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Strange SX-1250 problem.

Robisme

Robisme

Sansui Enthusiast
Subscriber
I have an SX-1250 here that I restored 4 years ago. The owner has been using it during the last four years on a regular basis.

The other day he reported that the sound cut off after about an hour of listening. He shut it off and tried it again later and got nothing but the lights.
I asked him to listen for the soft start relay then the protection relay on power up. Neither picked.

Now for the strange part. He dropped it off and I plugged it in, with him watching. I hit the power switch and the lights come on, then quickly we hear the soft start relay, then a few seconds later the protection relay.

I have been playing it for about an hour now while monitoring the bias, which I set to 50mv, and it's rock solid. It was always 50mv since it left here.

So that's where I'm at. I figure its not the soft start relay contacts because he said the relay never picked. The only other thing I could think of is a bad rectifier that supplies the DC to the relay.
But, it's working for me.

He was using JBL L100's at the time of failure.

Any thoughts?

Rob
 
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Gremlins. My friend's Pioneer PL-51A platter wouldn't rotate when powered on. It moved ever so slightly but that was it. Took it home with me to find out what's up and it never failed to work right the entire week I had it.
 
Check your molex connections really good. Make sure their pressed all the way down on the pins. Did you happen to clean both the pins and connectors while restoring?
 
Not knocking your work, it just seems odd that the unit now works with the only thing occuring was being moved. If it's not a connection it's a trace or solder joint somewhere most likely.
 
As mentioned above, could be a bad connection.
Had a receiver that went into protection. Opened it up, everything checked out ok. Put it back together and it worked fine...for a bit.
Turns out it was shorting out when it was reassembled and had time to sit, warm up, then flex enough to short.
 
All connections were cleaned. All connections are on all the way and tight. It worked fine for 4 years.

The soft start relay should not drop out during normal operation. Over current is handled by the protection circuit.

I will check the circuit that feeds the soft start the DC voltage.

Ran it for two hours yesterday with no issues.

I will recheck everything.

Rob
 
Maybe inspect the bottom trace side of AWR-107 board for poor connections?
Resistor R1 could have an issue?
Probably check some other things while you're there.
There could of been a previous repair to the soft-start resistor and fuse? Some thermal fuse butchery causing a power issue?
I usually remove one (Blue?) wire, pin and all, so I can flip it completely over and get good access to the connections. Clean the dust off.
 
I sold a restored receiver some years ago to a fella that lived in the next town. He notified me that it lit up but would not work. I knew there was nothing wrong with it, so I told him to move receiver to another wall outlet to test. He did, and it worked. Then I had him check his wall voltages, and they were all screwy. He finally said, "yah, its a really old house, and could use some repairs". Parents house had a bad breaker box, some of the old breakers were half burnt, like the one tied into the dryer. A few years ago, I had it replaced.
 
Well, here is an update. It finally quit, but, it's only in protection. When cycling power the soft start relay does pick. Guess he didn't notice that because it's pretty simultaneous with the power switch. It does take a few seconds to let go on power down though.

Any how, disconnect left amp and it works fine. With left amp connected voltage on the green wire fluctuates. I will look into that.

So not as strange as I first thought.

Rob
 
I looked back through the messages and see that I just did what I thought should be done to make it reliable. I did not replace every transistor in the receiver.

So now I replaced the 2SC869 on the left driver board and it's working fine. 2 hours in and all is well. Now to change the other channel 2SC869.

For reference, here is a pic of what I originally replaced.

1766339481508.png
 
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