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I've had it sitting around the house for a while on my list of projects to get to at some point, and the time had finally come. I already had found the user manual and "service" manual only - this has the schematics, board layouts and parts lists...but no real troubleshooting or service advice in there. After looking everything over the last couple of days and taking some measurements, I found that one of the ICs on the amplifier board was not getting supply voltage. The fusible resistors R4397 and R4398 were open (10 Ohm, 5%, 1/4W fusible resistors), and that led me to believe that IC4301 (Sony PN 1-212-857-00, UPC2581V, for the surround channels) had failed. No other components seem to be an issue (no bulging capacitors, no visually overheated components), and the B+ and B- supply voltages were making it to the other similar ICs (4201 and 4101 - their fusible resistors were also OK).

As all 3 of these ICs feed in to whether to activate the PROTECTOR! mode, I figured that this is a likely culprit and a good place to start to try and make a repair.

While I was waiting for the UPC2581V for IC4301 to come in from China, I decided to double-check my findings by comparing measurements for shorts with IC4101. Doing so, I did confirm that I have a short between Pins 15 (VEE) and Pins 14 (VCC), and also Pin 2 (+VOUT1) on my IC4301 - my IC4101 did not measure shorts.

On the fusible resistors, I tried to order these from Sony's support site, but they ordered was canceled a day or so after I entered it, as the parts are no longer available. I also really didn't find a good source online, either. This is for Sony PN 1-212-857-00 . So, I'm going to use 10 Ohm 1/4W 5% resistors I have on hand in my shop, and I ordered some 200mA fuses to connect in-line with them as a combination will sufficiently mimic the original parts.

The UPC2581V ICs and 200mA fuses arrived, so I replaced IC4301, and R4397 and 4398. No dice...the unit still goes into Protector! mode.

I've cheked the voltages at IC4101, 4201 and 4301, and they are all at the values outlined in the service manual.

So now I'm measuring around the detection circuitry for the Protector! mode to try and locate what is tripping the signal back to IC357.

I will post my findings - good, bad, or frustrating - as I try and fix this receiver.
 

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To close this thread out, after putting about 10hrs. into investigation and troubleshooting on this one, I decided to junk it and move on. I have too many other projects around the house.

I tried disconnecting the different boards attached to the control board with IC357, and PROTECTOR! was always still triggered. This leads me to think that there was an issue somewhere on the control board, too, but I didn't want to spend more time on it.

I wish good luck to whoever next tries to tackle one of these.
 
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