[R-390] Intermittent problem.

[k2cby]

Intermittent problems are the hardest of all to find (solve).
I don’t like to pour cold water, but a cable/connector is one of the last
places I would look. In normal operation, they don’t move, are not subject
to heating & cooling and typically either have continuity or don’t.
With an intermittent the first question to ask is whether you can provoke or
cure the condition by manipulating any of the controls. If so, dirty switch
contacts or a mis-aligned wafer are high on my list.
Second question: does the problem occur at all control settings? Band
switch, selectivity? If it is confined to one band or everything below (or
above) 7 MHz I’d suspect a crystal. If it is one selectivity setting,
suspect a (gulp !) mechanical filter.
When does the problem occur? When doesn’t the problem occur? When turning
the set on from a cold start? After it has been operating for a while. When
the room is cold. When the room it hot. When you drop the front edge of the
receiver ¼ inch onto the bench.
Try to isolate the stage. The quickest check is to feed a 455 kHz signal
into the IF deck. Obviously, if the intermittent re-occurs, the problem is
in the IF, Audio or power supply. If it never affects a signal originating
at the input of the IF deck the problem is in the crystal oscillator, the
VFO, or the RF deck.

I assume you have checked/replaced the tubes. By far the best test is
substitution of a known good tube. I would be particularly careful about
those in the crystal oscillators and the VFO. The R-390 uses triode mixers
that need a considerable oscillator signal at the injection points. When an
oscillator tube starts to lose its manhood the mixer will sometimes drop out
altogether.
Finally, I would offer the somewhat esoteric suggestion of checking whether
the antenna changeover relay contacts are dirty or the antenna trimmer
assembly is oil-soaked.

 

 

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