[R-390] R390 Band Switch Alignment

[Dan Merz]

Hi,  ok,  here's some more data to chew on.  The 390 non-a seems to have
settled into working pretty well on the lower bands except for the 7 to 8
Mhz range.  At least I can consistently hear the calib oscillator on most
others. I put my scope on the oscillator input to the cathode of the 2nd
mixer tube - the rack gets in the way using a tube extender on the 1st mixer
so I focused on the 2nd for now.  The oscillator output should be 20 Mhz (10
doubled) for this band.  It is,  but the waveform is badly distorted and at
least half the amplitude of most of the output for most of the other bands.
If I look at the 17-18 Mhz range where the same crystal is used,  again the
waveform is distorted and even smaller in amplitude.  The distortion is
mostly of the nature that every other cycle never goes down very far so the
wave is almost like a 10 Mhz wave with a notch instead of a complete cycle
in between to make the 20 Mhz form.  The 27-28 Mhz range,  which uses the
third harmonic of this same crystal,  doesn't look so distorted,  go figure.
If I can figure out how to get a tube extender into the 1st mixer socket,
I'll look at the 1st oscillator output,  but the 10 Mhz xtal for 7-8 Mhz for
that oscillator is also used on 2-3 Mhz and that band seems to be ok.

So it looks like the xtal or it's connection is the problem.  Are the
crystals in the 390 soldered in or are they in sockets?  I tinkered with the
trimming caps on the back a bit but that didn't resolve anything.  I suppose
the bandswitch could still be the problem but the waveform on the scope
doesn't show any evidence of intermittent or inconsistent connection when I
change bands.  But I should check the 1st mixer tube for the signal from the
first oscillator also.

In looking thru the Field and Depot Maint Manual (last digits 35) and the
Organiz. Maint Manual (last digits 20) I mostly understood the xtals and the
conversion scheme in 35 and which xtals were used on the various bands but
failed to understand one table in 20 that identified which xtals were bad
based on which bands didn't work.  That table is in Sect 14 under
troubleshooting in Manual 20.  Does anyone recall a problem with this table.
Also in Manual 35,  alignment of  2nd xtal oscill. Sect. 77 advises which
trimming caps to adjust for each band.   Cap. number 16 seems to be left out
and Band 17 seems to be left out.  Anybody recall this omission?

  More later,  Dan.

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Flowertime01 at wmconnect.com
Sent: Wednesday, April 27, 2005 7:08 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] R390 Band Switch Alignment

John,

Thank you for the archive link. I need that and did not know it.

Mark Huss,

Thank you for putting up the post from my soap box.

Dan Merz,

You may have a band switch alignment problem where the 4, 5, 6, 7 setting is
not making good contact in one of the wafer sections.

As you have Cal tones on each band, the oscillators and crystals are good.
The only things in common with the four bands being weak is the RF switch
and the set of tuning coils. The rack, the slugs, the switch, Check the
slugs and rack from the tops side before you pull the RF deck. Do peak the
second crystal deck caps.

One additional idea before you go into the bottom of the RF deck. 
Pull the slug rack by releasing the two springs (one front and one rear) on
the rack and lift it out of the slides. Insert a #1 Philips screwdriver down
the slug hole and remove the 6x32 screw that holds each coil can in place.
The coil cans plug into the deck with several pin contacts. Wiggle them up
free and clean the contacts and pins. These contacts have been known to
oxidize like tube and crystal pins and sockets.

Mark Huss put up a soap box of mine about why we should pull the RF deck and
do the band switch alignment by eye. It looks like you have a classic case
of band switch just not quite making on one position. Some time if you tune
from high to low or low to high the switch will make for you going one way
and not the other. Some time you can do 8, 7, 6, 7 and get the 7 Mhz to
work. some times you can do 3, 4, 5, 4 and get the 4Mhz to work. These are
clues the switch lash is to far out and needs adjustment. May as well get
your self a some deoxit or other favorite switch contact cleaner and go
after the band switch. As long as you have to pull the RF deck, you may as
well do a full PM under there and be done with for a couple years.

Plan to clean the switch, set the switch adjustment, give the deck a bath,
and check the state of the capacitors under the deck. If you have some brown
beauties in there, change them out.  Just do not jump in real fast. work
with the receiver, Use it a while, work on the other parts of the alignment
and see how it works out. After you get all the other easy top side items
fixed up, then plan a Saturday to do the RF deck. By that time you will have
gotten the tools and materiel together for the task.
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