[TMC] Pal 350/500

Thu Jul 22 23:44:44 EDT 2021

Chris, keying the primary circuit of the HV supply may be used with complete confidence for ham radio-type PTT break-in operation. As I had mentioned in an earlier post, the TMC GPT-750 is but one example of a medium power transmitter (1000 watts AM/CW/RTTY/SSB) output) where this feature was incorporated, and as I had further mentioned, TMC did not even include soft-start for this, their thinking being that this was not necessary for reliable operation of the equipment. This equipment, which has accrued 100s of 1000s of hours of combined operation, has proved them right.

My homebrew kilowatt-class AM rig (2x 4-400A modulated by 2x 833As) has been used reliably and with zero issues for 42 years by keying the 240 VAC primary circuit on and off every time I make a transmission, and that is for quick break-in AM operation. Yes, I did include soft-start for this, and it does further enhance the reliability of the equipment when it is keyed for transmit this way, and you may want to include this feature in your PAL-350.

Every commercial and military medium power AM transmitter designed for PTT operation keyed the primary circuit on and off for transmit, not to mention every plate-modulated ham radio rig, such as the Heath DX-100 and Apache, the Johnson rigs, the Collins KW-1 and the 32V-series.... the list goes on. And AM operation is very stressful on any transmitter, much more so than CW or SSB operation. None of these rigs used a soft-start circuit for the HV either.

As such, I think your fears are unfounded.

73,

Bruce, W2XR

73,

Bruce, W2XR 

> On July 22, 2021 at 11:30 PM Chris Bolkan <bolkyboats at gmail.com> wrote:
> 
> 
> 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
> >
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