Yamaha RX-A3030 DC PRT - need advice

[Max60Watt]

Dear all,

I have a Yamaha RX-A3030 reciever that I have been working on for a while - it has an issue I can't figure out, so I hope that one of you can help me further!
The SR (Surround Right) channel is outputting 0.85 VDC, while all the others read -1.0 VDC. The amp shuts off after a few seconds due to protection mode with the error DC PRT. I can however start it up in protection mode to do measurements.

I have basic electronics understanding, I have tools, and I have repaired things in the past - but I can't figure out which part is the culprit in my receiver! I have narrowed it down to one channel and the part of the circuit is misbehaving. All resistors read specified values, all capacitors test ok (ESR meter) and I don't find any shorts in the transistors (also checked all the SMP). Diodes are testing ok, the zener I have not tested. I have measured voltage on differtent parts of the channel and compared with the other channels. One part of the circuit is reading positive voltage (see orange lines in the schematic below) where the others are reading negative. Would you know why? I have tried swapping the (medium sized) transistors around, resolder - no sign of cracking or broken connections.

Matching values are in green, deviating in orange (disregard the two yellow rings in the bottom circuit). Channel on top is the faulty one:

I will appreciate any advice, thank you.



Best regards,
Max

 

[mbz]

What were the nnn digits ie DC PRT nnn, vaguely recall they should be present, could be wrong.

The SR (Surround Right) channel is outputting 0.85 VDC, while all the others read -1.0 VDC.

I would have thought either would be sufficient to trip protection

Will look at your measurements

 

[sae-7125]

photos of the cadaver in question will be helpful.

like most multi-channel eventual doorstops (pardon the natural snark from a two channel addict) there is an age/hours-of-use threshold before self destruction becomes highly probable. i have had an unusual amount of success with the handful of multi-channel systems that have gone across the bench. some of the fixes were rather weird/odd/unthinkable but...

thermal camera helps big time but learning how to use the camera effectively takes a while.

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looking at the manufacturing age there will be more than a few cooked caps/electrolytics. using a mains cord power meter might help evaluate system inrush and power up idle pre/post protection mode although its a trivial measurement at this point.

looks like there is a 47k resistor on each amp that feeds into the protection sense circuit line. upstream is a transistor which is fed by a darlington package through an 18k. service port cb10# measures across emitter resistors for bias adjust. maybe turning down the dc bias for the oddball channel might help, unknown at this point.

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*for those who want to see a 225 page big picture, page 159 has amp channels

 

[Max60Watt]

Thank you for your input! I know someone who has a thermal camera, the type you connect to your phone. I will check if something heats up. Also I will try adjusting bias on the faulty channel. I have checked all capacitors with ESR meter (in circuit), can they measure good (correct uf, low ESR) but still be "bad"?

PS: sorry - I did not realize that the resolution on the picture I uploaded was lowered. On my pc, the image is detailed. I have posted them again below

 

[Max60Watt]

Here are some pictures "of the cadaver"
Nothing burnt, no cracked solder, nothing that smells.

It did also have a fault on the mainboard, I have replaced a 7812 voltage regulator (ICI180) in the power section.

 

[sae-7125]

thank you for posting the revised measurement schematics and the machine itself.

photos 164656 and 144652 show an interesting design flaw, predictable hot spot located below electrolytic caps. last time i checked heat flows upward, from a convective cooling point of view. recapping the amp board would be a good idea even though there are other boards/cards/modules that will start causing issues at any time.

the thermal camera will help see from a different perspective or technological vantage point. which helps since humans have difficulty looking at the flow of power/energy in a circuit hence the need for diagnostic equipment.

identifying hot spots and then counting how many electrolytics are near/inside the thermionic blast radius (parts cooker ground zero) is helpful, especially with tube gear even though your current project is solid state.