2235B Pop on switch off

VintageMad89

Active Member
Hi folks, I have a small problem when powering off my receiver. Basically when I release the power switch, I get an audible 'pop' through both speakers (on any input or volume and speaker A or B)

It has always done this in the 2 years I've owned it but the problem persists even after a full recap and relay burnish...

I'm thinking perhaps a relay driver transistor may be going bad?

Any suggestions would be great!

Scott
 
If the speakers don't "pop" on turn on, but only on turn off, it may be that the DC offset drifts as the unit warms up.
 
Relay drive transistor merely energizes the coil to close the relay, nothing to do with audio. Sounds like dc offset which can drift if the differential pair gain is out of whack. Have you replaced the differential pairs?
 
If the speakers don't "pop" on turn on, but only on turn off, it may be that the DC offset drifts as the unit warms up.
Thanks for the suggestion, I have measured offset after 30 mins and both channels stay around +-1mV between switch on and warm up so don't appear to be any problems there.
I have also tried some deoxit on the power switch (just in case) with no improvement.
 
Relay drive transistor merely energizes the coil to close the relay, nothing to do with audio. Sounds like dc offset which can drift if the differential pair gain is out of whack. Have you replaced the differential pairs?
That, I have not done. Are they on the power amp board?
 
Yes. As well, try shorting the main in jumpers and see if it does it. Transistors in the pre can get noisy causing issues in the main amp(s).
 
Yes. As well, try shorting the main in jumpers and see if it does it. Transistors in the pre can get noisy causing issues in the main amp(s).
Thanks, I'll try that also!

Pardon my ignorance, but do you happen to know where the pairs are on the board? I have the schematic in front of me...
 
A quick update-- I haven't got round to removing the differential pairs (that's tomorrow's job!) but looking at the power switch again, I've noticed there isn't a suppressor cap across the pins. Should there be?

Regards
 
Some units have it at the back where the power cord enters - usually around the unswitched outlet or wherever the ends of the power cord connect.
 
UPDATE

So, here goes. I have replaced both diff pairs on the amp board and still, the POP remains.

I can't seem to find the suppressor cap Steven was talking about- the only thing I can see that resembles this is in the attached picture

I'm not sure what it is the orange disc cap is connected to, maybe someone can shed some light on that, thanks.
 

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You are looking for something similar to this blue cap+resistor. It's either on the power switch or in the back near where the power cord comes in.

IMG_2074_zpsxorujqeg.jpg
 
Thanks Steven, unfortunately there is nothing that resembles the picture. Similar or otherwise.
It's probably a long shot that this is the problem anyway. But it is shown on the schematic as G001 across the power switch, S001 (although it could still be mounted in the back.). I wonder if it is missing? :dunno:
 
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