[TMC] Pal 350/500

[Chris Bolkan]

Sent from my iPhone

> On Jul 23, 2021, at 11:22 AM, Chris Bolkan <bolkyboats at gmail.com> wrote:
> 
> 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
>> 
>>