Pop, then fading buzz in left speaker when turning off power

[Stink Bear]

I've searched the site and found similar problems but not quite like mine.
When I turn the receiver off I get a pop from the left speaker and then a slowly fading buzz from both speakers.
I also get a pop at power on, but much weaker than power off.
I just started using this receiver. After I got it 6 months ago I cleaned it and tested it, if it was popping and buzzing then I didn't notice it.
Once today I heard a couple of pops shortly after I turned it on but other than that it sounds pretty clean.
If I turn the speakers off before I kill power there is no pop or buzz.
The pop stays in the left channel when I put it in reverse mode.
Volume down, same symptoms.
Pull the amp board and start checking transistors?

 

[pete_mac]

The million dollar question here is... which model receiver?

What you describe is symptomatic of an early receiver which doesn’t have an soft-start and active protection circuit.

 

[Stink Bear]

Wow! I'm almost astonished that I didn't think to give any info about what I'm working on. I really screwed up this initial post. Thanks for not being sarcastic and making me feel even dumber.
The receiver is a 2000A.
I've read about what you describe and I think some pop may be normal, but I'm getting a fairly significant pop, and the fact that I'm only hearing it in the left channel makes me think there is a problem.

 

[tnsilver]

The 2000A service manual is here. A loud pop on turn off suggests a failure of a snubber / anti-arc capacitor. I'm not familiar with this unit but I've gone over the schematics and parts list looking
for such capacitors and there are only 4 documented with sufficient working voltage. Those are C008 (0.033uF/600V) behind the transformer, C009 (0.0047uF/600V) probably near by, and
C013/C014 (0.01uF/400V) behind the F-1195 ripple filter PCB. There may be a another (undocumented) anti arc capacitor on the on/off switch too. I'd suspect the last two 400V caps first,
especially the left one.

As for the residual buzz, I would probably check / replace all components on F-1195 too, starting with the electrolytic capacitors, the transistors and diodes (including Zeners), as well as verify all
resistors are still up to snuff.

Merry Christmas and a Happy New Year

Tom

 

[Stink Bear]

Thanks for the reply Tom! And thank you for the advice. As someone who has a limited understanding of electronics but likes to tinker and see vintage equipment work I really appreciate you sharing your knowledge and expertise.
I'm getting ready for work now but maybe this evening I can look at the service manual and schematic and get a better understanding of what you're looking at. It will probably be at least a few days before I get a chance to do any troubleshooting. It may have to wait until I've fixed another unit I'm working on.
Merry Christmas & Happy New Year to you as well!

 

[KingBubba]

Just create a startup/shutdown routine and you will be fine. Before turn on, make sure the speaker switch is off, turn on and wait maybe a 5 count, then turn the speakers on. Turn off will be the exact opposite. Turn the speakers off then follow with the power switch. This has worked fine for me for many year with all my receivers.

 

[Stink Bear]

Thanks for the advice KB. That's what I'm doing for now, but when I get my main receiver repaired and back in the stack I think I will look into the 2000A. I think there's something amiss there.

 

[tom3]

Also might 'exercise' all the switches and controls right before shut down. A dirty control will sometimes cause noise as the voltage drops. Quite often a noise cap though.

 

[Stink Bear]

Good idea. I was thinking I might try cleaning pots and switches again. I'm pretty sure I cleaned the pots well, but I don't remember if I cleaned the switches.