Saturday, October 24, 2015

McIntosh MA5100 Spotlight

McIntosh MA5100 Integrated Amplifier 

We had a pretty ruff McIntosh MA5100 come in a few months back after bearing some rough Wisconsin winters. The chassis had yet only minor pitting evident with the screen printing all intact and undamaged! The MA5100 was a integrated two channel 45 watt/CH sold in the late 60's to early 70's. Amongst McIntosh fans of even the neediest requirements, the MA5100 is held in regards by many.  I wanted to highlight the PCB sections here beyond the cap stuffing.

Pre-Amplifier PCB 043-723 & Filter 043-722 PCB
One thing I like about these MC's is the use of quality Mallory axial capacitors. Typically axials seem to be of inferior construction compared to their radial counterparts so finding a capacitor with high expectations an be tricky. I like the TVX axial offered by Nichicon but in this case I decided to go with the Sprague ATOM axial series which is a low leakage high quality long life axial available from 6V-500V. After trying several MKP and WIMA PT and PP type I settled on using Panasonic ECW polypropylene type films for the rest of the stage with impressive results and sticking with carbon comps.   

Like the symmetric pre-amp design, I decided with the filter assembly to utilize ECW's polypropylenes again and carbon comps. Whats nice is the ease of access here, undoing the standoff screws you can open-book the PBC's forward, this also gives you great room to buff the chassis here without depopulating the entire chassis. 

Drivers 043-724
The driver sections are a fairly simplistic push-pull topography. Again I went with Sprague Atom axial capacitors with a polypropylene ECW compliment, I preferred the ECW's to say the MKP's I use in Marantz pre-amps. The differentials pairs were mismatched due to a repair in its history so matching Hfe On Semi BC556 were utilized in place. Also noted were mismatched output devices MC#132-513, On Semi manufactures some excellent replacements MJ15003 20A/250W BJT devices.


PCB Assembly 043-724

Auxillary 100/200 microfarad Mallory caps for the coupling/drivers outboard were replaced with Sprague Atoms and .01 microfarad PT type ECQ' Panasonics. New #1864 bulbs were installed while we work on a proper 28V LED replacement with proper disbursement. I'm a big fan of using Flitz polish for points around the can cases, knobs and RCA buffing. Sometimes short of re-plate theres not much you can about pitting but prevent further oxidization.

I was quite impressed with the final product and will be looking forward to another, it paired quite nice with an array of speakers we tried. Each stage seems to consistently blend well with no significant lower end lag or top out. 

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