[TMC] My PAL-500

[Roy Morgan]

At 11:01 PM 12/29/03 +0000, telegrapher@att.net wrote:
>It lives again!!!!!!
>
>Finally got the PAL-500 power supply fixed and now the amp is running again.

Larry,

Hooray for you.  I have a PAL-350 here with no power supply.  (NOTE: Power 
supply wanted!)

>I  Mounted the 3 screen voltage dropping resistors on terminal strips
>bolted to the fiberglass.

Were they missing?  Is this a TMC power supply?  The TMC supply for the 
PAL-350 has dropping resistors (two 40K 50 watt) and VR tubes for the 
screen supply.  I wonder what the plate voltage is in the PAL-500.  (It's 
2000 volts in the -350.)

    Fired it up last night into a dummy load and was able to get about 175 
W of
>carrier out of the 4CX-250 finals.

The PAL-350 is supposed to have 4CX350's in it. A pair of 4CX-250's is 
capable of some

>I found that tuning the beast can be a touchy job because the procedures in
>the manual are not the clearest.

Touchiness is not a hallmark of TMC gear in general.  Is there something in 
there that needs  more fixing??  Granted, the tuneup procedure is very odd!

>One of the things that is still a little funny is that there is a SSB/CW
>switch on the back of the AMp.

One version of the PAL-350 manual I have here (for the C model) says:

"..If it is desired to operate in the CW mode only, greater efficiency can 
be obtained by setting the SW/SSB switch, on the rear apron of the RFA, to 
the CW position.  The 4CX350 tubes will now be biased to cut-off.  At the 
same time, an additional bleeder load (R124, R125, R126, and R127) is added 
to the high voltage supply by relay K104. The CW/SSB switch also changes 
the scale reading of the multimeter to prevent the needle from pinning in 
the RF drive position because of the greater drive required to produce full 
rated output."

The actual PAL-350 I have has the CW/SSB switch located on the front panel, 
apparently factory wired that way.  (It has no label, by the 
way!)  Cleaarly, this switch manages changes in voltages and such in both 
the amplifier and the power supply. This is why it's a shame to have a PAL 
and it's power supply separated!  Home brewing all the functions, including 
the protection circuits, is a non-trivial job.

>  I'm driving it with a GPE-1A for AM operation.

This is "low level modulation" where the exciter signal (at a hundred 
milliwatts or so) is AM modulated and the amplifier is run in linear mode - 
that is SSB mode, NOT CW mode.  Standing carrier has to be set at one 
quarter of the maximum power (one half rf voltage output at the antenna) 
with no modulation.. Then full modulation will drive the thing to near 
cutoff in the negative direction and to it's full output in the positive 
direction in linear fashion.

Note: It is well recognized that the human voice is 
non-symmetric.  Reversing the mike leads in your set up may put the higher 
peaks in the upward direction instead of the downward direction. This is 
smart test for and change if needed.  A fine point of AM operation is to 
standardize mike connections to produce positive voltage with increased air 
pressure on the mike, and arrange all electronics chains to increase 
carrier power on positive voltage at the mike input.

If you had a plate modulator, you could run the thing in CW mode at 
whatever maximum carrier you can get out of it and away you go.  The 
external modulator would supply the needed modulation power.  Management of 
the screen voltage under these conditions may be tricky - it's NOT good to 
drive the plate voltage below the screen voltage. EIMAC advises using 
unregulated screen voltage derived from the modulated plate supply via a 
dropping resistor.  "Care and Feeding of Power Grid Tubes" is the required 
reading for this topic.  Get your copy at: http://www.cpii.com/eimac/  The 
operating conditions for SSB and plate modulated AM are very different.

>Its been a struggle but its also been a good learning curve....

Indeed.. and "good on ya" as they say in Ireland.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan@nist.gov --