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Old 24th February 2014, 08:08 PM   #11
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Hi,
The output should read max 2.6mv. So I do not know if .2 volts would enable the amplifier protect mode. Another check it is to check the voltage at the base of Q 119. A positive voltage means that one of the channel it is enabling the protect mode. Your are right the protect mode should be high or 4.9 volt. Check pin 23 of the micro IC301. Normal It should read 4.9 volt. Do not forget that the center channel also can enable the protect mode. Check the voltage in the emitter of Q117C and Q117A.
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Old 25th February 2014, 05:10 AM   #12
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Hey guys,

On the emitters of of Q130 A/C and Q130 A/C I have 37v, with 37.7v on the bases. This voltage drop sounds about right however it should be from 0.5 to 0V (I think)?

Measuring L101 and L102 gives me the a similar voltage to the emitters of around 37V. By the time this is dropped over the resistors R157 and R158 this is giving me the 0.7v to Q119 that is pulling PROTEC low.

Should I work my way back through the required voltages on the service manual from the bases of the power transistors and see where these high voltages are coming from? From the manual most of the transistor/cap network is low (0,0.6,0.5,-0.5,-1.1v etc) except for those on the supply rails.

It can't be too far back the problem though however as CB101 (input PCB) is disconnected as well as CB701 (Centre and Rear power amp) as I wanted to narrow down the fault. Is there any reason for these to be connected? Am I perhaps getting funny measurements (Eg the 37V's) because they aren't? I don't think so but...


Tauro - I may be missing something here but Q117 is a single transistor (No A or C) and how does this connect to the centre power amp?

Sbrads - Entirely possible about the drink, however the surfaces of all the PCB's are in pretty good shape, a little bit of dust however no obvious staining...

Thanks again!
Felix
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Old 25th February 2014, 05:53 AM   #13
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Hi,
Sorry, Does transistors should read Q717A and Q717C. I think I need new glasses. I can not read too good the schematic. Any way I think you found the problem. The voltage reading for the bases of Q130A/C should be .5 volt. If they are reading 37 volts means that it is possible that Q110 is shorted or something it is driver it to 37 volts. Or Q130C it is shorted. Check the base voltage for Q110. It should read 1 volt. Also read the emitter and the collector voltage.
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Old 2nd March 2014, 05:33 AM   #14
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Hey guys,

Been having another look at this and trying to trace back as far as I can where all these high voltages are coming from! It seems all the transistors should have around -1 to 1 voltages on the bases and ALL seem to be taking 30-40v....

I measured the test points TP101 and TP102, which is where it gets interesting. TP101 has around 30.5v, whereas TP102 has 0! This has made me think it's something to do with the left channel, however, I am getting some really weird readings when I measure other voltages which should be the same if the schematic is to be believed.
For example, TP102 has 0v, but the legs of C104, Collector of Q132 and R116 all have ~31v even though they are all connected in parallel! How can this be? TP101 does not have this anomaly though as they are all at around 30.5v.

Secondly, on the attached diagram a similar thing occurs with R121 and R122 (marked in green). They have 36v on one side, 0 on the other but then the next resistor has 30v...
Perhaps the PCB is broken or a track shorting?

Thirdly, the bridge between the bases of Q132 and Q131 is reading 4.4v, where it should be -14.1. This leads to the CB102 connector and Q305, which is tied the the FMUTE pin on the IC. As I have 0v on this pin, Q305 is pulled high to 5.1 and subsequently delivering 4.4 volts to the bases of Q132 and Q131, which puts the amplifier in "Mute mode". I guess normally this is what happens then after the amplifier has checked PROTEC it will unmute the amp and drive the outputs? Could it be because of this (the amplifier 'muted') why I am getting weird measurements? I doesn't really explain where this leaking 30ish volts is coming from though. I have been tempted to desolder R329 to disconnect this muting operation as it would supply the correct -14.1 to Q132 and Q131... thoughts on this?

It seems the problem is common the both channels, which is annoying as it's hard to cross reference. It might be worth noting that R109 and R110 have the correct 14.5v on one side, but have 17v (instead of 0.6) on the other, which to me sounds like a shorted C121 and C122, thoughts?

Thanks HEAPS again for your help! =)

Felix
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Last edited by FelixAUS; 2nd March 2014 at 05:35 AM. Reason: Forgot diagram!
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Old 2nd March 2014, 06:30 AM   #15
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Hi,
Does reading in green maybe the resistors have some king of varnish and you are not making good contact. Try to clean them with a knife and read them again. I do not think the two channel have the same problem.
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Old 2nd March 2014, 07:52 AM   #16
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Hi Tauro,

I just went and re-measured each of the resistors and I have a 36v (or -36v if I reverse the leads) drop across both of them, meaning that I am getting a clean contact. It's hard to test them in circuit, would it be worthwhile de-soldering them and making sure they haven't gone open circuit, or is this voltage drop to be expected?

Cheers!
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Old 2nd March 2014, 02:46 PM   #17
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Hi,
Okay, check resistance or continuity from the R115 and R121. This will check that you may have an open copper run from R115 and R121.
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Old 2nd March 2014, 04:08 PM   #18
sbrads is offline sbrads  United Kingdom
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I'm looking... and looking - your voltage readings are not exactly nonsensical but not far short.

Initial thoughts:-
TP101 and TP102 are connected together - not sure why Yamaha would do that, doesn't seem right.
You can't have 2 vastly different voltages on the same track, i.e. left of R101 and also R102, unless that track is open circuit. In fact, there's 3 different voltages on that line.
Perhaps the schematic has errors though.

The 2.5v across C121 is also across R153 (330R) which just means there's 7.5mA going through R153. C121 can't be s/c.

I would be looking at those tracks where you have 3 different voltages again, around R121 and R122. Power down and just measure ohms for s/c between what should be connected points in the circuit, I reckon something is o/c and perhaps it's repeated across the 2 channels if it's a weak point. Are there via holes? They can go o/c.

There is another possibility though, one or both amps are oscillating at HF, you'll get all sorts of strange DVM readings if that's the case.

Last edited by sbrads; 2nd March 2014 at 04:35 PM.
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Old 3rd March 2014, 11:21 AM   #19
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Hey guys,

Did a fair bit of probing and circuit tracking and it turns out the schematic is fairly wrong!

Here goes for an explanation, if you like I can do you up a new schematic...

1. Turns out that the left hand side of R121, R122 and TP102 are connected to 0v, hence the weird measurements.

This means that there is no connection between (R115 & R121), (R116 & R122), (TP102 & C104).

2. TP101 is correct, it connects in parallel to R101, R102, R115, R115, C103, C104, C127, Q131, Q132, CB101.

3. C127 in series with R162 connects to ground as expected.

4. The right hand sides of R121 and R122 connect as expected from the diagram.

Besides that I think everything is correct...

Having a think now, I am measuring all this without CB101 or the Rear Power Amp connected as I didn't want to introduce any input to the system? This shouldn't be a problem?

Regarding the different voltages I had on the tracks of R101 and R102, it's highly likely that this is due to my poor measurement techniques. It's hard to get a good reading when the unit only stays on for 3 seconds! When I fired it back up this morning I was only measuring ~25v on the TP101/R101/R102 etc track, however after the amp powering up and back off again enough times this slowly rose back up to around the ~31v mark. I assume this is something to do with some large cap's charging, should we think anything of this?

There are no Via Holes on this board, it's a nice and simple single layer PCB =)

Regarding the HF oscillations how likely is this? I have a DSO so I could have a look for some HF action. Would it be fine to connect this to some of the components? I have only normally used it for 9v/low current stuff and would hate to blow it up ha!

I guess the real question is where to from here? Does this incorrect schematic mean anything? It doesn't explain where this stray high voltage is coming from. Perhaps I should continue tracking back from each of the 47V rails on both channels and see what I can find?

Sorry for the long post, thanks for the help!

Cheers,
Felix
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Old 3rd March 2014, 04:04 PM   #20
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Hi,
I think you should go back to post #12 and work backward. You post that the voltage reading for the base of Q130 it is 37 volts and it should read .5 volts. This will force a high output voltage to the speaker. I would check the voltage at the base of Q110 and Q112 and both emitters.
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