[R-390] Antenna Trim trouble

[Jordan Arndt]

Hi again Jacques and the group..

I ran the tests with the Ant Trim assembly completely removed and 
disconnected from the RF deck.

My best VTVM is a fully restored and calibrated Marconi TF 1041B and it was 
used on the 500 Mohm scale and wouldn't even twitch during the 
testing...Testing for capacitance and insulation resistance with my TO-6 
also showed no problems during testing.

I think I'll go ahead and re-install the RF deck and see if I've solved the 
problem..

P.S...Does anyone have some spare connectors which I could use to build an 
extension cable for the PTO...?  I might give adjusting a pair of Cosmos 
PTOs for linearity a try. They are fine across the 1Mc coverage, but are 
1khz or so out at some points along the way...

73...Jordan VE6ZT



----- Original Message ----- 
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'Jordan Arndt'" <Outposter30 at shaw.ca>; "'Collins R-390 HF Receiver 
List'" <r-390 at mailman.qth.net>
Sent: Monday, September 30, 2024 3:04 PM
Subject: RE: [R-390] Antenna Trim trouble


> 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
>