[TMC] Pal 350/500

[Chris Bolkan]

Hi Bob!
Thank you for the diagram! It is interesting that in that
implementation the screens are grounded by the same relay that was
implemented to change the bias. That is different from the
implementation TMC did on the PAL500 to key using bias change. I am
going to print it out and try to figure out everything that is going
on with that scheme. Yours involves both control and screen grids in
the switching whereas the PAL schematic only changes bias on the
control grids. I have come around (learned) that keying by the power
supply is not only acceptable but common. I get it now. But I am still
curious what TMC had in mind with the grid biased version, if it was
ever actually used that way and how it worked.
Thanks again!Chris

On Fri, Jul 23, 2021 at 8:33 AM Robert Nickels <ranickels at gmail.com> wrote:
>
> On 7/22/2021 10:30 PM, Chris Bolkan wrote:
> > hand written notes of how yours is wired for bias keying
>
> Here's a link for anyone who might like to have a look:
> https://www.dropbox.com/s/xl7th1zf6b7n9vh/TMC%20PTT.jpg?dl=0
>
> It's a copy of a pencil drawing to begin with so the quality is about as
> good as I can do unless I figure out how to make my scanner work with
> linux.    This was done by someone other than me, after tracing out the
> wiring in the power supply and realizing that it wasn't per the
> published schematic I can't remember if I verified that this is how it
> was wired or not.   That's the problem with having more projects than
> time - things get pushed out and details that seemed indelibly etched at
> the time fade away.    But this method follows the conventional approach
> for amateur linear amplifiers by applying cutoff bias in receive mode
> and switching to operating bias when PTT is keyed.
>
> It's true that broadcast transmitters are designed with latched AC
> contactors and don't go on/off frequently, but many have been converted
> by hams for AM operation and  they simply  rewire the primary power to
> allow the plate contactor to be turned on and off with PTT.   The
> original step-start is retained to limit inrush current and overload
> protection relay contacts remain in circuit.    It's a good idea to use
> some form of sequencing to make sure the antenna is the first thing
> connected and the last thing disconnected to protect the transmitter.
>
> High power HAM transmitters designed for AM operation such as the
> Johnson Viking Kilowatt, Globe 500, etc. do not have step-starts and
> simply turn plate voltage on/off with PTT.    But their power supplies
> are typically choke-input and use much smaller values of filter
> capacitors - for instance, the filter cap in the choke-input Desk is
> 8uF.  My PAL-350 schematic shows a 2-stage L-C-L-C filter where each cap
> is 4uF so it is designed to work the same way.
>
> 73, Bob W9RAN
>
>