Yamaha M-85 Bias Voltage Does Not Increase In "Auto A" Switch Position - diyAudio
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Old 5th March 2014, 10:53 PM   #1
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Default Yamaha M-85 Bias Voltage Does Not Increase In "Auto A" Switch Position

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Last edited by 69sixpackbee; 12th March 2014 at 03:43 PM.
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Old 6th March 2014, 04:20 AM   #2
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The many small caps in the 'auto class A' and 'current detector and bias controller' sections have been blamed for this apparently common fault. It's an old amp. now, so if it hasn't already been done fully, now's a good time and there's the place to start.

Recap both channels there for a chance at a proper solution, as I recall being advised in a recent thread elsewhere. I should say I haven't done this myself but it looks like an entirely sensible approach for those without the time or expertise to nail the small number of most likely culprits, when actually, all original electrolytics will be past their use-by date (around 28 years old).
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Old 6th March 2014, 06:21 PM   #3
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Thanx.
I have totally recapped several receivers in the past which had an inordinate amount of e-lytic caps. I made it a priority to swap out the tants as well. This one has very few but I will look at them. However, I just purchased a very good copy of the SM that I can actually read. Now I will try to bonehead through. More than likely, however, I will probably just end up sending it back for a refund. I have enough projects and this Yamaha is just a boat anchor as currently is. I'll mess with it for a little while but I don't know. All I can say is all of the functions, aside from the auto A switching circuit are in working order. But when there is barely enough volume coming out with a good input going in something is out of whack. The LED meters might be saying I have 200 Watts coming out but my ears say otherwise. 7.4V out at the speaker terminals is not 200W of power!
Thanx,
Bud
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Old 6th March 2014, 07:48 PM   #4
kouiky is offline kouiky  United States
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This model of amplifier was well built, but it is a popular one to find inoperable because of the condition of the supporting components. There are many things that can lead to failure, and many of the owners didn't take proper care of them. While it nothing is absolute, I have found that owners of more expensive amplifiers tend to take care of them, where as owners of inexpensive amplifiers may be more likely to abuse and neglect them. The problem you are having may be the result of years of overheating because past owners placed them in confined places, or stacked other components over them. The heat resulting from current flow and environmental conditions is what causes the capacitors' electrolyte to dry and possibly eventually boil out.

The meters read the output from the drive stage, that they would indicate that those sections of the amplifier are at least functional. You need a dummy load and appropriate measurement configuration to measure power, but only after every section has been gone over. This is where most technicians go wrong, becuase they assume everything is operating optimally. Then when the voltages increase and serious current begins to pass through these components, they stop working. At full drive without a dummy load, you should see a 20V sinusoidal waveform on an oscilloscope that roughly corresponds to the meters, once they are calibrated to the output voltage. As you can see, each section has to be gone over, otherwise there will be inconsistencies.

No amplifier lasts forever, and they all eventually require servicing. I know form experience in working on other electronics that for the condition your amp is likely in, it will be impossible to attain any solid or consistent feel for the sound. If one thing is out of tolerance, everything else is. Yamaha had a pretty wide spectrum of amplifiers ranging from the common low end, up to some true exotica. One thing that I have found consistent within the brand was that the designs were always spot on. I think that if you dedicate time and care into this rebuild, instead of rushing for the finish line, you may come to find it was more than worthwhile.

Last edited by kouiky; 6th March 2014 at 07:51 PM.
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Old 6th March 2014, 07:56 PM   #5
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The meter range switch already is in the less sensitive position, right? (Otherwise you'd be looking at 20 W rather than 200.) Failing this, is that 7.4 V with or without speakers? In the former case I'd guess that the protection relay contacts have seen better days...

BTW, older Yamaha power amps are notorious for (a) bad solder joints and (b) aggressive glue (originally used to keep the big electrolytics in place) that can eat up components it makes contact with. When restored properly, however, they're actually fairly nice pieces of kit.
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Old 6th March 2014, 08:16 PM   #6
kouiky is offline kouiky  United States
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The meter should be in the 1x position, thus reducing the sensitivity to allow for full power metering. I am going to say it again, rebuild the amplifier and verify each stage before you start pushing it. If you do not, you will run the risk of inducing further damage. As to their solder quality, I have not encountered this problem. I have seen the white glue on power capacitors in lower-mid grade preliminary amplifiers that is used to keep any sudden impact from stressing the joints, but I haven't seen any corrosion as a result. I use sensor-safe silicon in devices and directly on copper without any problems.
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Old 7th March 2014, 03:40 PM   #7
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Quote:
Originally Posted by sgrossklass View Post
The meter range switch already is in the less sensitive position, right? (Otherwise you'd be looking at 20 W rather than 200.) Failing this, is that 7.4 V with or without speakers? In the former case I'd guess that the protection relay contacts have seen better days...

BTW, older Yamaha power amps are notorious for (a) bad solder joints and (b) aggressive glue (originally used to keep the big electrolytics in place) that can eat up components it makes contact with. When restored properly, however, they're actually fairly nice pieces of kit.
Hello,
I had the switch in the "x1" position as it was definitely 200W with a 1Hz tone coming into the amp via my iphone at the time, with everything at MAX volume & that is what I measured at the "B" speaker post WITHOUT a load .

Thanx!
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Old 7th March 2014, 07:00 PM   #8
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How odd. A non-capped iPhone ought to be good for about 1.2 Vrms, giving 150 W / 8 ohms (34.something Vrms) with this amp.

If the amp honestly "thinks" it's giving out that much, the corresponding voltage swing must be present somewhere after all, it just doesn't make it to the output. Sounds like an interrupted ground connection somewhere towards the output side. Check for continuity between input ground and speaker return.
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Old 8th March 2014, 05:27 PM   #9
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Originally Posted by 69sixpackbee View Post
It was supposed to work. The guy even sent me a video of it in action prior to when he shipped it to me and it was working. I trust the seller and he is doing the legwork to get the claim settled for the damaged goods. I figured I would hang on to it as it now surely is a "fixer upper" sadly. I intended to re-cap the thing and check it all out so I guess I will have to do some extra work. Who's to say if I was to send it back and get my money back that the next one I find will turn out to be a lemon as well.

Now then, moving along...I started checking the power and what I have found, right off the bat, is the voltage from the power supply is weak on one rail.
The schematic shows 88.1(-88.1) on the H rail which is exactly what I have. However, on the L rail the schematic calls for 49v and I am getting 35.6v.Click the image to open in full size.
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Old 8th March 2014, 05:29 PM   #10
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Originally Posted by 69sixpackbee View Post
Now to further, the secondary taps from the xfrmr are as such:
Y=28.3 Vac
O=28.3 Vac
R=66.2 Vac
BL=66.2Vac

What would cause the discrepancy in the voltage? I am not sure but if the schematic is calling for 49V and I am producing 25% less I suppose this would create a problem, yes?
Click the image to open in full size.
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