[TMC] Pal 350/500

Robert Nickels ranickels at gmail.com
Wed Jul 21 23:45:33 EDT 2021


On 7/21/2021 9:21 PM, Chris Bolkan wrote:
> standard practice to turn the HV supply
> completely on and off to key amplifiers is not what I expected.

Not only amateur but broadcast transmitters were controlled that way, 
but a step-start often was incorporated to limit inrush current.  That's 
still a good idea when the high voltage is going to be applied continuously.

This discussion served to remind me that it's been a year since I got my 
PAL350 going and now I can't remember how it was done ;-) I do know that 
that it was actually wired differently than the schematic showed,  so 
that HV was applied continuously and bias was changed from cutoff to 
operating bias levels via the external keying circuit.    I used my 
FT-817 for testing (which still put out too much power even at its 
lowest setting) and keyed the amp with a manual switch that actuated a 
small relay.

My unit is a very early model built by TMC Canada (in fact I think it 
may well have been a factory demo model) which doesn't conform exactly 
to any of the published schematics, but came with handwritten notes 
showing the "modification for PTT operation".

To the original concern about controlling HV transformers by switching 
the primary voltage, of course the original designers had no alternative 
but to use electromechanical relays and arcing caused by the collapsing 
magnetic field caused contact deterioration and failure.   Solid state 
relays with suitable capacity are readily available and inexpensive and 
would eliminate contact reliability problems and should apply less 
stress because they will switch at zero crossings.  Here's a reference:

http://www.crydom.com/en/tech/newsletters/solid%20statements%20-%20ssrs%20switching%20types.pdf

73, Bob W9RAN



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