[R-390] RF deck questions

[Paul H. Anderson]

On Tue, 20 Aug 2002, Andy Williams wrote:

> I finally had time to start work on the R-390 I bought last year.
> Last weekend, I discovered that the RF deck will not mechanically
> tune past the 25 MC band. Since this unit has had some bad repairs
> done in the past, I was wondering if this could be the result of
> installing the RF deck without the green gear? Or is this likely to
> be a mechanical failure?

One of the cams in the 390/391 RF deck will reach a mechanical limit if
the unit is not aligned properly.  On one of my R-391's, this means that a
motor can drive the cam to the point of failure (clamp or gear on that
camshaft).

> Which brings up another question: What do I use to clean and
> lubricate the gear train? Will SuperTech General Purpose Lithium
> grease work?

As Barry mentioned, a synthetic oil won't dry out quite so quickly.  I
have used Mobil-1 to good success.  Someone here was suggesting Mobil-1
90W gear oil, which I think is a good suggestion, too, but I haven't tried
it.

This may be heresy, but I used a dishwasher to clean several of my RF
decks (391 and 390A).  If that amount of water is going to cause problems,
they were about to break anyway.  I'd rather spend time fixing what the
water broke that trying to clean the damn thing as well as the dishwasher
can do.  I do carefully rinse with distilled water and alcohol afterwards,
followed by air hose and dessicant drying.

> So far I replaced 1 tube socket in the power supply, the 4 47 ohm
> resistors in the power supply and half the power supply wiring
> harness. In the audio module I replaced C607 as a precaution, the 4
> 47 ohm resistors, fixed several bad solder connections and replaced
> some of the wires. The B+ regulates at 190v after replacing R626.

I recently worked on an AF deck that I had to replace all components in
the VR portion of the circuit.  There are a couple of caps (maybe C607 is
one), but also a mica cap, I think, that seem to give problems simply from
exposure to heat.

I think you should aim to do better than 190V, simply because if the
components are to correct value, and the tubes are ok, you should get
180V.  Better to fix now than fix later, in my opinion.

Good luck - it is a great radio to work on!

Paul