[R-390] RE: Antenna Trimmer

[Laird Tom N]

> I don't have my R-390A yet, but I was wondering about it anyway. Does 
> the R-390A have an antenna Timmer? If so, what is it's characteristics?
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here are some old posts from my archives:

Tom Laird WC9M
Moline, IL.

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>Got around to installing the worm gear assembly back onto the antenna
>trimmer bracket so I could adjust this from the front panel (I had been
>turning the gear with my fingers from the top). I now notice something:
>if the teeth of the gears are engaged, the signal increases about 10
>to 15dB. I noticed this when I rocked the knob back and forth. It goes
>from engaged to not touching to engaged in the other direction with the
>signal drop occurring during the disengaged portion.

There are a couple of things you need to watch out for
on the antenna trimmer. One is that the insulating bushing and
thrust washer for the shaft that goes to the front panel are
intact and oil and residue free.

The second is that the insulating washer for shaft that enters
the can itself isn't cracked or mispositioned. It's got a tiny
little step on it that is supposed to center it in the hole. It
usually doesn't. I've seen number of RF decks where it was
mispositioned and would allow a short when the gear was loaded
in one direction or the other resulting in sensitivity going South.

Kill the power to the set and measure the resistance of the
shaft and gear that goes into the can to ground. I don't have
the manual in front of me but it should be very high, hundreds
of K maybe. Keep the lead on the center of the shaft/gear and
start rotating the trimmer knob. If the resistance reading
drops, you've probably got a misaligned insulating washer or
some debris in the can.
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The worm gear on the shaft was touching the shaft, thereby grounding it.  
I left the gear too far forward allowing it to touch the shoulder on its
shaft. Not only did this ground the gear, it left way too much axial play
in the shaft. I loosened the locking screws and pushed the gear away from
the shoulder (about 1/16" or more) to where there is almost no axial play
in the shaft. This stopped the intermittent grounding and now the shaft
stays in the same position along its axis.