Batboard

Hey folks,

Picked up an IFR 1200S. When I started going through it comparing it to my HP 8920A, I noticed that the generated signals jumped around by 1k. Here's a video of its shenanigans...

https://www.youtube.com/watch?v=OrlWU8UsveY

I think I've isolated the problem down to the Fast Low Loop board. Does anybody know of a specific place to look? Could this be caused by the 77-79 MHz VCO, the 18.8 MHz oscillator, or the 100-200 VCO? I'm thinking something is going crazy in the steering voltages. None of the unlock lights come on, though.

Thanks,

Jared

Turned out to be the 18.8 MHz oscillator. Steering voltage that runs the varactor was varying up and down.

I changed the zener diode that feeds VCC voltage to the phase comparator (6.9V) that runs the varactor on the 18.8 crystal. Just in case, I changed the tantalum cap that's on the zener and also changed the varactor. It was somewhat difficult to change because I had to remove a couple of shields on the solder side of the board. All in all, it ended up costing me about $6 in parts on ebay and about 6 hours labor (that includes studying schematics, diagnostics, disassembly, repairs, and reassembly).

Here is the schematic of what I replaced.

Here is the board layout of what I replaced.