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I’ve been given a faulty Yamaha RX-V765 Receiver. Parts are no longer available for this unit, and I’m trying to convert (i.e. hack) it directly into a Power Amp.

I have been able to get some audio out of the unit using a standard Line input source, but it is of poor quality and nowhere as loud as it should be. I have to admit that I am pretty much out of my depth (but want to learn and also not spend any money). I'm only guessing where the pre-amp circuitry ends and the power-amp input starts. For debugging / starters I am working only on the Front Left (i.e. "FL") channel.

Link to the schematic for the main circuit board that I am working with.

I have annotated the above schematic with where I am adding an input source as per the image below:

partial schematic diagram
(The Orange dot is the signal from the audio, and the Green dot is ground.)

A video showing what I am getting at the moment is available here. This has been recorded with the input source at the maximum volume (which is too quiet for what the amp should be able to do and is also distorted). The Red lead in the zoomed in video corresponds to the Orange Dot.

The full service manual is available here.

Any help as to where I should be connecting the input signal would be most appreciated :-)

Thanks in advance.

Update:

Tested the DC voltages across all the main output transistors and they are all fine. I'm also getting the nice slight hum out of all speakers (i.e for channels) which makes me think that the actual Power circuitry is fine. When I repeated my input signal injection for FR rather than FL I got the same result (very low volume with some distortion). This made me think that something must be wrong in circuitry that is common to both - so started looking for readings around the "star ground" circuit. This is what I've found so-far - erroneous readings around one of the Voltage Regulators "IC102". (My readings in Green - those supplied in the Yamaha Service Manual in Red.)

Oops - the voltages below are incorrect, see Update 2 Below Debug

It's difficult trying to get readings with both the main board and one daughter board installed (for the speaker output), so I'm going to extend the leads so I can get the daughter board out of the main enclosure (but still electrically grounded where needed). Hopefully then I'll be able to get some more readings.

Update 2:

It was only after I got good access to the underside of the main board that I realised I had the pin-out of IC102 wrong. (I thought I was reading "IN" but was actually reading common.). The corrected voltages are as below:

Debug2

Update 3:

IC102 turns out to be a standard -12V 1A regulator. I've replaced this using a good one from my local electronics shop - now the readings around this area are better, but there are problematic voltages downstream from this area. Around Q1072, Q1073 and Q1074 the voltages are now fine - but around Q1067 and Q1070 the voltages are incorrect. (Note: I wasn't sure what B/C/E readings I was measuring on these transistors because I was tired/lazy; I just noted the three voltage readings on the pin-out. The values provided in the square brackets for these components may therefore be out-of-order.)

With one of the speakers connected, when I try to measure the voltages on Q1070 (with my properly grounded meter) I get an audible "buzz" when measuring one of the pins. This seems to be the only place on the board where this happens so far. I'm not sure what this means, but perhaps it means Q1070 is bad ? (Two of the voltages around Q1067 look okay, but not the third.)

Debug3

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  • 1
    \$\begingroup\$ Welcome to EE.SE! This appears to be a reverse engineering, modification, or repair question. Please be aware that such questions must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being discussed, so that you can ask specific, focused questions that can be answered concisely. Otherwise, the question is far too broad. More information can be found here: Is asking how to fix a faulty circuit on topic?. \$\endgroup\$ – Dave Tweed Jan 29 at 21:03
  • \$\begingroup\$ @DaveTweed: Thank you for the reply. I'm pretty sure that the actual circuit being discussed above is not faulty. (The daughter board that was faulty on this receiver is the HDMI / Dolby prologic board, and that has already been removed from the unit.). I'm guess I'm wanting some help to make an educated guess as to where I should be injecting an analog input to get the unit to function as a standard stereo amplifier. \$\endgroup\$ – Patrick Rynhart Jan 29 at 21:14
  • \$\begingroup\$ Yes, that's called reverse-engineering and modification. I'm just saying that you're on the verge of being either too broad or off-topic for this site. \$\endgroup\$ – Dave Tweed Jan 29 at 22:41
  • \$\begingroup\$ @DaveTweed: Noted - I would still appreciate some help or pointers from the community if at all possible as I'm not really sure where else I could ask. I am keen & interested and am happy to do further investigation if someone could point me in the right direction. Thanks & Apologies, Patrick \$\endgroup\$ – Patrick Rynhart Jan 29 at 22:45
  • \$\begingroup\$ If you are getting -18.8V on one side of R1238, and 0V on the other side of it, methinks it's bad - possibly something on that -12V rail overloaded the regulator and that resistor failed due to excessive power dissipation. I'd look around there. \$\endgroup\$ – Adam Lawrence Jan 31 at 21:02
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For what its worth, your orange dot is definitely NOT the correct place to inject a signal. It's inside the feedback loop, and the circuit is going to fight you.

The correct place is the pins identified at the left edge of the schematic, for example, "FL". These are the input to the differential pair that is the first stage of each output channel.


EDIT: Actually, I'm not sure what those "POE" pins are for. The correct way to apply the input signal is between the "FL" pin and the "star ground" line that you have identified with your green dot.

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  • \$\begingroup\$ I should have mentioned in the OP that I had already tried injecting a signal at "FL" and "POE". If I do this, then I can hear audio, but it is incredibly faint (as though the signal has been muted by the preamp circuitry). The "hum" from the poweramp through the speaker is about the same volume when I hold the speaker this close to my ear. It is for this reason that I tried moving "lower down" in the circuit - hence the orange dot attempt (which is clearly the wrong place if I'm in a feedback loop). \$\endgroup\$ – Patrick Rynhart Jan 30 at 0:01
  • \$\begingroup\$ See my edit above. \$\endgroup\$ – Dave Tweed Jan 30 at 2:44
  • \$\begingroup\$ Thank you very much, I will try that when I get home this evening. I really appreciate the help. I'm pretty sure that POE and the "Green Dot" / Star Ground line were actually electrically connected both to ground though (using a continuity test on my meter). I will give it a shot this evening though and post an update. Thanks again, Patrick \$\endgroup\$ – Patrick Rynhart Jan 30 at 2:52
  • \$\begingroup\$ No luck unfortunately. Have moved back to using FL and get the same result with POE / "Star Ground" as Ground. The result I get sounds like this drive.google.com/file/d/1xVTUjMNHcliGIUSWRJxH8cKlVL0R_8ip/view. Does this indicate failed components even on the main board ? (I guess I could try repeating with another channel for comparison.) \$\endgroup\$ – Patrick Rynhart Jan 30 at 9:26
  • \$\begingroup\$ Repeated the injection test for another Channel (FR) and am getting the same result - have posted an update above. Thanks for your help! \$\endgroup\$ – Patrick Rynhart Jan 31 at 19:17

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