Sony GX59ES... Keeps going into protect?

[mattstech]

Got this one for free, from neighbors cleaning out the basement. It was apparently part of a "stereo system" but... this is all that's left! Wasn't powered on for almost 15 years according to them
From the early 90s, 80wpc I believe. Thick metal face, metal knobs. Part of Sony's ES line, which leads me to believe that this is most likely a decent piece of "Vintage" (Okay not really) Hifi equipment
Has some crazy Dolby simulated surround effects, but it's clearly just a two channel amplifier with stereo inputs.

After it's been on for about 5 minutes, it switches into protection and locks up the receiver. I've double checked that nothing external is causing this, including using different speaker wire and speakers, selecting the right impedance, But... still goes into protect.

I've disconnected everything from it except for power, and even switched off the speakers
Low and behold, something still triggered the protection circuit. Even with the speaker selector switch on the "off" position!

I'm yet to dust it out, but so far I have no idea how to solve this.
Is this receiver even worth it to repair?

 

[slimecity]

Im sure its worth repairing, it just depends on the complexity of the unit and therefore the job required to fix it.

The service manual appears to be downloadable on the net for this model - would suggest you get it.

Second thing - while the unit is working - and the speakers are working - connect a meter to the speaker terminals and in mV measure the DC offset and let us know

 

[mattstech]

Im sure its worth repairing, it just depends on the complexity of the unit and therefore the job required to fix it.

The service manual appears to be downloadable on the net for this model - would suggest you get it.

Second thing - while the unit is working - and the speakers are working - connect a meter to the speaker terminals and in mV measure the DC offset and let us know

Thanks! Will get the service manual.
This was probably really dumb, but i was confused about doing it with speakers plugged in or not, I hooked up a multimeter (No speakers were plugged in, just multimeter) and I got 570mV and 572mV on the left and right channels respectively
I'm really not experienced, will do it with speakers plugged in if you like

 

[audiotemp]

The protection threshold for DC offset voltage protection is a diode drop, which is more or less what you have reported.

I'm not the one to walk you through sorting out the issues with this, but you can start by checking to see if the supply voltages are OK. They should be +/- 59V, I think. Note that that is enough to be dangerous. (I don't know your experience level, apologies if that is obvious.)

This is an early surround receiver and does in fact include a rear channel amp but it is small (a chipamp or two) and not particularly useful now. It should make a good quality stereo receiver though if you can repair it.

 

[Binkman]

Sony note says to use the https://www.hifiengine.com/manual_library/sony/str-gx57es.shtml
includes the 67es, too.

http://sportsbil.com/sony/STR/STR-G/STR-GX59ES_GX69ES.pdf
Denotes a few differences.

The ES series are actually very nice sounding amps. ( I have the 67es to repair but in no hurry ) 90% is the big transformers and IMO poorer small power supply circuits, but effective and just very fragile.

Your noted cut off into protect is after warm up.. good. Not blowing fuses.
Few observations:
-This is not an amp for a beginner! You can get by with a good dmm but a scope would help a lot.
-No diagnostics with anything hooked up select Aux. Not too concerned about offset but over 100mv is questionable.
-These es amps have 'fusable' resistors that can go 'open'.
-The protect circuit(s) cap(s) might in itself be faulty, usually low v cap or transistor.
- Generally speaking lot of testing yet to be done and most at component level. Just note those fuseable resistors. usually 10ohm.

 

[audiotemp]

http://sportsbil.com/sony/STR/STR-G/STR-GX59ES_GX69ES.pdf
Denotes a few differences.

I had not noticed the 57 vs. 59 difference. Binkman's linked dicument seems to indicate that your receiver has both rear and center channels, like the 67.

 

[SVI2004A]

Quite an amp... this was one of their better units.... it could well be a dry joint or a dodgy transistor (most likely in differential/cascade)

When protect triggers check voltages at Emitter resistors - DC will be appearing on the channel affected, check for dry joints.... freeze spray will be useful to find a twitchy transistor if that is the cause

 

[Binkman]

I have the 67ex in off storage but I recall the p.s. is on the main board. then another p.s. branch off to the digital side. All the circuits for the digital chip side checked out okay. I unsoldered the outputs to test all, but other stuff came up so shelved. Got it for free but guy I got it from kinda messed stuff up.

SV12004A above etal as noted it does work and protect works. Also to note these traces are very thin but soldering on the 67 was pretty good, do recommend a temp controlled iron. Again those fusable resistors can open and have been reported troublesome.

 

[SVI2004A]

Transistors in differential and cascade sections of amps I have seen cause the symptoms displayed.... works for a bit, then trips out... as transistors warm up glitchy ones cause the fault... freeze spray is useful for tracking them quickly.... dry joints can surface in the same way... as set / components warm up, expansion/contraction can make issues appear after some time on.

This is based on experiences of this symptom on various sets over the years

 

[mattstech]

Thanks for all of the recommendations! I've been working all day on this thing and sadly havent found the culprit.

Now, these "fusable resistors" in the unit I suspect could be part of the problem. The emitter resistors are measuring a good 10Ohms, reflowed the solder on most of the PCBs, corrected the BIAS adjustment (it was just a tad off on the left channel), and I checked the power amp ICs for the main speakers A+B (not surround) and I can't detect any shorts

What I find interesting is when I unplug the surround amp board entirely. It stops clicking into protection, but also stops the main speaker relays from activating.

Either it's the transistors on the surround amp, or there's a fault in the protection circuit

 

[audiotemp]

What I find interesting is when I unplug the surround amp board entirely. It stops clicking into protection, but also stops the main speaker relays from activating.

It's a bit late here for schematic reading but it looks to me like the voltage supply for driving the speaker relays comes from the B+ supply on the surround amp board. It's routed through a 6-pin cable that goes from there to the surround speaker board. From the surround speaker board it goes through a 4-pin connector to the main speaker terminal board. So, no surround amp board, then no relay clicks.

You may well have a bad surround amp chip. Have you measured the DC levels at the surround amp board outputs?

 

[mattstech]

It's a bit late here for schematic reading but it looks to me like the voltage supply for driving the speaker relays comes from the B+ supply on the surround amp board. It's routed through a 6-pin cable that goes from there to the surround speaker board. From the surround speaker board it goes through a 4-pin connector to the main speaker terminal board. So, no surround amp board, then no relay clicks.

You may well have a bad surround amp chip. Have you measured the DC levels at the surround amp board outputs?

I've put the project aside for a day, but I suspect this is the case. I don't know how to read the voltage on the surround ICs as they have 5 or 6 pins instead of the "standard" 3 pins configuration.
Will post some pictures I took yesterday of the inside of the unit

 

[SVI2004A]

Surround ICs pin 4 is output.

Check for dc offset - probably easer take from R608 (rear) and R658 (centre)

They also have overcurrent detect circuit based on R607/Q601 and R657/Q651

Check voltages there and the pcb for dry joints....