Monkeying with the MON-KEY

Skip Freely, K6HMS

I recently acquired a Mon-Key to replace one I had from 1959 through 1962. It is in great shape, but after plugging in into a Variac and bringing it up slowly to 120VAC, it just hummed loudly and refused to respond. Since my key collection is a working collection, I decided to fix it. My comments and information might be helpful to others.

When I removed the cover (3 screws; 2 at top of control panel, 1 under unit at center of opposite end) for inspection, everything appeared to be in good shape, except for the wax-filled capacitors, which obviously needed replacement. The resistive line cord was in fair shape, but a kink in the middle was carefully worked out, and some shrink tubing added for strength. I measured the resistance of the cord to be 360 ohms cold, and remembered that the voltage drop across the cord resistance was about of the total of the cord and the filaments of the 3 tubes.

If you every used a Mon-Key, you occasionally experienced accidental contact with the key lever or other metallic terminal, which gave you a nasty shock you never forgot. After recovering your senses, you also sent mental lightning to Wiley Wenger, the designer of this diabolical beast. I have a 1:1 power isolation transformer (Triad N-53M, 115/115VAC + /-5%, 0.74a) I use for testing of transformerless electronic devices like the MonKey; one side of the 120VAC line is attached directly to the chassis, so you get a nasty shock from contact with the circuits. The isolation transformer permits grounding of the chassis while working on the unit, and eliminates the risk of electrical shock.

N7CFO provided me with a 5 page document on the Mon-Key. While it contained an incomplete schematic, it did not provide the component values of locations within the unit. To support my work, I generated a schematic diagram and physical layout with more information, which were helpful. I hope to find more time in the future to identify and list all of the other components, particularly the resistor strings Ra, Rb, and Rc.

I removed capacitors (Cl, C2, C3, C4, C5, and C6) from the unit, and determined that the values were as follows:

Cl, C2         Industrial Condenser Corp., Chicago, IL; .01 mfd., 60OVDC; 3/8" diameter x 1 3/8"                      long; type PT135.

C3              Mallory; .003 mfd., 600VDC; 7/16" diameter x 1 " long; type TP406,

C4, C6       Mallory; .01 mfd., 60OVDC; 7/16" diameter x 1 " long; type TP406.

C5               Cornell-Dublilier; 40/40 mfd., 1 5OVDC dual electrolytic; 1 " diameter x 1 3/4" long.

It is hard to find exact replacements for these older components, since most electronics parts stores carry lower voltage capacitors suitable for modern solid-state circuits. After looking for exact replacement parts at several swapmeets, I substituted the following components:

Cl, C2, C4, C6   Sprague; .01 mfd, +/-5%, 50OVDC; 7/16" dia. X 1 1/4" long; type 7l5P.

C3                      Sprague; .033 mfd, 60OVDC; 3/8" dia. X 1 1/4" long; type 6TM-S33.

C5                      2 each Cornell-Dublilier; 40 mfd, 45OVDC; 7/8" dia. X 2" long; type                            WBR40-450.

After replacing these components, I again brought the unit up slowly, using the isolation transformer. It still did not work, and I checked the tubes (2 - 12AU7, 1 - 35W4) on a transconductance tube tester. The tubes were pretty well shot, so I obtained "new" tubes at a swapmeet. After replacing the tubes, the unit worked fine. As an aside, the capacitors Cl and C2 can be changed to .01 5 mfd, or the present .01 mfd capacitors paralleled with .005 mfd capacitors to lower the speed range if desired.   A little time spent cleaning up the key paddle mechanism, and careful adjustment, has brought back the feelings of old. The MON-KEY is dated and nostalgic, but when set up properly, can produce excellent code. 

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