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

[skippy124]

With the scope I traced the pop to 2611 at pins 18 and 19. I indicated where I am getting the pop with the red bursts on the image below. The pop does not exist where the blue "X" is shown.
View attachment 1017443

Hi Zeke,

Are the measurements on the base and emitter of TR02 correct?

 

[Zekeman]

Bias was dialed-in correctly a couple of weeks back. I'll recheck it.

I'll post the other findings tonight. Thanks

 

[vigman]

A spark means either Hi current or Hi voltage being removed from a load.. at amp idle you should have neither....

 

[Robisme]

Ok so how does this work?
How can you hear a POP when the protect relay drops out instantly ?
Is it possible that the protect relay has arced? and is not releasing as fast as needed??

I have been watching this thread hoping for a clue for an issue I am going to be chasing. I have a 7900Z receiver doing the exact same thing.

So it can happen. I don't see how though. DC offset is 14mv, but with tone on, a very loud pop on power off. Relay opens instantly.
Both channels. Tone off, no pop.

My first thought is power supply issue because it's in both channels, or a bad ground.

Time will tell, I hope.

Rob

 

[Zekeman]

The power supply is OK it's just a tad high. Not quite sure if that is significant but its not wildly off and it is consistent across all values.

When I get home I'll try the latest suggested ideas toward resolving this.

 

[Zekeman]

Hi Zeke,

Are the measurements on the base and emitter of TR02 correct?

Yes

 

[Zekeman]

There is no DC offset pot in a 7900... tell me the DC voltage @ one side of R-73/74 to gnd with the amp @ idle , power on AND if there is a difference with filters on or off

To answer your other question - the amp is properly biased at 18mV.

Regarding R-73/74....I'm getting some fluctuating values of approximately -21mV, -19mV

 

[vigman]

Sorry 2596 the protect board.. Input to the relay...

 

[vigman]

Again so I'm clear the POP only happens on power off and you are in tone on or tone and filters on .. but is OK with both tone & filters off?

 

[Zekeman]

Exactly yes

 

[vigman]

If the amp is running and you switch in & out of tone and or filters No pop ?

 

[Zekeman]

No pop

 

[skippy124]

Yes

TR02 is NPN, 10V on the base and 11V on the emitter?

 

[Zekeman]

Yes I believe I have a c1845 in there .

 

[skippy124]

Orientation OK?

 

[vigman]

Wow this is strange......Measure the PS for the output section, does it glide down or DROP OUT when you kill power ??

 

[Zekeman]

Will do

 

[Zekeman]

TR02 is NPN, 10V on the base and 11V on the emitter?

Edit: This morning (Sat) I am getting Orientation OK. 11V base and 11V (stable) on emitter and 20V collector

 

[Zekeman]

Wow this is strange......Measure the PS for the output section, does it glide down or DROP OUT when you kill power ??

Glides back to 0

 

[Zekeman]

Here are some additional values taken today and points where the pop occurs.