IFR 1200S Fast Low Loop Problem

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ke4zdg
Posts: 85
Joined: Fri Jun 30, 2006 6:05 am
What radios do you own?: Spectras, Sabers, MCS/MTS2000

IFR 1200S Fast Low Loop Problem

Post by ke4zdg »

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
User avatar
ke4zdg
Posts: 85
Joined: Fri Jun 30, 2006 6:05 am
What radios do you own?: Spectras, Sabers, MCS/MTS2000

Re: IFR 1200S Fast Low Loop Problem

Post by ke4zdg »

Turned out to be the 18.8 MHz oscillator. Steering voltage that runs the varactor was varying up and down.
User avatar
ke4zdg
Posts: 85
Joined: Fri Jun 30, 2006 6:05 am
What radios do you own?: Spectras, Sabers, MCS/MTS2000

Re: IFR 1200S Fast Low Loop Problem

Post by ke4zdg »

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. Image

Here is the board layout of what I replaced. Image
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