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FelixAUS 22nd February 2014 06:31 AM

Yamaha RX-V390 - Circuit Protection
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Hey Everyone,

I have a RX-V390 which will turn on for about 3 seconds and then turn off due to the short circuit protection (apparently). I have read a heap of other threads of similar receivers doing the same thing which have made me narrow down what I am looking at however I just can't seem to make my mind up on what's wrong. I have pulled out and disconnected the Tuner, Input and Rear Power Amp PCB which has narrowed it down to the front control board and the main power amp. I have measured the speaker outputs and there is no voltages on any of the channels and all the fuses are intact.

Can anyone explain these attached circuits to me about how the detection circuit works?
From my understanding, pin 'PROTEC' should be high (5v) however mine is low, hence why it shuts down. Tracing back from the pin however gets me to the main power board with transistors Q118 and Q119 (see attached). How does this work? The collectors of the transistors are connected to the PROTEC pin, however being an input I don't see how this helps.

Anyway, testing the rest of the voltages in the 'PROTECTION' area has lead me to the readings marked in red on the diagrams.
Looking at Q127 has me to believe that a fault is here as I have 46v on the collector which isn't tied to anything (I have disconnected the rear power amp connector).

Any ideas? Does this protection circuit look for leakages into the ground rail or something? It's hard to get proper measurements with the largish caps (C132/C133) as they are still charging up until the 3 seconds where the main power relay clicks off from the IC. This does mean that transistors Q117 and 116 don't turn on which could potentially be why I don't have any voltage drops over R171 D106 and Q127 perhaps?

This is probably really vague however ANY help would be appreciated, if you require anything else (the entire service manual?) let me know! =)



sbrads 22nd February 2014 09:08 AM

My guess is you have 0.7v on the base of Q119, this would make PROTEC low, the 5v pullup voltage coming from IC301 I guess but being pulled down in your case by Q119 being turned on. If so, you're then looking for a DC offset at the output of the power amplifier being fed into the base of Q119 via one of the various resistors and diodes at the amplifier output, eg R165 (overcurrent of left OR right amp output stages) R157 (front left amp DC offset), R158 (front right amp DC offset).

Q127 gets its 46v via a relay coil, something to do with turning the centre speaker on. If Q117 collector goes low after a few seconds I don't think you have a problem in the C132 mains input detection time delay circuit.

My guess is a blown output stage.

FelixAUS 23rd February 2014 08:29 AM

Hey sbrads,

Thanks for your reply, that makes a lot of sense now, I have no idea why it didn't occur to me about a 5v internal pullup...

I have started measuring the front power amp circuits and you are right, there is a 0.65v DC offset at the R165/R157/R158 junction. I tried tracing back further than this and comparing to the schematic however it's hard when the amp will only stay on for about 3 seconds and a lot of the circuits are yet to reach a steady state.
To determine which channel was blown I de-soldered R158 and the same problem occurred (still had a 0.7 offset), so I also de-soldered R157 and it stayed on! This made me think it was a problem with the left channel so I re-soldered in R158 (right channel) and the problem came back!
This meant that I had to have both left and right channels disconnected for it to stay on. I then removed R165 (the overcurrent protection) and it made no change, it would still turn off unless both R157 and R158 were disconnected.

So, what does this mean? It's not a problem with the specific channel and it's something common between them both?

Let me know what you think!



sbrads 23rd February 2014 12:43 PM

I suppose the -46v supply might be missing. Not likely though.

I think at this stage it might be best to leave the power off and go looking for shorted transistors around the output stages as they usually go short when they fail. I would be a bit careful about removing protection for test purposes unless you can put 10R - 100R 1/2W - 1W (depending on quiescent current) resistors in series with each supply as sacrificial lambs. Meanwhile I'll give it some more thought.

sbrads 23rd February 2014 01:38 PM

Common to both amps where it would cause +ve offset at both L/R outputs:-

1. Open circuit track on the -ve rail. Ohms test from the bridge to the output trannys.
2. Low capacitance on -ve reservoir cap C141, lots of ripple. Parallel it with another cap.
3. Both amps blown. Could a speaker have been wired between L & R?

FelixAUS 24th February 2014 02:45 AM

Just went and measured, I have both +-46v supply so that's fine.

I checked for continuity on all of the transistors in the front power amp and none were shorted.
Resistance testing from the negative side of bridge to the collectors of Q129A and Q130A gave me 0.2 ohms (the same as measuring from the positive side to Q129C and Q130C) so it's not an open track.

I'll try and test C141 if I can find a large enough cap (I would assume I would want something of similar size… I have a heap of old computer PSU’s kicking around and one has a 470uf 200v one, would that suit?)
Is the best option just to solder this onto the bottom of the board? I think my multimeter only measures capacitance up to 100uf, so wouldn't be worth unsoldering it?

Both amps could be blown, that is entirely possible. I found this receiver on the side of the road (hard rubbish haha) so I have no idea the history of it. I had been wanting to get a 5 channel to run some speakers out into our kitchen and couldn't believe my luck when I found it in such good condition (besides it not working...)
So yes, anything could have happened.

I have checked for open circuits on the 4 fuse resistors and they all seem fine...
Is there any way for checking capacitors easily in circuit (just to get an indication of dead/not)?

Thanks a HEAP for your help so far!


tauro0221 24th February 2014 04:24 AM

If you check the collector of Q129 and Q130 it will tell you which of the channel make the amplifier goes into protected mode. A high voltage in the collector means that you have a bad output transistor. It should read almost zero. I do not know if the 3 seconds allow you to read the voltage but you do not lose anything by trying it.

FelixAUS 24th February 2014 04:41 AM

Hi Tauro,

Shouldn't the collectors have a high voltage as they are tied to the 47v and -47v rails? Or do you mean as a voltage drop across the transistor (from C to E)?
I measured the collectors with respect to the 0v from the supply and I have 47 or -47 as expected...


tauro0221 24th February 2014 01:19 PM

yes, sorry it is my fault. It should be the emitter. Any voltage in one of the emitter the channel it is bad. Do you lost the power to the transformer also?

sbrads 24th February 2014 06:15 PM

So, is there any voltage >0.2v on the outputs? i.e. L101 to 0v, L102 to 0v.

I've just thought of something else that could affect both amps. Someone spilt a drink into the vents and the board is contaminated nearer the input end, forcing offset at the output(s). Seen it happen!

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