Does anyone know where I could download a Hickok 615 sweep generator schematic? I checked all the usual places and can't find one.

Chris:

PM your e-mail address to me and I'll send you the manual and schematic.

Thanks to Tim I now have a schematic.
Resistor 109 is dangling by one wire. I see from the schematic it is supposed to hook to the "mod unit". This mod unit looks like a miniature gum drop and has two leads coming out of it, one going to the 6J6 plate and the other going to the coil. I see no lead to hook to the resistor. It must be broken off at the body, but I cannot find a stub. Does anyone know what is in this mod unit? The parts list gives no description other than Hickok part number 3110-52.

Anyone have any ideas on this mod unit?

It was hard to get a picture of it nestled down in there. This thing is about the shape and size of a LED. If I could find out what is in there, maybe it could be replicated with individual parts.

No one knows?

After studying this thing some more, I still cannot find the stub of a broken lead on that mod unit. It only has 2 wires coming out of it. If someone with 615 could take a look and see what R109 is actually hooked to it would be much appreciated.

Anyone?

Chris:

I have your answer. I pulled my 615 out and opened it up. The 100K resistor connects from the feed through to the terminal of the coil that physically faces the feed through. Same terminal that the small ceramic cap (C110?) connects to.

Like yours, that yellow plastic blob only has two leads coming out of it.

I'll leave it open for a few days in case you need something else.

Tim, thank you very much!

I hooked the resistor to the coil and now it seems to be functioning correctly.

Thanks again.

Glad it is now working Chris. I'll button it up.

I tried to use this thing the other day and found it didn't have much sweep width. I opened it up and found resistor 109 had drifted from 100K to 190K, so I replaced it. The new resistor gets hot and smokes. (Guess I found why the old one had drifted. ) If I unhook the "mod unit" the resistor stays cool, so it looks like I need to figure out what that thing is. Below are some pictures of the "mod unit" with the last two showing what can be seen through the plastic. Could this be some type of varactor diode? It measures 15K ohm either direction.
Note that the schematic shows R109 going to the mod unit. It actually should be shown going to C110 and the variable inductor.

I don't know for sure, but my guess would be a varactor too.

I took a quick look around at various electronics suppliers and didn't see any varactors that would even come close to the specs needed here. It would need at least a 1500v diode. I did a quick test with two 1n4007 diodes in series in place of the mod unit, and of course the oscillator would not start. I'll try bypassing the diodes with a small capacitor although I don't expect the rectifier diodes to work very well.