Bringing up a M-85 after repair, Advice please

R

RFWidgets

New Member
I am now at the point where I can attempt to power up my repaired M-85. Since the Left channel was not damaged at all, I have now soldered the connections to the
power transistors for the Left channel only. I have an 8 ohm speaker connected to the Left channel only. No speaker or load on Right channel. Also all of the power
transistors for the Right channel are NOT connected. I was troubleshooting a DC offset problem with Right channel and believe it is now fixed. More about that fiasco in
another thread. So for now I am monitoring the AC Line input current, plus the bias test point for the Left channel. With the Bias Set potentiometer set to zero ohms,
what should I be seeing for bias current at the test point? Right now with 120 volt AC applied I am seeing zero milliamps. Voltages on Left Channel Output transistors
Q131 +37.3vDC Q133 +37.3vDC Q135 -36.7vDC Q137 -36.7 DC offset on Left channel output 1.7mv

Comments???

Karin
 
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RFWidgets said:
I am now at the point where I can attempt to power up my repaired M-85. Since the Left channel was not damaged at all, I have now soldered the connections to the
power transistors for the Left channel only. I have an 8 ohm speaker connected to the Left channel only. No speaker or load on Right channel. Also all of the power
transistors for the Right channel are NOT connected. I was troubleshooting a DC offset problem with Right channel and believe it is now fixed. More about that fiasco in
another thread. So for now I am monitoring the AC Line input current, plus the bias test point for the Left channel. With the Bias Set potentiometer set to zero ohms,
what should I be seeing for bias current at the test point? Right now with 120 volt AC applied I am seeing zero milliamps. Voltages on Left Channel Output transistors
Q131 +37.3vDC Q133 +37.3vDC Q135 -36.7vDC Q137 -36.7 DC offset on Left channel output 1.7mv

Comments???

Karin
Click to expand...
When you move the bias resistor does anything happen? If not check the bias transistor, the drivers and predrivers. I would guess with dc balance drivers are okay, but never assume. Review the schematic
 
So the idle current verification on these are actually a DC voltage test at their respective test points. It's a Yamaha thing.
Chasing voltage is the best way to shoot trouble IMO, opposed to the fuss work of breaking open ckts injecting amp meters in series.
That's why voltage specs are peppered across the service manual schematic, vice current.

ABC

Always Be Chasing Voltage

Voltage too high? Chase opens. Too low? Chase shorts. Shorts are fun cuz they smoke guide you. Opens suck and take some time.
 
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RFWidgets said:
what should I be seeing for bias current at the test point
Click to expand...
Should be zero at full counterclockwise null position. Voltage! Not an actual current test. If you're seeing some voltage at full null swap the bias transistor and/or it's nearby buddy per post #2. The 2 trannies physically closest to the bias trim pot.
 
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I was just thinking about the same thread idea. Should have started long ago. Will keep it here for future posts more to come.
With respect to Soundscape's comment. I did not want to experiment with getting too far ahead of myself yesterday by setting bias currents
without some comments. Today, just a few minutes ago I did advance the bias potentiometer and I am able to set the bias current on the left channel.
I have not connected the right channel transistors yet. I will get around to doing that today. So far so good. Have not put any audio thru the amp
on the Left channel just yet. I don't want to get too complicated just yet.

Karin
 
I believe success has visited. I have just set the bias on both channels after installing the power transistors in the Right Channel.
Both channels are now fully functioning as far as static bias current. I have not attached any audio source as yet. I have to set up some more
equipment to monitor the distortion and audio output waveforms. That is going to take a few hours but for now it looks as if this year long
project may be coming to a close. I can't thank this group enough for all of the moral support and especially Avionic for material support.
Karin Anne Johnson
 
Just got done with some audio testing. Balance between channels is very good. No cross over distortion on either channel. Did not want to max out
gain as I am using 3 amp rated variac and was concerned about blowing fuse in variac. Still have to test Class A bias but I think that is going to be OK.
Did find some dirty potentiometers on input level adjust for each channel. I have some DeOxit which will take care of that.
So far all looks good.

Karin
 
Amp is working fine. But the two input potentiometers have some problems. I will have to remove the small circuit board to repair or replace
the potentiometers. Both of them have intermittent spots. It is a bit difficult to get any DeOxit sprayed into them given their position on the unit.
Complete removal is necessary. If the pots are bad I will need replacements. Before I go searching for them.
Any suggestions for a replacement part???

Karin
 
Weird: Worked on 10-12 M-80/85's in the last year, never had bad pots, not even dirty ones. Was your unit very dusty?
 
RFWidgets said:
Amp is working fine. But the two input potentiometers have some problems. I will have to remove the small circuit board to repair or replace
the potentiometers. Both of them have intermittent spots. It is a bit difficult to get any DeOxit sprayed into them given their position on the unit.
Complete removal is necessary. If the pots are bad I will need replacements. Before I go searching for them.
Any suggestions for a replacement part???

Karin
Click to expand...
To do it properly, you need your remove from the unit.
Your pots are probably doing this ( maybe not as bad)



Here after cleaning
 
Has anyone come up with a replacement cap for c162 and c163 on a m-85. I have the nichicon 1000uf 100v.
Mounting is the ? wondering if there is a better fit out there.
Thx
 
rottalpha said:
To do it properly, you need your remove from the unit.
Your pots are probably doing this ( maybe not as bad)



Here after cleaning
Click to expand...
Nice vid's Rott. :thumbsup:I promise I will not comment on them. One of those Oops moments " Shiite happens" It won't happen again. :thumbsup:Please forgive me..:bowdown: :rflmao:
 
dpeters688 said:
Has anyone come up with a replacement cap for c162 and c163 on a m-85. I have the nichicon 1000uf 100v.
Mounting is the ? wondering if there is a better fit out there.
Thx
Click to expand...
SECRET - NOFORN

If I tell you, you know the rest. :rflmao:
 
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