Author Topic: More Yamaha Amp issues...  (Read 2784 times)

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Offline andy206

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More Yamaha Amp issues...
« on: April 14, 2015, 11:48:30 pm »
Hi all,

thanks for the awesome videos and a great forum for a start! Still new here but already a fan :)

So Google turned up the post about the yamaha amp fix, and I seem to have a problem with my trusty rx v661 also. Unfortunately not C405 I don't think - it turns on fine, but goes into protection mode due to low voltage on one of the monitoring circuits. I traced it down to a +12V line, which shows +4V. Great I thought, replace that and I'm sweet - but no cigar. Turns out the old regulator still regulates out of cicuit, and after the replacement I'm still at 4V.

Then tried to isolate on which board this is happening, and turns out when I disconnect the dsp board - taddaaa - back to +12.

So I'm a reasonably good hobbyist, certainly no professional... hence my question, what can cause a voltage drop like that? Looks like an almost-short somewhere I would have thought, right? I assume burnt caps can do this, they all look pretty good though. Nothing obviously went up in a puff of smoke. Any other suggestions on what to look for?

My other question, I have measured all the voltages through from the supply onwards, according to the service manual, and for a number of them (mostly AC but a few DC ones as well!) they are a fair bit higher than in the manual. Can't remember any exact numbers at this point, but what sort of tolerance would you expect? Is higher ok? They're not hideously over the top, but maybe 10% at times.

Would really appreciate pointers in the right direction, thanks heaps!
Andy


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Offline Grapsus

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Re: More Yamaha Amp issues...
« Reply #1 on: April 15, 2015, 12:30:42 am »
Then tried to isolate on which board this is happening, and turns out when I disconnect the dsp board - taddaaa - back to +12.

One simple way to find the shorted component is to look at temperatures. If something sucks up all that power, it must heap up.
 

Offline andy206

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Re: More Yamaha Amp issues...
« Reply #2 on: April 15, 2015, 12:58:38 am »
Thanks Grapsus... good point. I have been feeling around a little but not specifically. Gonna try that first thing tonight. That's assuming this is the reason for the low voltage, is that reasonable even? Any other reasons it could be so low (input voltage is sufficient at about 20V, on an 18V line - one of those examples I mentioned where there's a fair bit more than I would have expected...)?

Thanks!


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Offline obiwanjacobi

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Re: More Yamaha Amp issues...
« Reply #3 on: April 16, 2015, 09:12:58 am »
Caps do not have to be burnt to (almost) short out. So check each one of them.
Also, what is the resistance on the supply rail (measure both ways / swap probes)?
Try to measure resistance on all bypass caps...

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Online tautech

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Re: More Yamaha Amp issues...
« Reply #4 on: April 16, 2015, 10:20:43 am »
With those symptoms the first thing I would look for is dreaded Tantalum caps.
Bead type or SMD. If found make sure to replace with a higher voltage rating if possible.

Re Voltages.
Does the SM stipulate how the amp is to be set for the test voltages to apply?
Any SM specified voltage tolerance?
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Offline andy206

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Re: More Yamaha Amp issues...
« Reply #5 on: April 16, 2015, 02:44:56 pm »
Thanks for the hints guys. Tried to find anything hot, but nothing really over the top... the bridge rectifiers are all a bit warm, but I think that's all within limits. At what kind of resistance would you call a cap to be burnt out? Literally almost 0 Ohms? Anything a few kOhms+ would be considered normal for in circuit, correct?
So while I was looking a bit more though I found something else interesting. There is a 7812 and a 7912 on the regulator board (among others), with the respective +/-12v going through a connector. The pin next to them is gnd, labelled E, and there are another 4 pins labelled the same. That closest E is same potential as the chasis, so thats what I measured against. and that's how I initially got -12 ok but only +4 odd. I measured the in and outputs on the regulators this time, and it turns out the +12v is actually fine, but the -12v is only -4! Turns out E is not the same as the next E! Pulled the 7912 out and tested, and it actually appears busted. Didn't have one around to test yet, but hey! Something! :) Even tho I still don't get how there are different grounds that happen to give me the opposite result initially...


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Offline andy206

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Re: More Yamaha Amp issues...
« Reply #6 on: April 18, 2015, 08:22:52 am »
So I did some more troubleshooting today... replaced the 7912, and voltages are all measuring fine now. But, I'm still getting ps1 low warnings.

I've attached the schematics for the offending PRV1 line. You can see where it goes into pin 135 of the ic, it should measure in at 3.2V, but I'm only getting 1.2 odd. If you follow it through the other 2 pictures, it connects to various power rails, which all measure as they should - high if anything. How can that be? There must be something I'm missing, right? I have checked the D804 zener and c808, they're fine.

Getting really frustrating... anyone got any advise on how to continue?

Thanks heaps.
Andy





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Offline andy206

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Re: More Yamaha Amp issues...
« Reply #7 on: April 18, 2015, 02:45:48 pm »
Guys... ignore it. thanks for all the comments. Turns out, it really only was the 2 voltage regs - everything else was grounding issues! I should have picked that based on my own description... ground was a common theme really! No idea what I forgot to hook up, I even ran wires to each board when I tested with the unit apart, but obviously something was missing.
Never mind - so glad it's finally working again :) Hope it helps someone else at some point.
Thanks again!
Andy


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