[R-390] Antenna Trim trouble

[Bob Camp]

Hi

The easy way to spot a short is to look for a badly bent plate in the cap.

Typically, the problem is not a short.  Far more likely is that the contact to the rotating plate is hitting an oxidized spot on the rotating plate. Yes, it also could be plating that has vanished over time. 

Simple answer is to spray it with whatever you favorite contact cleaner / de-oxidizer happens to be.

Bob

> On Sep 30, 2024, at 5:04 PM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> 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
> 
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