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Yamaha A-6a / A-760II No power

avionic said:
Does the service manual procedure tell you to short out the optocoupler ?
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It doesn’t but otherwise the adjustments cant be done at all. I get 0 volts in both adjustments, which i figure is from the chassis ground being shorted to the p1. Any ideas?
 
In some Yamaha models (M-50, M70, etc.), shorting of some pins are part of checking the integrity of the control board. The SM for A-760ll does not have such a section and no mention of shorting any pins, optocoupler or anything else, is made. Check your solder connections, you may also have a bad optocoupler although that’s unlikely, Avionic would know how likely that is.
 
Sparkplug said:
If you remove the photocoupler, do you still get a shorted P1? Have you checked D501?
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Pulled out D501 , reads fine. Ill check the optocoupler later tonight. I checked the transistor in it but not the led, since I figured they hardly go bad. Checked all the line level stuff with my scope on the primary board other than the optocoupler.
 
Levi8000 said:
Pulled out D501 , reads fine. Ill check the optocoupler later tonight. I checked the transistor in it but not the led, since I figured they hardly go bad. Checked all the line level stuff with my scope on the primary board other than the optocoupler.
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With D501 removed, did you try to see if P1 still read a short?
 
If removing the photo-coupler doesn’t remedy the short, you need to trace back the P1 line and check D327, that’s where P1 comes from. Check the solder joints wherever P1 touches.
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The unit is fixed! The chassis ground short was caused by a pretty embarrassing oversight. The mica sheet on one of the outputs was missing! Starts up nice now. Haven’t had the chance to use it yet. The infinity speakers I have need new woofers, which i’m waiting on. In the mean time, I was considering changing the op-amps, but it doesn’t really seem so worth it. Any suggestions?
 
Levi8000 said:
The unit is fixed! The chassis ground short was caused by a pretty embarrassing oversight. The mica sheet on one of the outputs was missing! Starts up nice now. Haven’t had the chance to use it yet. The infinity speakers I have need new woofers, which i’m waiting on. In the mean time, I was considering changing the op-amps, but it doesn’t really seem so worth it. Any suggestions?
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Great news … :thumbsup:
Don’t change anything, listen to it for a while to establish your present reference point. Also changing Op-amps can problematic, if gains don’t match, you could introduce hum and other noises. If it ain’t broke, don’t fix it.
Enjoy.
 
I was poking around in the amplifier to check the service manual adjustments, the amplifier was really hot, and I noticed the supply voltages were 54 volts! The big issue is that the variable resistor is at the very lowest. The optocoupler current is good. Do you guys have any advice? The unit is 100v running through a stepdown rated for 500w at 120vac.
 
Levi8000 said:
I was poking around in the amplifier to check the service manual adjustments, the amplifier was really hot, and I noticed the supply voltages were 54 volts! The big issue is that the variable resistor is at the very lowest. The optocoupler current is good. Do you guys have any advice? The unit is 100v running through a stepdown rated for 500w at 120vac.
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You should not run any Japanese units on 120VAC, they’re not designed to operate on it.

You should adjust the supply voltage down using VR303 and set the optocoupler using VR501, at the same time. You need two voltmeters. No need to check optocoupler current if you’re not replacing it (it’s in the SM).
 
Its not running on 120vac its running on 100vac through a 500w stepdown transformer. Vr303 is as low as it can be and the voltage is 54vdc, when on 120v it reads 60vdc.
 
Levi8000 said:
Its not running on 120vac its running on 100vac through a 500w stepdown transformer. Vr303 is as low as it can be and the voltage is 54vdc, when on 120v it reads 60vdc.
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Good, your initial description threw me. What voltage do you read on the optocoupler?
 
1.2v, Also would rail voltage that high kill it? I changed all the caps with 105c ones, because I heard these ones ran hot.
 
Levi8000 said:
1.2v, Also would rail voltage that high kill it? I changed all the caps with 105c ones, because I heard these ones ran hot.
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No, it won’t kill it. Remember it’s a Japan model so some parameters will be different than export models. Supply voltage on my B-50 also runs a few volts higher than its US equivalent M-50, that doesn’t mean there’s necessarily something wrong. I know I was the first person to open it up so I put it down as design variation. My control board is also slightly different than M-50, I wouldn’t be surprised if yours was that way too. Now if your components start blowing or getting heat damaged, that’d be cause for investigation.

If you have a nice legible schematic (I don’t), you can carefully look at voltages around different parts and see if they comply or how far off they are. With supply voltages higher, the power resistors in PSB and various parts that drop down voltages and dissipate power will run hotter. Up to a point that’s tolerable, beyond that, well … no.

I’d check the voltages across R450, 451, 452, 453, R458, 459, and R474, 475 to see what they look like against the schematic.
 
Alright, thanks for your help. I’ll take a look. The heat dissipation problem is probably from the turntable on top of it having really low feet that block airflow.
 
Levi8000 said:
Alright, thanks for your help. I’ll take a look. The heat dissipation problem is probably from the turntable on top of it having really low feet that block airflow.
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If I had to give one serious advice, that’d be to remove the turntable and make sure nothing interferes with the air convection for your amp. Yamaha and all other manufacturers expressly warn about the danger and degradation it could cause to the amp operation. Also you’re slowly cooking your turntable :D.
 
Sparkplug said:
If I had to give one serious advice, that’d be to remove the turntable and make sure nothing interferes with the air convection for your amp. Yamaha and all other manufacturers expressly warn about the danger and degradation it could cause to the amp operation. Also you’re slowly cooking your turntable :D.
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Lol the turntable was warm noticeably. Unfortunately I don't really have the table space for the both of them. The first sign I noticed of a major problem was the slight warp of a year old record. Would building a desk riser type thing above the amplifier with a couple of inches of clearance be wise?
 
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Levi8000 said:
Lol the turntable was warm noticeably. Unfortunately I don't really have the table space for the both of them. The first sign I noticed of a major problem was the slight warp of a year old record. Would building a desk riser type thing above the amplifier with a couple of inches of clearance be wise?
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Yes, give it a clearance of at least 4 to 5 inches (~10cm) each side, preferably more on the top side. It’s easy to build one out of wood. This is very important, please don’t take lightly.
 
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