[R-390] The adventure begins (and a capacitor question and a R-390 question).

[Bill Smith]

----- Original Message -----
From: "John Saeger" <john@whimsey.com>
To: <R-390@mailman.qth.net>
Sent: Wednesday, May 22, 2002 11:15 PM
Subject: Re: [R-390] The adventure begins (and a capacitor question).

> > Could be open mica capacitors.   The red-rectangular versions in my
R-390
> > (NA) are monsters.  They leak and open just like the paper tubulars you
> > correctly replaced in your father's radio.   Hope you also checked the
> > resistors, by the way.
>
> Hmmm...  Good point about the mica capacitors.  Maybe they should be gone
> through too.  But if I'm taking out a mica, I'm wanting to put in another
> one.  Is there anything really better than a mica?

Good question.  I happend to have new old stock that "looked" like the
originals, and didn't dare check them first, but the receiver is working.  I
have heard that dipped silver micas are just as bad.  Does anyone have any
suggestions?


>
> Good point about the resistors.  I did check a few, and for the most part
> they were at least double what they should have been.  I began to wonder
if
> they had changed the ohm or something.  ;-)  I didn't do anything about
it.
> Maybe someday I will, but after doing the capacitors, they're about the
only
> original parts left.  So I thought I'd let them run till one breaks.
>
That reminds me of the number of radios I took apart as a youngster, gaining
a proudly hoarded large box of wax capacitors and another of resistors.   A
friend came by, and after glancing at the box, laughed, and said that
probably most of them were bad.  I wouldn't believe him.  Same with
resistors, I couldn't figure out why the meter worked sometimes, and at
other times didn't.  The resistance scale kept changing.  I thought maybe
there was a cold solder joint in the Knight-Kit VTVM I had put together,
after all it was only my second kit that I had assembled.

I never imagined that resistors could go bad.

Of course those were the days of my first amplifier design.  I wanted to
build a simple microphone preamplifer, so I picked out a 12AX7.  I found a
transformer and rectifier, and build a power supply, but when it came to the
amplifer circuit, I couldn't figure out why it had all those extra parts.
For example, there was a capacitor coupling to the grid.  Why not a direct
connection?  Also, there was a resistor from grid to ground.  What purpose
could that serve, it would just take away the voltage from the microphone.
Out it went.  Then there was a resistor in the cathode of the triode.
Couldn't figure out why it was there, so rewired the cathode with a direct
connection to ground.  Then, there was a resistor in the plate circuit.  At
that time, I really believed that the tube "amplifed" the signal, and all
you had to do was apply voltage, sort of like giving food to a cat.  So out
came the plate resistor too, just wired it to the power supply.  I had to
leave the output capacitor in, because otherwise the circuit sparked too
much.

Well, I hooked it all up, and applied voltage and a signal.  Unfortunately,
it didn't work.   Seems that the tube warmed up a bit too much, too.  Well,
it was back to taking broken radios apart and adding sections of spare wire
to the working receiver's outdoor antenna.

Back to the R-390 (NA).  I have a plate that at least looks original,
showing the mfr. as Motorola, and a serial number of 4700.  Were R-390's
made with serial numbers this high, and is there any guess as to the date of
manufacture?  Think I asked this question before, and dates are found on the
PTO and elsewhere inside the set.  I'll look for those, but is there any
information regarding the sequence of serial numbers and mfr. date?  Or did
each manufacture make up their own?

What is the last (highest) serial number known of the R-390, and for that
matter, R-390A?

73 de Bill, AB6MT
billsmith@ispwest.com



> John
.