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Yamaha M-4 DC Offset

Q for Avionic! I have picked up a M4, gave it all my love. (Full recap,transistor cooling paste/isolators replaced/lamps replaced)

Everything is working fine. Right side everything adjusting just fine. Left side 65v adjust fine.
But when i try to do the bias adjust my meter is showing null. What it going on?
 
Look over your work on the left channel. Look for cold solder connections or solder bridges. Check your emitter resistors and there solder connections. Check solder connections for the test pins.
 
Amplifierz said:
Q for Avionic! I have picked up a M4, gave it all my love. (Full recap,transistor cooling paste/isolators replaced/lamps replaced)

Everything is working fine. Right side everything adjusting just fine. Left side 65v adjust fine.
But when i try to do the bias adjust my meter is showing null. What it going on?
Click to expand...
Did it work OK before you did anything to it ? Are you getting a relay "click" at power up ?
 
avionic said:
Check your emitter resistors and there solder connections.
Click to expand...
R439,441* ,443 and 445. Also look at R437 and 461 solder connections and resistance values.

* emitter resistor thats being used to adjust bias.
 
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What does the DC offset look like in the left channel ? Adjustable ?
 
Amplifierz said:
The only thing i cant do is adjust bias.
Click to expand...
Is the left channel , noticeably, running warmer or cooler than the right channel ?
Still.. I'd be going through with checking all your solder connections and resistors mentioned earlier.:thumbsup:
 
Hello Avionic, it's been a while. But i am back at it! I checked all resistors you mentioned earlier. They all checkout just fine. Re-did all my solder work on that PCB.
That side is running a lot hotter then the right side. DC Offset can be adjusted and can be measured.
Only BIAS cannot be measured,when i connect my multimeter to correspondent measure points.
 
Turning the bias trimmer on the left channel CCW a little will cool it down.It almost sounds like a cracked trace or cold soldered connection around the test point pins. If the amplifier is working and sounding OK.
 
Well it's 2023 and it's about the right time to resurrect this thread, lol. I just picked up one of these great amps to use with my AR3As - and so would like to thank avionic for posting the info to correct the mistake on the service manual for proc. #3.

It had me scratching my head for sure.
 
skypuppy said:
Well it's 2023 and it's about the right time to resurrect this thread, lol. I just picked up one of these great amps to use with my AR3As - and so would like to thank avionic for posting the info to correct the mistake on the service manual for proc. #3.

It had me scratching my head for sure.
Click to expand...
Your welcome..:thumbsup:
 
avionic said:
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...
Click to expand...
Thank you, sir. Super helpful.

PS was exactly 65Vdc both sides, idle current were both 0mVdc (but I adjusted) so I guess someone set them wrong, DC balances were generally off and it took a while with those frigidity things to get a good +/- range of +-10mVdc.
 
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