IFR COM-120B Generator Programmable Attenuator

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ke2d
Posts: 31
Joined: Thu Mar 01, 2007 6:24 pm

IFR COM-120B Generator Programmable Attenuator

Post by ke2d »

Hi All,

I have a COM-120B that someone put 100w into the AUX GEN input by mistake. It took out the 18.2 and 301 ohm 1/4w surface mount resistors in the Power Termination unit, right where the AUX GEN port enters the unit. I was able to replace the resistors and repair the Power Termination unit, but it appears the high level of RF also made it into the GEN Programmable Step attenuator unit. The 10, 20, and first 40db stages still work fine, but the second 40db stage that was inline when the 100w of RF was introduces has been damaged. The generator output on both the RF IO and AUX GEN ports is now off by about 30db on anything less than -50db, but once you get to -50 or higher, it is right on the money, based on the levels being measured into another analyzer. I scribbled down a "truth chart" my measuring the control voltages into the four stages, and the table confirms the issue tracks the one bad 40db stage. The bad stage is now measuring approximately 70db loss rather than 40db.

Has anyone attempted to repair one of these step attenuators?

From what I can see by removing the one and only cover, it looks like the board is laser etched to derive the resistance values for each stage. There may be other parts on the other side of the board, but have not yet figured out how to get the rest of the unit apart, and do not want to risk damaging ir further. Anyone have experience working on these attenuators?

Anyone have a spare attenuator unit lying around that they wish to part with?

Eric
K2CB
jry
Posts: 448
Joined: Fri Feb 06, 2004 3:14 pm

Re: IFR COM-120B Generator Programmable Attenuator

Post by jry »

I would just replace the whole unit.

There are used ones available at a reasonable price.

trying to repair that unit is not something I would do unless there wasn't a supply of used ones.

New they would be a small fortune but that model is NLA as new
ke2d
Posts: 31
Joined: Thu Mar 01, 2007 6:24 pm

Re: IFR COM-120B Generator Programmable Attenuator

Post by ke2d »

I agree, I'll probably just find a used "pull" to complete the repair, but I'd like to try and repair the original unit, if at all possible.

Has anyone actually repaired the stripline board in these things?

Eric
K2CB
lc4600
New User
Posts: 16
Joined: Mon Aug 03, 2009 7:29 am

Re: IFR COM-120B Generator Programmable Attenuator

Post by lc4600 »

I remember IFR days at work and I have seen the RF relays replaced but if the laser trimmed resistors were burnt then the whole unit was done. N0PXJ
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