[TMC] Pal 350/500

[Chris Bolkan]

Hi Bob!

Thanks for your reply. Can I trouble you to send me an image (scan,
take a pic with your cell phone, I don't care) of the hand written
notes of how yours is wired for bias keying? I would like to know if
you followed the PAL 500 biasing scheme and see how you implemented
it. If you can, please email it to bolkyboats at gmail.com.

I am coming to the conclusion that with the choke input power supplies
and the overdesign that these amplifiers may indeed be keyed on and
off by a relay connecting and disconnecting mains voltage of the high
voltage supply. I have looked at the system drawings which incorporate
PAL500 amps and it appears that when they are keyed on to transmit, it
is accomplished by connecting and disconnecting mains voltage from the
primary side of the HV transformer(s). What I don't know about these
systems is if they were used for continuous or very long transmit
cycles (broadcasting) or if they were used conversationally like for
ham radio. None of the systems I could find contained receivers which
is suspicious to me that maybe they were used in long duty cycle
transmissions.

The big AM transmitters I think people are referring to which turn on
and off mains voltage to the power supplies to transmit, are broadcast
transmitters that were originally keyed on continuously like a radio
station. In that case there was no original design intent to have them
keyed on and off frequently. I still am having a hard time digesting a
linear amp being keyed frequently (as in a casual conversation) would
do so by turning the power on and off every time one talks. But I am
relatively new to this hobby and there is so much I don't know!

Best regards,

Chris


On Wed, Jul 21, 2021 at 8:47 PM Robert Nickels <ranickels at gmail.com> wrote:
>
> 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
>
> ______________________________________________________________
> TMC mailing list
> Home: http://mailman.qth.net/mailman/listinfo/tmc
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:TMC at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html