Realistic STA-7 receiver

[rpapps]

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

 

[Netlist]

Replace D501, D502, D503, D504 in power supply, even if they check good.

/Hugo

 

[rpapps]

STA-7

Hi Hugo
Thanks for the reply.
Wouldn't a power supply problem affect both channels?
My fault is in one channel only.
Cheers
Rob

 

[anatech]

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

 

[rpapps]

STA-7

Hi Anatech
I'm not at home right now.
I'll post chip info later.
Thanks for your help.
Rob

 

[Netlist]

This info comes from an paper lying around here, I have no experience with the amplifier. It also says to check for a short or nearly short between pin 1 & 3 of the output IC's. It's not a big deal to change four diodes, one never knows.

/Hugo

 

[anatech]

Hi Rob,
Asked a friend about your amp. The original chip is HA-1370 - NLA. Check the solder connections and traces for one. Check the 1uF coupling caps, actually, remove both to see if the fault remains.

-Chris

 

[rpapps]

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]

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