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Power Output Boards

Posted: Sat Nov 05, 2016 10:47 pm
by jwarrenNRE
I had an MC2100 that was missing the power output boards PN 043-899. They're still available from McIntosh. These are plug in boards that are also used on the blue meter version of the amplifier 2105. When you get the boards, they're the last revision of the board over the long production history of the amplifier. A photo of the new board and one pulled from a customers early S/N unit is shown below (someone replaced the caps on the older board).

Looking at the new factory board, many composition resistors are replaced with film type. The two circular heat sinks identify the complimentary power transistors 2N5322, 5320. These serve as emitter followers which buffer the voltage gain stage providing a lot of current gain. The voltage gain stage is the NTE175 in the TO66 package, upper right.

The two small signal devices just below the NTE175 comprise an emitter coupled PNP differential input pair (Q101, Q103). The signal from the preamplifer board is sourced to the base of Q101 whilst feedback is sourced to the base of Q103. The schematic shows these devices require, from L2R, a C-B-E pinout and the 2N5087 low noise, high gain amplifier is a solution used for many years. The boards were provided with 2SA1016K devices which are low noise, high gain PNP transistor perfectly suitable for this application. The 1016K however is a B-C-E L2R pinout and thus unsuitable for the circuit trace layout used in the boards.

Re: Power Output Boards

Posted: Sat Nov 05, 2016 10:54 pm
by jwarrenNRE
Here's a photo of the board with the 1016K diff pair and the 330uF feedback capacitor excised. The center pin of the 1016K is a collector. The square solder pad is used by McIntosh to identify the emitter which, on the 1016K, is the correct pinout. The board trace layout however requires the center solder pad to be a base(!). I suspect that the technician that assembled the board for this order simply failed to "contort" the pins to accommodate the proper connection to the board traces(?).

Re: Power Output Boards

Posted: Sun Nov 06, 2016 2:59 pm
by jwarrenNRE
I checked the V-I of the 1016Ks pulled from one board, measure only to -5V C2E over a range of base currents (1ma to 5ma in 1ma steps). I really didn't take the time to check the pair over the expected range of operation but more to see if they were reasonably well matched which they seem to be. Ran each at 5 cycles before taking the final plot (to get then good and hot).

Re: Power Output Boards

Posted: Sun Nov 06, 2016 5:16 pm
by jwarrenNRE
So back at it, I installed a pair of "exceedingly well matched" 2N5087s replacing the 1016Ks on the factory board. I also replaced the factory Elna feedback caps with a Nichicon (the Elnas, which are good caps! were damaged when I spooled the amp up with the factory power amplifier boards installed).

With both boards "repaired" it was time to test the results on the distortion analyzer. Installed a new, repaired board in the L and an old, reworked board in the R. The rework was nothing fancy, new electrolytics, new matched 2n5087s and replacement of three composition resistors with composition replacements.

I gave this amp a good go over. The entire output stage was reworked, new OTs, filter caps, emitter resistors. The multicap was reworked and a new preamplifier board was designed and installed. Thermistors and all rectifiers were replaced. The stereo-mono switch was repaired and new ceramic insulators installed.

It turned on and nothing went "pop" so on to the distortion analyzer!

Re: Power Output Boards

Posted: Sun Nov 06, 2016 11:25 pm
by jwarrenNRE
Ran 1kHz and 20kHz distortion on both boards and the results are below. Analyzer was Krohn-Hite 6900B distortion analyzer with 30kHz low pass filter.
Results indicate the new boards with closely matched 2N5087 differential pairs outperform the older vintage boards by a small margin and both 1000 and 20000Hz. The preamplifier board is the R2 redesign discussed here:

http://www.northreadingeng.com/McIntosh ... amp_R2.htm

For a 45 year old, quasi-complimentary, first generation solid state unit, this amplifier is pretty damn good!

Re: Power Output Boards

Posted: Wed Nov 09, 2016 11:20 pm
by pjameson
The 2SA970, like the 1016K, is a b-c-e configuration and is what Audio Classics was providing for the 132-056 replacement part. Forming the leads to match the right pad locations was necessary as well.

Nice amp, btw.