[R-390] Question from a newbie

[k2cby]

You can certainly make extension cables for an R-390A, but for my money it
is not worth the effort. 

 

It is definitely NOT worth cannibalizing a salvageable R-390A mainframe or
subchassis to get the plugs and sockets to make one. That’s a mortal sin.

 

I have had an original Motorola 1954 contract unit (S/N 1200) since the
early 1980s. I lovingly tore down, cleaned, lubed and reassembled the
mechanicals. 

I recapped all the “brown beauties” -- which, in retrospect, is something of
a waste except for “the infamous C-552 (0.01 µF 300 dcwv) that couples the
plate of V501 to the mechanical filters. Because its failure will fry the
filters, it should ALWAYS be replaced with a top quality new part with a
voltage rating of at least 350. It's also easy to reach, and there is plenty
of room to fit a replacement.” 

I had to replace one of the IF filters which developed a short to ground,
killing the AVC bus. I also added an in-rush protector, solid-stated the
rectifiers, and substituted a resistor for the ballast tube. Beyond this,
all I’ve done is to replace tubes whenever the performance started to fall
off and do an alignment every 4 or 5 years.

These are remarkably durable and reliable radios. They were built to take a
beating and to last forever. You shouldn’t have to pull out any of the
subchassis. By the way, with a little effort, you can turn the IF deck and
the audio deck upside down for servicing without disconnecting the power
cables.

The most useful service tool – for this and most other “hollow state” radios
– is a set of tube socket adapters. These will let you measure tube pin
voltages from the top of the chassis. They were common as dirt back when
every town had a TV repair shop, but now they command premium prices on
e-bay. The best commercial ones were made by a company called “Vector”.
General Cement also made good ones. The R-390 Army service manual shows how
to make poor substitutes from tube sockets, but keep your eye open at
hamfests for the originals. If you have to choose, the most useful is a
7-pin, next a 9-pin, finally an octal. The esoteric ones are a waste. 

Best of luck. Enjoy the radio.

 

Miles B. Anderson, K2CBY
16 Round Pond Lane
Sag Harbor, NY 11963
Tel.: (631) 725-4400

Fax.: (631) 725-2223
e-mail: k2cby at optonline.net