MA6100 clean up

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With the power supply board replaced in my own MA6100, and the thing still on the bench, a cleanup seemed to be in order.  The preamp and output stage chassis screen cages are easily removable, there's two sheet metal screws holding each in place.  Remove those and the cages will fall free from the chassis.

This unit has been in continuous used since 1976, the year I purchased it new from Tech HiFi, Cambridge, MA!  I've only been in it once, to replace the large filter cans and a few of the electrolytics.  I also brought it to a McIntosh clinic, it was opened up there as well. 

[nre]

There are five boards located in board edge connectors, each is held in place with a L-bracket that pulls the board against the chassis, remove the screws and wiggle-lift the boards off.  A squirt of electronic grade alcohol where the board is inserted in the connector makes this effort much easier.  The finger contacts are Pb-Sn plated.

The two in the bottom of the photo are the preamplifier boards, the two above those are the power output boards and the one at the top is the phase inverter/voltage regulator board.

[nre]

The boards are single layer, no silk screen.  The component layouts are straightforward, all parts are easily accessible.

Preamp board


power output board:


inverter/regulator:
\   

[nre]

On one power output board, there's a transistor missing solder cups.  These are staked into the board from the top-side using a simple, handheld fixture.  I believe they were removed during a clinic visit, I didn't remove them and I'm the only owner of the amp, bought in new in 1975.  The transistor is an MPSA05, the complimentary to the MPSA55. 

That said, I have a few boards that I can cannibalize to extract these NLA parts from.



 

Done:

[nre]

Chassis with boards removed. The amp is in near perfect condition.

[nre]

Here's the parts list to replace all of the EVOX MMK (metallized polyester) with WIMA MKP10 (double sided metallized polypropylene).  WIMA lead spacings are equivalent to the EVOX.  The cases are a bit wider.

[nre]

There are eleven (11) Tantalum caps in the preamp and phase inverter boards.  The factory used ROE caps that, at the time, where considered high-end tantalums.  ROEs are good quality manufactured in Germany and now owned by Vishay.  There's nothing particularly high end about them.

Tantalums are higher in cost than Aluminum can electrolytics of equivalent voltage-capacitance.  The capacitance however of solid Tantalums is less sensitive to frequency, voltage and temperature effects.  The equivalent series resistance across is typically lower than an equivalent Al-cap which reduces losses in small signals, i.e. preamp signals.  DC leakage current is lower too.

For the MA6100 I'm using the moderately priced AVX TAP series.  They're clearly laser marked with polarity, capacitance, voltage and tolerance and easy to install. 





 

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One of the two preamplifier boards with new WIMA polypropylene and Ta electrolytics.  Tantalum caps are about 6-10x the cost of Al-electrolytics and the factory was using Al-electrolytics where Ta electrolytics were beign specified in the SM.

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There was little to do on the power output boards.  I removed the heat sinks from the P-P complimentary driver stage pair and  VAS stage, cleaned the sink and the mica insulator and applied compound.  I apply a "dose" of compound to one side of the insulator, locate in on the heat sink, lighty press it down then apply a second dose to the top side.  Slide it under the transistor case and then fasten it with the screw.  If done correctly a small amount of compound should squeeze out.

The VAS gets quite warm, it's providing the majority of voltage gain in the amplifier. 



 

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Installed all the boards and gave it a hard run under the distortion analyzer.





LEFT channel %THD



RIGHT channel %THD



L and R channel bandwidth

   

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There's a typo in the MA6100 SM I have, the original from the day I purchased it new in 1975!

The 100uF/16VDC bypass capacitors C123 and 124 in the preamplifier boards are shown correctly configured in the schematic of each, they're not correctly configured however in the part identifier board layout for each.  I've shown the discrepancy below.



If the cap is installed as shown in the schematic, all's well otherwise you'll see severe, distorted sine wave signals at the output of the preamp RCAs.

Note the factory board below (Nichicon, light blue Al-cans).  Negative pole side of the can oriented left, both cans.  The factory used Al-electros whilst the SM calls out Ta-electros.  Surprisingly the cap I installed didn't burst!



Note also the solder flux residue on the board, it's everywhere.  It's visible at the base of the transitor solder cups.

In the reworked board, a part/human friendly solder flux remover followed by electronic grade IPA rinse was performed.  The back side of the boards were heavily laden with residue.  They clean up nicely.