MA6100 Power Supply Board

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I was asked to repair an MA6100 that is missing the power supply board.  The owner said he purchased it from a buyer that told him the amp was missing the board.  I actually own an MA6100 and, when I serviced it some years back, took a few photos of the power supply board.

Here's a photo of the board, a turret strip type.  Simple but not that straightforward to replicate.  The board itself is custom made for the amp.  This will take some thought.

 

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I've completed the PC board layout of the new board.  I intend to install one in my amplifier as well so I removed the board from my MA6100 and used it to design the new one.  The customer's unit is here, still packed up. 

Here's the board pulled from the amp.  More typical of a tube era build using a turret strip and relatively low cost parts.  The two wire wound resistors have large voltage drops across them and, as a result, get very hot and scorch the board.  They're 5W rated.  My unit shows some distress but I've seen it much worse on other MA6100s.



 

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Here's what you have to work with once the board is removed, leads are prepped to accept the new board. 

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Here's a photo of thermal distress that the factory board will experience (pulled from the internet).  The step down resistors are causing the board to bake, the leads are also involved in the distress.

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R1.0 demonstrator board fits, always a good thing!

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R1.0 demonstrator/prototype Complete!



 

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Ready for bottom chassis cover.

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The demonstrator worked flawlessly.

I have another customer interested in a power supply board for his MA6100 so I've revised the R1.0 to 1.1 configuration.  The R1.1 will incorporate solder pads and silkscreen footprints for AC pulse film capacitors in the output stage bridge rectifier circuit and general purpose polyester film in the output stage RC circuit.  The thu holes for the transformer leads are larger in diameter to facilitate easier looming of the leads.  The holes will also accommodate passing thru of #6 forks which could be soldered to the ends of each lead and allow screw disconnects of the lead to the Keystone terminals.  The traces are duplicated on the bottom of the board resulting in 2X the total copper shown in the figure below.

I would prefer to use wire wound, non-inductive power resistors for R313/314 but cannot find them.  I did manage to find a supplier for NIB Sprague KOOLOHM, non-inductive resistors and ordered a box of 1500 Ohm, 5W axials for this project.  The 1500 Ohm resistor (R314) operates at its rated power and gets quite hot.  The orginal McIntosh parts are simple wire wound, power resistors potted in ceramic boats.

In this design iteration, I've provided a location to use a PCM 5 film cap across each of the mains diodes.  Either a ceramic disk Z5U or Polyethylene film such as the WIMA FKP2 would be suitable. 

Top copper:

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Here is the R1.1 version of the power supply board.

To make the board easier to install, the thru holes for the transformer leads are larger.  There's a footprint for each capacitor across the main power supply rectifiers and the RC filter at the output.  Both will accommodate either a ceramic disk or any PCM5 film type such as a WIMA AC Pulse polyethylene for the rectifiers or bypass type for the filter. 

Board top:



Bottom:



Installed:

   

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Getting back to the Sprague Kool Ohm resistor (1.5k Ohm, 5W).  I measured the impedance of the three purchased.  Then compared the response to a Dale simple wire wound, also a 1.5k Ohm, 5W.

Here's the two resistors.



Here's the impedance, GRN is the Dale and the Kool Ohm is RED.

Not what I was expecting!  Both the Dale and the Sprague are resistive to about 10kHz until the reactance begins to become measurable.  The result implies the same winding method in each.