sta 960 "loud pop"

[loose ends]

I have a Realistic sta 960 with a loud pop or thump when you turn it off. Even with the volume turned down. Any ideas anyone?

Thanks,
Roy

 

[ecluser]

Can you take a measure of the DC voltage at the speaker output, for each channel?

I worked on a STA-850 recently and I draw a partial schematic. If you could take a picture of the inside it would help.

 

[loose ends]

I have it apart but I think I might be able to turn it on. I do that and take photos tomorrow am. I also have to figure out how to post a photo on here.

Thanks

 

[ecluser]

Click on "Go Advanced", a new window will open. You will see an icon (paper clip) to join files.

 

[loose ends]

Let's see if I can do the photo thing.

 

[loose ends]

Hope this worked.

 

[ecluser]

The second picture shows one power amplifier channel. The trim pot visible on that picture is for the bias current adjustment. There is no offset adjustment. The two transistors TR601 and TR602 are most probably the ones for the input differential stage. It is something that you can easily check. If you follow the circuit from the base of one of the transistors (probably TR601), you should reach the terminal 1 or 2 visible near the edge of the board. These terminals seem to me to be the input terminals for that channel, one for the signal and the other for the ground. The emitters of TR601 and TR602 are probably connected together under the board.

The important point is that TR601 and TR602 (if what precede is exact) must be matched (have the same hFE) in order to have the least amount of DC voltage at the output. This is this DC voltage which is responsible of the thumb noise at turn-off. But this condition is not sufficient. Sometimes we need to change some components on the board. We will see later what you need to do exactly, but your first action at this point should be to take a measure of the hFE for the differential transistors. TR601, TR602 on one board, and the corresponding transistors on the other channel's board partly visible on the fourth picture.

Do you have a meter with a hFE scale?

The STA-960 and STA-850 are very different from what I see on your pictures. Unfortunately, the schematic I did for the 850 is useless for your receiver. We will need to work from the pictures you will provide.

 

[loose ends]

I just have a regular dmm. It is a good one. I forgot to give you my voltage readings for bias.
Speaker a:
R 0.0mv L -17.8mv

Speaker b:
R -6.0mv L -17.7mv

I have to go food shopping with the wife (or I will suffer a fate worse than death) and will check as soon as I get back.

Thanks,
Roy

 

[HomeBody]

...or I will suffer a fate worse than death...

Ah, the life of a divorcee-

 

[AnalogDigit]

It might be normal for that piece of equipment. I used to own one. Turn off the speakers by switching the selector switch to out before turning off the receiver.

 

[ecluser]

This offset is too low to cause a loud thumb noise. There is another cause, but I have no cue.

 

[HomeBody]

It might be normal for that piece of equipment. I used to own one. Turn off the speakers by switching the selector switch to out before turning off the receiver.

That's a practice I still adhear to...Before I power down a unit, I'll take the volume to zero, then disengage the speakers, followed by a power down...and I'll unplug the unit when I leave the house.
No more voltage spikes screwin' up my stuff.