[TMC] Pal 350/500

Sat Jul 24 16:25:13 EDT 2021

I know I am late to the party, but had been thinking along Chris' lines...meaning, why beat that one contactor to death?  I would imagine that most of the HF installations where TMC rigs were used were multichannel RTTY and AM broadcast one that stayed key-down for hours if not days.  Yet, the SBE-2 sideband exciters include VOX and when running ham-like SSB I would think the contactor would be beat itself to death in relatively short order.  

Tetrode SSB transmitters such as the Central Electronics 100V and 200V simply apply switched -120v blocking bias to the driver and final amplifier stages to turn them off/on and include a provision to route that same blocking bias to an external tetrode amplfier.  PA noise is not an issue.  Plate and screen voltage remains applied.

As I eventually get my PAL 350 paired up with a SBE-2, that was my plan - at least, from an experimental standpoint.  If one was running RTTY and AM, then the occassional PTT and keying the transformer as-is would likely be fine since the number of key ups is much smaller than for amateur-like SSB-VOX connumnications. 

While we "seasoned" Hams all likely lusted over those TMC transmitter ads in QST, what Ham actually owned one in the 1960s??  TMC parts depots no longer exist and those contactors likely are not growing on trees, so why not experiment with the blocking bias concept?  

73  Nick K5EF

> On July 23, 2021 7:59 PM Chris Bolkan <bolkyboats at gmail.com> wrote:
> 
>  
> 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
> >> 
> >> 
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