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IFR 1200S Fast Low Loop Problem

This forum exists for the purposes for discussing service monitors (This includes but is not limited to Motorola, HP, Aeroflex, GD, etc). Additional topics allowed include test procedures, interpretation of test results, where to find information about specific tests, antenna VSWR, return loss testing, duplexer and filter alignment, etc.

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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

Postby ke4zdg » Mon May 23, 2016 7:55 am

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

Postby ke4zdg » Thu May 26, 2016 9:05 am

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

Postby ke4zdg » Fri Sep 09, 2016 6:13 am

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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