Yamaha TSR-7810/RX-V781 Protection Circuit Problem

[AgentDelta]

Hi All,

I have a Yamaha TSR-7810 (AKA RX-V781, from what I’ve been able to determine) (service manual available here: https://elektrotanya.com/yamaha_rx-v781_tsr-7810_rx-a860_sm.pdf/download.html) that won’t power on because the protection circuit trips every time I try to turn on the unit. It came to me with this issue, so I don't know when the issue started or what might have caused it.

So far I haven’t been able to track down the root cause of the problem (and while I’ve found plenty of threads about similar problems, I’ve been unable to locate one with the same diagnostic values that I’m seeing), so I’m hoping you’ll be able to point me in the right direction in terms of what to check next and what the issue might be. 

When I override the protection circuit to get into the self-diagnostic menu, all the values are normal except the value for PS1, which steadily cycles between 0 and 255 instead of staying within the range of normal values listed in the service manual (pg. 72).

I’ve traced the PS1 circuit in the diagrams in the service manual, and portions of the circuit are found on one of the digital boards (pg. 122 of the service manual) and the main board (pg. 132 of the service manual). When I unplug the cable that PS1 uses to travel between the two boards, the PS1 value listed in the self-diagnostic menu stops cycling and stays at a steady 255.

Based on my very limited understanding of how the circuit functions, this leads me to suspect that the issue is somewhere on the main board or the transformer. I haven’t seen any capacitors that appear to be bulging or leaking, and all the solder joints that I’ve looked at appear to be intact. I’ve tested the rectifiers (D1051 and D1040) and voltage regulators (IC103 and IC104) and they all seem to be working properly (so far as I can determine with just a digital multimeter; I don’t have access to an oscilloscope, unfortunately). The resistors that are part of the PS1 circuit (R1255, R1256, R1229, R1228, and R1227) are all functioning properly as well.

At this point, I’m not sure where to go next. I could start testing capacitors (C1116, C1117, C1078, and C1079 seem like the place to start on that front, as they appear to be the only capacitors on the AC side of things, though I’m not seeing anything visually wrong with them) or focus in on the transformer (though so far I’m not seeing much information about it in the service manual; it’s not even entirely clear what voltages are supposed to be present on the AC side of things, which is what it seems like PS1 is monitoring). Or it could be something else entirely, but I’m not sure what. Any thoughts or suggestions for what to focus in on next? Thanks in advance for any help!

[Jeff eelcr]

You should check outputs and power supply first as you started,  with a meter is fine.
You should not need a scope yet.
Look for any DC on the outputs. 1V or so would throw it into protect.
Jeff

[David_AVD]

Does it have any signs of corrosion on the PCBs? I see Yamaha receivers all the time that have corrosion and open circuit or intermittent vias. Once corrosion gets in too far, you could waste many hours trying to fix it and then still have it die weeks later.

[AgentDelta]

You should check outputs and power supply first as you started,  with a meter is fine.
You should not need a scope yet.
Look for any DC on the outputs. 1V or so would throw it into protect.
Jeff

Good thought, thanks! I hadn't checked the outputs. Just tested them with the unit powered on in self-diagnostic mode, and none of the outputs had voltage on them.

Does it have any signs of corrosion on the PCBs? I see Yamaha receivers all the time that have corrosion and open circuit or intermittent vias. Once corrosion gets in too far, you could waste many hours trying to fix it and then still have it die weeks later.

That's good to know; thanks! So far I haven't seen any signs of corrosion on the PCBs, and I've done a decent amount of disassembly on it. The unit appears to be in pretty good shape physically, which is a good thing overall, but hasn't made it any easier to track down the source of the issue it's having.