[R-390] R-390 zero adjust problem fixed and have more questions.

[Mark Huss]

For what it is worth, you have to be careful with the R-390 (non A) zero 
clutch. They changed the design in the R-390A because it is so easy to 
break the clutch on the R-390 by overtightening it.
1) and (2) are probably related to some extent. Go ahead and linearize 
the PTO, you will have to do it anyway. Another test, especially if you 
have a frequency counter, is this. Remove the shield from the Second LO 
Tube. Wrap about twenty or so turns of #28 or #30 wire around it about 
in the middle. Secure with tape, or if it holds its shape, just twist 
the ends to keep it compressed. Now, wire the ends to the input of the 
frequency counter. This will let you read the frequency of the second LO 
directly. Then run through the bands, monitoring how far off each 
crystal frequency is. The results may surprise you. Crystals not only 
drift with temperature, they age 2 to 5 ppm per year, and temperature 
accelerates this. After fifty years, drift would be about 4250 Hz on a 
17 Mc crystal. Since heat accelerates this process, and the crystals are 
in an oven, you can expect more. Maybe more than 8 kHz. One of the 
design compromises in the R-390 series was no trimmer. But then again, 
who knew these things would have an operational life of better than 40 
years, and still be going strong after fifty. :-)
(3) is the antenna relay. It is designed to switch the antenna input 
from grounded to input, normally grounded. With little current through 
the contacts, they gather oxidation, so you might have to tke one apart. 
Also, they are designed to close at 26VDC. The rectifier for the coil 
circuit is a selinium rectifier. It has a voltage drop of about 1.2 volt 
per plate. And this voltage slowly increases with age. So check the 
voltage at the coil. If it is less than 24 volts DC, you need to change 
it. Make sure you put a resistor in series with the silicon diode so 
that the voltage at the relay is no greater than 26.5VDC (average, there 
will be half-wave ripple at the pin)
(4) Squelch I can't help you with. Except to explain what is already in 
the manual

Bob Young wrote:
> I fixed the problem with the zero adjust that I caused the other day 
> by screwing the shaft in too far. I took off the panel with the help 
> of the manual which will be my 390 bible from now on.
> Basically the clutch was not releasing enough to engage again. I took 
> a screwdriver and applied pressure between the little round thing 
> between the big gear nearest the panel and the cotter(?) pins. I had 
> to do this more than a few times, but it worked, it's back together 
> again and working fine.
> Ok my first questions
> 1. Some of the bands are beyond the range of the zero adjust to zero 
> beat them, this is usually by about 3 KHz. Ex. band one 500-1000 KHz 
> is 3 KHz low at thehigh extreme of the zero adjust, band twe is fine, 
> plenty of room at both ends. Is this hopefully an alignment problem? 
> This couldn't be a crystal problem could it?
> 2. Related (maybe?) The Cosmos PTO in the radio is way off probably at 
> least 10 KHz end to end, I'm not usre if it is the same amount off one 
> each band.
> 3. The crystal calibrator works (barely) but the tuner section doesn't 
> shut off and it is hard to hear sometimes.
> 4. The squelch control doesn't seem to work either.
> I'll start checking tubes tomorrow as the sensitivity seems to be down.
>
> Any help will be appreciated, thanks,
>
> Bob Young
> Millbury, Ma
> KB1OKL
>
>
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Some people are like a Slinky .. not really good for anything,
but you still can’t help but smile when you shove them down the stairs.
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