Sony receiver STR-DB925 "Protector" on display -- please help

[max.wwwang]

Powers up but turns to "protector" on display after a few seconds ...

Help appreciated.

[Full service manual can be found here: https://servicemanuals.us/sony/audio/str-db925.html ]

[max.wwwang]

The DC rails voltages after the rectifier bridge are 42.7/-42.6V, significantly higher than those marked on the schematics 38.4/-38.2V.

[max.wwwang]

Voltages of the "protector transistors" are all going wild.

[tautech]

This occasionally happens with my STR DE485 shed stereo particularly over the winters damp cold weather.
Never opened it up and I suspect it's years of dust accumulation that becomes partially conductive in damp weather.

If the day comes it never clears with a reboot it will be time to open it up and clean the PCB's and check for tracks that may have corroded.....that's what I suggest you do rather than look for faulty components other than in the PSU which if tired could quite possibly trip the protection circuits.

Some say put the PCB's in the dishwasher then dry them fo a couple of days however I've never been that brave. 

[max.wwwang]

Power transistors have also gone wild. Maybe C521 near Q506 is the culprit (shorting B&C and ruining the all the neighbours)? And seems Q506 would definitely be dead with >80V drop over its B/E junction?

Interestingly, having turned the power off, I'm unable to get a connection reading across B/C of Q506 (and across C521). The readings, both polarities, are 5.85kOhms and 2.06kOhms. But these are not the right reading, I think, with both Q506 and C521 intact.

[max.wwwang]

... other than in the PSU which if tired could quite possibly trip the protection circuits.

Thanks. What do you mean here? The only possible anomaly here is the higher than specified rectified voltages. Would this possibly be caused by the PSU being 'tired'? 

[tautech]

... other than in the PSU which if tired could quite possibly trip the protection circuits.

Thanks. What do you mean here? The only possible anomaly here is the higher than specified rectified voltages. Would this possibly be caused by the PSU being 'tired'? 

Too much ripple....tired caps. 
I'd first be looking the control circuitry rails to ensure ripple was low and not likely to falsely trigger any logic thresholds.

[macboy]

You have DC on the output of that amp, that is obvious. The output is the center node of R527, the line with the arrow drawn on it.
Suspect that Q507 is shorted C to E and/or Q506 is open C to E, and/or possibly that one or more components in its bias circuit has failed (especially Q504/505 and R524/525), or the driver IC is failed. Your measurements indicate nearly 80 V across R524, so that resistor is definitely gone. It is a fusible/non-flammable resistor, and that is critical for fire safety, don't use an ordinary resistor there. Imagine what would happen to a 180 ohm, 1/4 W metal film resistor with 80 V across it.

Remove Q507 and Q506 and test them out of circuit. Note that they are high-gain darlingtons, not simple BJTs. You should be able to power up the receiver with those two transistors removed, and the protector error should be gone. Replacements for these are not critical, any comparable sized darlington NPN/PNP pair can be used (gain >=5000 or so).

[macboy]

Don't worry about the amp power rail voltages appearing to be a little high. In fact, if you switch the 4/8 ohm switch to 8 ohms, you will find they are substantially higher still.  The difference between the nominal voltage marked on the schematic and your measurements can easily be explained away by AC line voltage variations.

[bingo600]

I have a STR-DB930 that occationally (often) went into protection.
I opened it up , and resoldered all the relay pins on all the relays , that cured it.

/Bingo

[max.wwwang]

You have DC on the output of that amp, that is obvious. The output is the center node of R527, the line with the arrow drawn on it.
Suspect that Q507 is shorted C to E and/or Q506 is open C to E, and/or possibly that one or more components in its bias circuit has failed (especially Q504/505 and R524/525), or the driver IC is failed. Your measurements indicate nearly 80 V across R524, so that resistor is definitely gone. It is a fusible/non-flammable resistor, and that is critical for fire safety, don't use an ordinary resistor there. Imagine what would happen to a 180 ohm, 1/4 W metal film resistor with 80 V across it.

Remove Q507 and Q506 and test them out of circuit. Note that they are high-gain darlingtons, not simple BJTs. You should be able to power up the receiver with those two transistors removed, and the protector error should be gone. Replacements for these are not critical, any comparable sized darlington NPN/PNP pair can be used (gain >=5000 or so).

Thanks.
Q506 is gone. R524 (180 Ohms) has also succumbed to the high voltage drop over it as you diagnosed. Q507 appears ok, as well as R525.
There must other components dead.

Transplanted the same transistor and resistor from the rear amp board (thought not too bad without the rear amp and speakers). Still not working. Also two fuses F801/F802 blown. Likely to go beyond repair but will let it sit there for another while.

And yes, both Q506/Q507 are darlingtons.

[max.wwwang]

Having removed Q506 (dead) and Q507 (good), and a little more fiddling around, I realised the +-42V DC rails are necessary (at least the +42V rail) to get the Fluorescent display going (+32V supply through Q801). Replaced fuses F801 and F802. The display goes on and no protector errors any more. Will have bit more checking, particularly around the interface of the AM/FM tuner module.

[max.wwwang]

Thought I can leave the centre channel out using the unit as a stereo receiver. So I moved the same Darlingtons Q606/607 from there to replace Q506/507. Also replaced the diodes D504 and Zenner D503 the same way (they were also gone).

Turning on the unit is fine. What's not is that, when connecting the speaker to the Left channel (where the repairs are), the relay begins to rattle. It continuously turns on and off making noise. This does not happen to the Right channel which is however not completely ok---the same happens when the master volume knob is turned to approximately 85% of the full scale.

What's worse, after more fiddling around, the rainbow cable connecting the digital board and the display board failed due to fatigue. At one point the display lighted up when I applied some force at one bent end of it. 

I though at least I could use it, with a working display, to play around the tuner module which comes with the RDS (radio data system) PTY/EON feature which is new to me. 

[max.wwwang]

So I'm considering taking the tuner module out and making a standalone radio tuner with a microcontroller or an Arduino. Any one has any idea of how the pins of A2 should be connected and fed or interpreted? Connector L is straightforward, they must be 12V/5V power input and L/R for two channels audio output.

The tuner module is TM301 as in the service manual. It has a label on it with text "409-12" and "9319F7C". I googled but found nothing useful.

Schematics of the tuner board and pin designations attached.

[Edit] I found a useful article here -- https://www.onetransistor.eu/2015/01/how-to-identify-unknown-radio-tuner.html
Obviously, the "RDS DATA" and "RDS CLK" are associated with the RDS features. "GND" is, of course, ground. "MUTING" should be an input signal for muting (or not). "STEREO" is probably also an input signal for stereo (FM) or mono (or an output of the format of the signal). "TUNED" should be an output pin indicating the status of tuned to a station.

Also, as shown in the schematics, the RDS decoder chip is BU1922. Inside the tuner, LA1837 (Single-Chip Home Stereo IC with Electronic Tuning Support) and LC72131 (AM/FM PLL Frequency Synthesizer) are used.