Realistic STA-7 receiver

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rpapps

Member
Joined 2004
Hi all
This receiver developed a bad crackle in one channel which I fixed by replacing the output chip. Now it works OK until it is switched off and then a low frequency oscillation pumps the speaker really hard until the power supply caps discharge after about two seconds.
Any suggestions?
Thanks
Rob
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi rpapps,
Interesting problem. Was the chip an exact replacement? I mean the exact same part number - no subs or replacement brand. What was the chip number?

It sounds like the on board muting is not happening if this chip has it. A leaky cap used as a DC block may cause this also.

-Chris
 
R

rpapps

Member
Joined 2004
STA-7

Hi Chris & Hugo
The replacement chip is a genuine Hitachi HA-1370.
There is a swith-on muting circuit but it is common to both channels.

I have replaced the power supply diodes and checked all capacitors with an ESR meter. Even after >20 years, they still read good as new. (I hate to throw anything away)

I've gone over the board with a magnifing glass looking for dry joints or cracked tracks.
I guess the next step is to order another replacement chip.
Thanks for all your input.
Cheers
Rob
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Rob,
Just for chuckes and grins, pull the 1uF input coupling caps out and try it. They may still be leaky. I use a $6500.00 HP LCR meter to check caps, I don't trust anything less anymore. Your ESR meter may not catch leakage in high impedance circuits.

Besides, caps are cheaper than chips.

-Chris
 
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