MC2100 restoration

[nre]

The new rubber feet attached.  These have a metal washer molded into them, they're also rubbery.  The sheet metal screws are a bit longer, stainless steel.





   

[nre]

Back at the chassis.

Buffed out most of the light scratches using polishing paste.




I replaced the Cinch terminal block and replaced the hardware with stainless fasteners.  #6-32 x5/8"

   

[nre]

Soldering the terminal blocks.

Form the leads around the hook.



And solder, like we were never here!

[nre]

When removing leads from the edge connectors (Amphenol parts that are NLA) the "trick" to prepping the solder lugs for new connections is to first trim as much of the wire stranding and solder as possible from the lug using a sharp wire cutter. 

The connector can be destroyed by pulling or twisting the stranding from the lug and if that happens

you're screwed.

[nre]

The large filter cans can be replaced.

Do they need to be replaced?  Who cares, just replace them. 

[nre]

Filter caps installed.

And don't pay resellers on our favorite auction site for these capacitors.  They're about $63/ea from Mouser.

[nre]

The pots have to go, they work but are tamper resistant, ie they will not accept a knob and must be adjusted using a flat blade screwdriver.



They're CTS 500kOhm linear pots with a 470kOhm +/-10% resistor across lugs 1-3 to modify it to a 250kOhm linear.  The solder job to connect the pots to the circuit looked a bit sketchy making me think these were added by an owner.  There's nothing special about these and a panel mount, 250kOhm audio-taper with 3/8" bushing and 1/4" shaft will do the trick.  Due to the type of pot used, the amp doesn't have input level adjust knobs which will change too. 

A long-shaft bushing (similar to what's used in guitar pots) is a thought here as well.  It would allow for setting the pot back a bit from the panel and make it easier to service.



 

[nre]

AC simulation of 250kOhm pot with and without a taper resistor and how it alters the preamplifier voltage output from 5 to 95% cw rotation.  When the vertical axis is a logarithmic scale (dB), a log-taper should show, approximately, equally spaced output levels for equal, incremental turns of the knob, in this case each increment is 1/20th of a full turn.  The predictions on the right are with a taper resistor across the wiper to gnd.  The taper resistor is 15% of 250KOhm (~37.5kOhm).

[nre]

Will install two, 250k, linear, Clarostat/Honeywell pots to replace the ones on the unit.  These are pricey at ~$17/ea if you can find them.  They're sealed construction, so nothing gets into the slider assembly.  And made in USA!

Part Number: RV4NAYSD254A



Datasheet here:

35dfa41e9654ac5abb8722a713c095c2.pdf

[nre]

The Clarostats installed.

[nre]

Completed the right channel output stage.  Replaced all six transistors with ON SEMI MJ15003 NPN (quasi-complimentary output stage).



Datasheet

MJ15003-D.PDF

[nre]

North Reading preamplifier installed.



New power output boards and output transitors installed.



Vintage knobs,

 

All buttoned-up,





   

[nre]

1kHz %THD v. Watts



1kHz %THD v. Volts

 

[nre]

%THD measured using Krohn-Hite 6900B at 20VRMS out, one channel,

Voltage



%THD at 20VRMS



 %THD measured at 5VRMS out, one channel,

Voltage,



%THD at 5VRMS



The distortion number are similar to those measured in the CLIO FFT analyzer.  Note the Krohn-Hite unit has significantly higher measurement bandwidth (5Hz-1MHz) than the CLIO unit (10Hz-192kHz).

[nre]

On older McIntosh amps the sheet metal fasteners that hold the chassis bottom cover might no longer grab the chassis when tightened.  This can be remedied by first filing down the upset metal on the chassis (not the cover) being careful not to gouge the area around the hole and then replacing the #6 x 1/4" with a #8 x 1/2".  Both the #6 and #8 have the same size hex head, the flange being a tad larger (a good thing).