Sony STR-DE345 receiver repair, "PROTECT" (fixed)

[Chungalin]

This 5.1 Sony receiver / amplifier exhibits a protection trigger some seconds after powering on. Service manual kindly available at ElektroTanya.

First check on power transistors (5 pairs of SanKen MN2488 / MP1620) and nothing is shorted there, luckily. Then check for DC on outputs (before relays) and flat 0V on all channels.

Then I recall my previous case STR-K740P. Both protection circuits are almost identical. PROTECT line is 0.3V, while it should be about 5V in normal condition. PROTECT line can be checked at cathode of D731, D732, D734, D735 or D736 (all Rohm 1SS133). Again, like in STR-K740P case, it smells like a failure in protection circuit, instead of a real protection trigger. I check aforementioned diodes and then I detect that D732 (the one between Protect and GND) is conducing in both ways!! Not that is shorted, no: it shows apparently "normal" diode read (about "500") in both ways.

Replace it with a scrapped 1SS133 and no more wrong protection triggering.

This is the second case in 6 months of false protection switch due to bad diode in the same circuit section on similarly designed Sony amplifiers. One case is just a dot, but two cases draw a line.

[Guram]

Re: Sony STR-DE345 receiver repair, "PROTECT" (fixed)

Hello Chungalin,

I guess I have the same problem with SONY HT-DDW840
I have already checked output MN2488 / MP1620 transistors. They look OK.

Sorry, I am new in electronics. Could you tell me where to find these diodes on the board?

Regards,
Guram

[Chungalin]

Re: Sony STR-DE345 receiver repair, "PROTECT" (fixed)

Your real model is Sony STR-K840P, which is almost equal to STR-K740P (see another thread). You should check D731, D732, D733 and D734. 1SS133T are small glass diodes with a single yellow band. Download service manual for STR-K740P / STR-K840P and perhaps will be easier to look there before. I’ve found that D731, D734 and D733 are grouped near the rightmost coil. I cannot help you more.

[Guram]

Re: Sony STR-DE345 receiver repair, "PROTECT" (fixed)

Thank you very much for your quick response!

I have checked them with diode range of the multimeter. They are conducing only one way, which I suppose is normal. But when I checked the same type of diodes next to their coils(one per each) D701, D710, D610 and two more(I don't see their numbers well) they are conducing both ways with 187v and 422v(only one).
Based on this - can I conclude that they are damaged?

Kind regards,
Guram

[Chungalin]

Re: Sony STR-DE345 receiver repair, "PROTECT" (fixed)

You'll have to check schematic and see what is connected in parallel with these diodes. Maybe is normal, or maybe not. Also, sometimes these diodes show normal readings under diode test, but when checked "live" in DC voltage range they show its malfunction. I discovered the problem in STR-K740P through voltage test, not in static diode test. Have you read my other thread about STR-K740P? You should have posted there instead of here, because STR-K740P is almost the same as STR-K840P.

I attach SM for STR-K740P / STR-K840P, so you can check schematics.

[Guram]

Re: Sony STR-DE345 receiver repair, "PROTECT" (fixed)

I've unsoldered these diodes from the PCB and tested them. They are OK.
After that I've thought that may be I've missed something during noble resistors test. So I tested the middle lid of each noble resistor with DC voltage 'live' and I found one irregularity.
One resistor shows -052.6v DC while others show -3.383v DC. Does it mean that this one is bad?

Thank you,
Guram