Friday, December 23, 2011

Restoring the Heathkit Jr 35

Heathkit Jr Electronic Workshop "35", Model JK-18

The Heathkit Jr 35 evidently had been stored with batteries in the battery holder, which corroded badly. I replaced it with a plastic RadioShack 4 D-cell battery holder (270-389), wired into the negative power rail and the hot side of the power switch (which is part of the variable resistor).
Corroded battery holder
Replacement battery holder mounted
I mounted the replacement battery holder with a dozen "heavy duty" 1 inch foam double-stick mounting squares (rated to hold 900g), stacked in four groups each three high to overcome the various screws protruding through the main board.
The remote speaker station

The rubber foot used to protect the relay from being crushed when you turn the kit over was stuck to the remote speaker station. I put it in its rightful place.
Relay with the brown rubber foot
Earlier, I had built a couple simple circuits, which showed the meter and new battery pack working, but showed me the telegraph key switches were not making good contact. I used a Scotch-Brite scrubbing pad to remove corrosion from the key switches on the main board and the remote station.

I ran through the tests in the manual appendix to check out the lamp, power switch, speaker, earphone, antenna coil continuity, remote station speaker & telegraph key, slide switch, and relay.

I then jumped ahead of myself, building the 4-transistor AM radio experiment; I got slight hints of a audio signal if I wiggled and touched some of the wires. I'm highly suspicous of the electrolytic capacitors. I probably should have tested those (and the transistors) before using them in such a complex circuit. The wire connections to the springs are not all that reliable, and the wires might have some oxidation on them.