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.

Saturday, October 1, 2011

Heathkit Jr. 35

I just picked up a Heathkit Jr. "35" circuit experimenter kit on eBay. I had one of these as a child; I don't remember how it is that we came to own it, but I had an itch to re-read the manual. Buying the manual from would have cost as much as the kit + manual. I remember being befuddled by the descriptions of how transistors worked. It turns out the descriptions of capacitors and transistors were a bit sketchy: I think it is pretty much impossible to understand how the AM radio experiments actually function from the text. The operation of the ferrite-core antenna and the various windings are unexplained as well.

I was a bit surprised to find all four transistors in the kit are identical. Thanks to this Heathkit part cross-reference, I see that they are part 417-118, a.k.a. 2N3393 NPN transistors.

Saturday, September 24, 2011

Importing from

I'm working on importing my blog from apparently will let me use AdSense even with free hosting, and I should get to know more about AdSense and about the Blogger platform as well, as a Googler.

I used the export feature to create an XML file, and used the tool to convert it, as I was under the 1 MB size limit. The code behind that converter is evidently at

That tool plus Safari saved the result with an .xml.html extension, which resulted in a Blogger error code of "bX-uroqxm" when importing. Removing the extraneous .html allowed the Blogger import to mostly succeed.

It does not re-map my Wordpress author identity to my Blogger profile. It also does a pretty poor job importing my source-code formatted content. So I'll be slowly publishing the old posts, tweaking that kind of thing as I go.