[R-390] Antenna Trim trouble

[Jacques Fortin]

Hello Jordan,
You should have figured it already, but...
It will be impossible to diagnose temporary shorts between the plates of
C225 A+B as long as all the antenna coils are left in place, because they
have a so low resistance value on their secondaries that this will mask any
leakage coming from the variable capacitor sections.
Check the schematic, you will see it.

What I believe can be done by leaving C225A+B in place is:
1_ Tune MHz knob to 00.
2_ Locate T201 and remove the related slugs rack. Take care to not loose the
springs in the RF deck...
3_ Remove T201: with the MHz tune in 00 position, the A+B sections of C225
are connected in parallel between pins 3 and 6 of the T201 socket.
4_ Using a sensitive ohmmeter connected between those pins, any (even
temporary) leakage of the capacitors can be measured, and with a capacitance
meter connected to the same pins, the total value of the variable sections
can be checked for any rotation angle of the control shaft.

Let us know...

73, Jacques, VE2JFE in Montreal


Hi again group...
I'm ready to re-mount the RF deck temporarily in the radio, but before I do
that, I'd like to know if there is a test technique or method I haven't
tried yet.
I was getting some scratching noises and "dead spots"  at some positions of
the Ant Trim control.
After an attempt at repair, without being able to pull the variable cap out
of the can, I've used a 50 Mohm impedance VTVM with a 500 Mohm scale as well
as the Insulation test of my Sprague TO-6 and checked between every line and
each line to ground and I can't provoke even the slighest twitch of the
metering when rotating the cap
Is there anything else I should try..?

73...Jordan VE6ZT