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Post by serafis on Jan 21, 2024 3:59:43 GMT -5
Hi, I have a Yamaha RX-V473 that switches off randomly with protect error 'PS3 - 131L', indicating that the MCU has detected that the 5.5V rail has dropped below its acceptable limit. This happens after up to an hour or more of use. Switching straight back on again works for another random (usually shorter) period before switching off again. Service manual is at elektrotanya.com/yamaha_rx-v473_htr-4065_sm.pdf/download.html, and screenshots of the PSU and 5.5V section attached below When first powered on, the 5.5V rail measures 5.523V. Once the various relays click, this drops to around 5.507V. I've replaced the Schottky dual diode and all the electrolytics on the power board and around regulators on the HDMI board, just in case. I've checked the values of all resistors and caps on the power board and they're all OK with the possible exception of R5414 which measures 1.627K rather than 1.8K. I'll try replacing that with a new resistor tomorrow to see if it makes any difference. But I'm stumped otherwise - I'm thinking that the randomness indicates a heat problem, but at a loss as to where to start looking or how to diagnose further. I'm maybe slightly suspicious of IC543, the programmable shunt regulator in case it's not doing its job properly, but I don't have a spare for this in the parts box. Any help or advice gratefully appreciated. Cheers, Jon. 
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Post by serafis on Jan 23, 2024 20:32:41 GMT -5
I put the oscilloscope across the 5.5V rail, with a falling-edge trigger set to around 5V. After a random time, I get a trace like this  I'm not sure what might be causing this, and while the receiver didn't switch off, I suspect that it might be part of the problem. Any ideas? |
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Post by mastertech on Jan 24, 2024 20:25:55 GMT -5
What you always have to remember is that besides that rail voltage, the detection circuit also can malfunction, meaning, the rail could be fine but if the detection circuit is reading it wrong it can cause an error. Now as you indicated that error is for 5.5v monitoring which is the PS3 and the value is 131 but the normal range is (132 to 168). So you're just slightly under when it goes into protect. So I would be verifying the detection circuit. And according to the manual it is monitored by the MCU (ic21) pin 106, which is PS3_PRT circuit. Examine the circuit on page 92 and do some checking there to see what you have.
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Post by serafis on Jan 25, 2024 15:23:40 GMT -5
Thanks, good advice. I've examined the PS3_PRT circuit and simplified it in the image below.  R206/R207 is a voltage divider of the +5.5V source from the power supply, providing PS3_PRT of 1.96V (for +5.5V) to pin 106 of the MCU, via R223, presumably a current-limiting resistor. The MCU calculates the PS3 monitoring value from PS3_PRT, where reference voltage of 3.3V is 255, i.e. {PS3_PRT / 255 * 3.3V}. The valid range for PS3 is 132-168 which, by this calculation, is PS3_PRT of 1.71V-2.17V. Working back through the R206/R207 voltage divider, this equates to a valid supply voltage range of 4.78V-6.08V. PS3_PRT of 1.96V therefore gives a monitoring value of 151 (1.96/3.3*255), which is somewhere in the middle of the valid range. PS3 of 131 would indicate that PS3_PRT had dropped to 1.7V, or that the supply voltage had dropped to 4.75V. Pin 106 of the MCU is also labelled LMT_PS3 and goes to the emitter of Q206, whose collector goes to DGND. R201/R202+R203 is a voltage divider of the 3.3S source from voltage regulator IC24, providing 3.03V to the base of Q206. I don't know what the purpose of this part of the circuit is though. Anyway, I've checked all of the tiny SMD resistors (in circuit), Q206, and the various voltages which are all spot on when the receiver is operating correctly. So I still don't know what would randomly cause PS3 to drop below the valid range after up to an hour or more of operation. Assuming the power supply of +5.5V is constant, possibilities might be any of the resistors or Q206 going out of spec (e.g. due to heat), or the MCU calculation failing for some reason. The other possibility is that the 5.5V source actually drops below 4.78V, but I don't know what would cause this. I tried applying heat to the HDMI board and to the power supply board to see if it triggers the protection - it didn't. So still stumped. Any further help much appreciated. |
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Post by mastertech on Jan 25, 2024 15:41:28 GMT -5
I think your idea of applying heat to various locations to see if it will go into protect is the best course at this time. Besides that, you would have to try to monitor the protection line and try to catch an abnormality when it happens but that could be near impossible without it being consistent.
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Post by serafis on Jan 25, 2024 15:47:53 GMT -5
Heat (hairdryer) didn't do anything unfortunately and I got it pretty hot! In diagnostic mode, PS3 was 153-154 constant, well within range. I'll try a monitor across PS3_PRT and see if I can catch anything.
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Post by serafis on Jan 27, 2024 15:39:31 GMT -5
So now I'm thinking about the 5.5V switch mode section of the power supply again (P97), and the 'scope image I posted earlier, and what could randomly cause the voltage to drop below 4.78V to trigger the PS3 protection.
The output voltage is predominantly a function of the transformer input voltage, the transformer winding ratio, and the frequency and duty cycle of the switching PWM.
TL41 (IC543) is a programmable voltage regulator which, together with its surrounding passive components, provides negative feedback via an opto-coupler (IC542) to pin 4 of the PWM/MOSFET controller (IC541).
When the output voltage varies from 5.5V, this feedback circuit should signal IC541 to vary the duty cycle of the PWM accordingly, to maintain the output at 5.5V. This seems to work as it should in normal operation as the scope normally shows a flat 5.5V line.
But the scope image I posted earlier shows a strange oscillation in the 5.5V output voltage. The scope was set to single-shot, falling edge trigger of 5V, and the amp was left running for more than an hour before the trigger occurred. While the amp didn't switch off with PS3 protection this time, I could see that an event like this could possibly cause the protection to trigger.
So I'm thinking that maybe one of these components is beginning to fail, e.g. TL431, the opto-coupler or the PWM/MOSFET controller.
Any thoughts on most likely culprit, or am I completely on the wrong track here?
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Post by madmal on Jan 27, 2024 18:51:24 GMT -5
Isn't the standy PSU the same as the one in the RXV675 you have ? Could you swap to test. I've had a similar issue on a Denon where the power chip on the standby board was bad but it would work fine for a while and then randomly switch off, however that was a TOP268 chip but I guess the same principal could apply?....
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Post by serafis on Jan 27, 2024 19:43:17 GMT -5
Thanks, but the PSU unfortunately isn't the same - different PWM/MOSFET controller, different board shape, layout, pinout and output voltages. Similar principle though, I guess I could swap the TL431 and/or opto-coupler but because the issue is random, difficult to tell whether this is successful or not.
Update: Just sold the RX-V675 so that's not an option now.
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Post by serafis on Feb 8, 2024 22:51:37 GMT -5
Just replaced the TL431 (IC543), PWM/MOSFET controller (IC541) and the opto-coupler (IC542). 5.5V rail looks rock solid at the moment, so I'll put it back into service and see how it goes over the next wee while.
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Post by serafis on Feb 9, 2024 3:58:18 GMT -5
Well, I ran it tonight for about 4 hours without an issue, so hoping the problem is now solved. Interestingly, analog inputs weren't working properly, about 10x too quiet with a lot of hiss, except in Direct Mode, when they were fine. I thought the DSP might be on its way out although digital inputs were fine. Now, however, the analog inputs all seem to be working properly, so maybe there was a power problem all along.
I have an RX-V475 that suddenly started having a similar problem with analog inputs - volume OK, but very 'staticky' and distorted. Again digital inputs were fine. It has an almost identical power supply to the RX-V473, so I'm going to replace the capacitors and ICs in this one too and see if it makes a difference.
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Post by madmal on Feb 27, 2024 15:39:04 GMT -5
Will keep this in mind. Glad you got it sorted.
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Post by serafis on Feb 27, 2024 17:52:00 GMT -5
I have an RX-V475 that suddenly started having a similar problem with analog inputs - volume OK, but very 'staticky' and distorted. Again digital inputs were fine. It has an almost identical power supply to the RX-V473, so I'm going to replace the capacitors and ICs in this one too and see if it makes a difference. It didn't. I think this one is stuffed! |
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