thoughts on this latest development?
 
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Man I've been poking around the F-2599 board for two days now-probing with my scope for signs of DC pop and NADA! Except for where the signal comes in from the switch board into the connected /separated switch and out to the power amp board. But that was already known. This one is a real challenge!

:dunno:
 
With no other symptom, you bet it is. I would explore the grounding. The input RCA Jacks I know, typically need some fixing anyways.
 
Does it POP with the preamp and power amp separated? I haven't seen that mentioned yet.
 
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