replacement for a McIntosh C20 preamplifier.
The first step in this ‘endeavor’
is to have on hand the schematic for the Mac C20.
One source is Sams Photofact
#475 folder four. I believe the McIntosh company will also sell you
a schematic for the C20. Another source is http://www.vintageaudiomanuals.com/search.htm
The next step is to determine
the C20 model that you are working on. The early model has a brass
bar across the front panel ands the volume control does not have a cover
over it. I replaced the attenuator on an early model. See if
the unit does work. Now unplug the C20 from the power source.
Remove the knobs, front panel,
and the plastic indicators on the top row of switches and pots. Remove
the bracket holding the two lamps that surround the volume control
Now, the hard part.
Remove the wires from the volume control. Mark where the wires go.
I found that new coax has to be installed from the balance pot and
the mode switch. Note the location of ONE of the ground wires.
It SUPPLIES the ground to the control and also two resistors under the
chassis. This ground wire comes from the front circuit board. Cut
the power leads going to the power switch on the old control. Now
remove the old control.
Since the Gold Point attenuator
is longer than the original control, the electrolytic capacitor (C1 in
the Sams) behind the control will have to be relocated. I added an
insulated solder terminal and a ground terminal on the inner side of the
chassis, right under the old location of C1. I had to drill
a couple of holes in the side. I used two 40 mfd @ 450 vdc caps and rewired
them according to the schematic. I covered the hole with an aluminum
plate mounted from the bottom of the chassis.
Since I was under the chassis,
I replaced the filament rectifiers and its 2000 mfd capacitor. I
used a dual positive center rectifier assembly equivalent to NTE
6086. Use the hole the old rectifier terminal strip to mount the
rectifier assembly. A couple more terminals where the old 2000 mfd
cap was located, and you are operational.
You could used the original
grommets in the chassis, but I found the original ones very hard.
I replaced all of them in the attenuator area.
The mounting hole for the
attenuator will have to be enlarged. The Gold Point attenuator
will be mounted using both nuts that are furnished. A coupler is
required for the Gold Point – C20 modification. The coupler
fits between the chassis mount and the lamp bracket. The couple
is drilled for 1/4 inch shafts and the Gold Point shaft is metric.
A single layer of black plastic electrical tape will take up the slack.
Make sure the solder points on the Gold Point are facing up.
A shaft extension with a flat for the knob will have to be manufactured.
I imagine you could cut the shaft off from the [old] control.
offers a shaft coupler which you can use here. See: bottom of page:
I also made a bracket to
hold the rear of the switch. I used the mounting screws for the power
switch to attach the bracket to the switch and one of the holes that held
the original FP type capacitor.
Now, to wire the attenuator.
The first thing is to twist some #22 stranded wire and run it form the
AC receptacle to the power switch on the Gold Point.
Run a #20 solid wire through
the four grounds on the printed circuit boars of the switch and solder
Do the coax first.
The coax is grounded at the switch and NOT at the balance control.
I always like to add a short piece of shrink tubing to the ends of the
coax. You will need to cover the ground shields at the switch with
shrink tubing. Note that the rear balance pot center tap goes to
the front 100K printed circuit plate ’in’ terminal. The ‘out’ terminal
of the front plate (A) goes to the mode switch ‘single’ terminal.
But you know that since you marked the wires and cables. The (B)
output goes to the terminal with the wire jumper.
The final wiring is from
the AURAL COMPENSATION pot (center tap) to the ‘in’ terminal of the C plate
and ‘D’ plate. At the same time you have these wires to replace,
replace the ground wire from the from PC board to the ground buss on the
Run the ‘out’ wires from
the one meg plates to the correct capacitor inputs under the chassis.
Note that there is a ground wire from the junction of two 47K resistors
that has to be run to the ground buss on the attenuator.
You are about done.
After connecting inputs and outputs, power on the preamplifier. You
will note, assuming you made no errors, there is a click when the attenuator
is rotated. This is the make break of the switch contacts.
I do not know if there is a fix.
The last ‘suggestion’ is
to make a cover plate over the switch to help keep little fingers from
the power switch and deflect the spillage of beverages into the Gold Point
written by: Robert
Walters (503) 284-8693 email: firstname.lastname@example.org
4105 N.E. Alberta Court,
Portland, OR 97211