McIntosh MC2120/MC2200 bias adj. puzzle

[Jan Dupont]

Used some of the Easter holidays to test some different older circuits, and among them the power amplifier section of the McIntosh MC2120 and MC2200 (same circuit used in both but different no. of output devices).

What puzzles me is the bias adjustment. The Service Manuals just states you to measure the total power consumption of the amp during bias adjustment, but I measured the Voltages Base to Gnd of the output devices.

In the manuals are stated that with no input the Voltages on the Bases should be +0.4 and -0.4V respectively.......
I get app. -0.38V on negative side, but each time I try to adjust bias up so the positive side measures app. +0.4V the amp suddenly trips into Class A with app. +9 and-9V on the Bases ?

The funny part of this is if I keep the possitive Base Voltage below app. +0.3V and the negative at -0.38V it works perfekt using a 3R3 dummy load. Full output without a glitch of crossover distortion on the scope, symmetrical clipping and just a little bit of overshooting when tested with square waves.

The circuit has been checked again and again several times a day for many days without I could find any errors in my circuit (it is after all a quite simple circuit).

Any experienced McIntosh people here that has an explaination ??

Thanks in advance

[nigelwright7557]

I usually apply a 1 volt sine wave and monitor the output for crossover distortion with a 4 ohm speaker connected.

I raise the bias very slowly until crossover distortion just goes.
This has always given good results.

[Jan Dupont]

I raise the bias very slowly until crossover distortion just goes.
This has always given good results.

And it sure does And it is very stable....
Just wonder why it trips into Class A so brute. No middle way ??

[nigelwright7557]

If the bias is taken too far it forces a lot of current through the output transistors.

[llwhtt]

Jan,

I repaired an MC2200 last year and tried using the "total power consumption" method and couldn't get it to work that way. I finally ended up adjusting the bias to just eliminate of the crossover spike while monitoring the output of my HP 334 distortion analyzer. The adjustment was very touchy and easily went too high. This was after a total rebuild of both heatsink assys. using parts obtained from McIntosh. My "golden ear" friend was happy with the results so that's the way it is to this day.

Craig

[Jan Dupont]

Nigel;
Thanks I'm aware of that
But this just happens with a 1/1000 turn of the 100R multiturn pot....
Normally you have a little slope when adjusting bias.

[Jan Dupont]

Thanks Craig
Nice to know this is common reaction. Then I don't have to search more for errors in my circuit
Looking forward to listen to it within a week or so. Last time I heard a MC2120 was 12 years ago