[TMC] Pal 350/500

[Robert Nickels]

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