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R-390A Alignment




 
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W7SOE
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« on: March 15, 2008, 02:05:51 PM »

Working my way through the RF alignment and ran into a bad trimmer cap in an RF can.  These caps seem to be integral to the assembly.  Is there a way to replace them?  Are they available?

Also, some of the antenna transformer slugs are peaking VERY low, is this indicative of a bad mica cap?

Thanks

Rich


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w3jn
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« Reply #1 on: March 15, 2008, 03:34:28 PM »

If there isn't a nice sharp peak (with the exception of the antenna coils) you have a bad S/M cap or a bad trimmer.

I've never had to replace a trimmer in these and it's been years since I've had a can apart so I can't give you any advice there.

I have a 390A on the bench right now that'll peak at the bottom of the 16-32 MHz band fine but won't track up top.    I suspect it might be one of the S/M coupling caps so I'm gonna hafta pull the RF deck after I verify it isn't the bandswitch gone outta sync.
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WA1GFZ
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« Reply #2 on: March 15, 2008, 03:38:45 PM »

John, make sure the slug is ok and not missing a hunk of metal.
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WV Hoopie
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« Reply #3 on: March 15, 2008, 04:28:19 PM »

Rich,

The trimmers should give two peaks, either peak will work. A peak with the slug low in the holder is a sign one or more of the silver mica caps are bad. The silver has migrated through. Mouser or Newark has them and I've replace several in my R-390/A's.

The trimmers can be disassembled by sliding the retainer to one side. The top section that turns will have some silver material which has been transfered onto the bottom surface. A q-tip with some De-Oxit will remove the plating that has rubbed/scrapped onto it.

The epay place has em for auction every so often.

73's
wd8kdg
Craig
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W7SOE
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« Reply #4 on: March 17, 2008, 03:17:52 PM »

The trimmers can be disassembled by sliding the retainer to one side. The top section that turns will have some silver material which has been transfered onto the bottom surface. A q-tip with some De-Oxit will remove the plating that has rubbed/scrapped onto it.

Indeed, I was able to bring three dead rf cans back to life by disassembling and cleaning the trimmers.  On one of them I broke the coil wire so it lost a couple turns when I fixed it.  It aligned fine though.

Thanks for the good advice, the alignment is done!

Rich
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