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Yamaha M-4 DC Offset

Thrak

Thrak

New Member
Picked up a M-4 power amp. Appears to be clean and fully operational, the input level pots will need a shot of Deoxit but that is about it, I think. The DC offset is 35mv and 77mv, not terrible from what I have read, but could be better. I see a couple of pots marked "DC Balance" that are just yelling to be twiddled to adjust the offset but I have no service manual and I am not touching anything until I get the OK. I also see the +/- 65v adjustment pots along with test points and the bias adjustments. Help!
 
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Wait for the SM. Careful in there...don't forget to use insulated clips with your DVM at those test points to avoid the "third hand", "zztt" or "magic blue smoke" syndromes (trust me on this).
 
M-4 Adjustment procedure (illustrated)

The DC balance is the Offset.The service manual is a little misleading as to where to connect the Volt meter.Your offset is a little high.I'd try to get them below +/- 10mv. I attached the DMM probes to the speaker outputs and adjusted the DC balance pot for minumum offset. Step 1 PS adj.,Step 2 Bias/idle adj. Step 3 offset/balance adj. :scratch2:

Correction of misprint in the service manual.....

Step # 3 DC offset/balance - probe test points should be CT and E on the Drive PC board or you can use the speaker output jacks as long as the protect relays are working. If the relays are not working then you must use the testpoints.

I photoshopped the procedure to reflect the corrected ( offset ) test points...
 

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avionic,

Thank you so very much! I checked the PS voltage and both sides were spot on. Idle bias was low on both sides, after adjustment both now reading 15mv and DC offset is now between 0 - 2mv per side.

Thanks again for the invaluable information :thmbsp:

Thrak
 
wajobu said:
Beautiful--complete with the photo roadmap!
Click to expand...

Exactly, could not have asked for any more than that! And I managed to make the adjustments without letting the magic smoke out. Thanks for the much needed advice wajobu.
 
I made up the photos some time ago to help out someone in the other forum DIYAudio.Guess they came in handy again.Your welcome!:thmbsp:
 
Bump for Rajoo..:D See post #3..
 
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if you clean the pots use the special version of deoxit...

i think they call it fader lube, the one with no residue and no abrasives or whatever.

the pots in the back of the m4 have some carbon plate in them that the regular deoxit might mess up.
 
wd409 said:
if you clean the pots use the special version of deoxit...

i think they call it fader lube, the one with no residue and no abrasives or whatever.

the pots in the back of the m4 have some carbon plate in them that the regular deoxit might mess up.
Click to expand...

D100L is fine.Its the D5 that has solvent in it that may foul carbon resistance pads.
 
Hey, Avionic: thanks again! My DC offset is at 43 and 45 mV. Your great instructions are sure to be helpful later tonight.
 
DC offset now at <10 mV, but both channels seem to drift back and forth quite a bit, like from 1 mV to 8 mV and back in a few seconds. The L channel's LED meter reads "hotter" than the R channel on the front panel display. It takes quite a bit of pot turning to even up the bars in the display, like 100% for the R channel and turning it down to 50% on the L channel.

I'm pretty sure I'm liking what I hear, but that's just the placebo effect, no doubt. :D

Update: I'm surprised, but the "B" set of speaker outputs are behaving much better than the "A" outputs: L is 0.8 mV, R is 1.7 mV, without the disturbing fluctuations. The LED meter shows both channels tracking much closer together at 100% input for both. Listening now, and liking what I'm hearing, especially on vinyl.
 
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Update: I'm surprised, but the "B" set of speaker outputs are behaving much better than the "A" outputs: L is 0.8 mV, R is 1.7 mV, without the disturbing fluctuations. The LED meter shows both channels tracking much closer together at 100% input for both. Listening now, and liking what I'm hearing, especially on vinyl.
Click to expand...
You have fouled relay contacts or cold solder connections on" SPKR A" relay.
 
I had tried to redo the settings on my M4 last night & was thinking the manual wasn't being correct. Then later my feeble brain remembered there had been a thread on this which I had used a few years ago to get the setting correctly. So thank you again, avionic, from the future!!!
 
Would like to add my thanks to Avionic:beerchug: My M4 was reading about 30mV on each channel, which wasn't cause for alarm, but running into this thread made it seem so easy to bring them down that I had to give it a try. Took me all of 10 minutes to get the top off(the screws were the most difficult part of the procedure:)), the meter hooked up and the offset reduced to between 0-5mV give or take. Can't say I hear any difference, but it was good to get in there, get remove a lot of the dust, and bring the levels down for peace of mine at the very least.
 
Yes I know this is an old thread however it is invaluable for its instruction. Thank you I just got my m4 sorted. All transistors R channel all fusistors replaced and the transistor with 1 screw holding it to the heatsync was shorted. The black one not the green one.

Sounding great on kappa 6's to the humpty dance to test the bass

All so good thank you!!!
Phil
 
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