[TMC] SBT-1(XX)

[TELEGRAPHER at att.net]

One of the things i wanted to make sure all of you are aware of is that there is NO antenna relay associated within the structure of the TMC gear as it comes from the factory unless it was available as an add on.  All this gear was designed for commercial service whether Point to point or split frequency and maybe some other types of operation.

I don't think Amateur operation was really considered to much because of the cost.  This was big bux in the late 50's early 60's time frame although i never saw a price on any of their stuff i bet it wasn't cheap.  I dreamed of having some of that beautiful gear when i was just a Sr. in high school and a new conditional class op.  Yeah right!  I'd probably still be in hock.  Do you suppose it cost as much as a small house back then?

When you throw the breakers on for the HV and the Plate voltages, you operate any antenna relay that is wired to the antenna relay terminals on the back of the PS-4 on the SBT-1 series or the GPE-1(A) in the smaller PAL series gear.

My theory for the way this gear was operated was that the radio set in the radio room or wherever with the HV and Plates energized.  No RF was being generated until the Mic was keyed if it was using PTT or a key was pressed if using CW.  This was done via the exciter whether using the GPE-1 or SBE-X units. 

In the case of my GPE exciter that i use with the PAL-500 amp, i have had problems when using the PTT feature with the HV breakers turned on and the Plate Voltage turned off.  When i keyed the unit using PTT it would sometimes trip the HV breaker off.  Wear and tear on the breaker and possible arcing within a set of relay contacts.  Sometimes i don't have that problem at all and it works just fine.

If you turn on the breakers and Plate switch you'll see that you don't generate any RF but then pushing the talk switch on the mic will cause the unit to generate RF and when you do this you'll hear a small relay operate.  The antenna realy if hooked up has been operated since the HV and Plate voltage switch were operated so hopefully you've muted the receiver by now.  With the GPE-1A unit i have operating the Mic PTT causes K1 to operate which then completes the circuit thru the interlock wiring and changes the bias arrangement on the finals.

Another thing which you've probably already discovered is that there is no "MUTING" feature in the transmitter, meaning the SBE or GPE-1 so there is an additional requirement.  Wasn't really a hot item in Point to point or split frequency operation.  Amateur service now dictates you come up with this feature as well.  TMC may have offered some other options that i'm not aware of.  

With the GPE and PAL-500 units there are a couple of screw terminals on the back of the GPE that are marked "Transmitter  on-off" which are a set of dry contacts inside the GPE used to key the PAL-500 turning it on.  These are in series with the interlocks in the amp and power supply.

No provision for muting though so i would use a double pole or triple pole (future considerations) antenna changeover relay with a set of contacts operating the mute function on your receiver.  The second set of three sets could be used to ground the receiver input when in transmit mode.

With the SBE-2/3 etc. units, the same arrangement is there.  Connections on the back terminal strip of the SBE-2 etc, wire to the MV power supply so that when the TX switch on the SBE, the PTT function or the VOX is activated, the amplifier is turned on.   This of course happens if your interlocks are all operating properly as they are all in series with the keying arrangement.

I would suggest that the equipment was in the operate condition at all times and the only thing that happened when the mic was keyed, a button pushed or a switch thrown  or whatever method of operation was selected, a relay operated in the exciter, whether in the SBE or GPE-1  and as long as the interlock circuit had good continuety the amp would be operated.  Since your getting good power out of your SBT unit all the wiring must be in place.  I wouldn't want to have to key the unit in CW mode operating the relays and turning the HV on and off.  That would be really tough on the components.

Now you need to fix up a Muting circuit for the receiver and figure out how your going to remove the antenna relay from control by the HV-Plate voltage switch arrangement.

Another item, the grid current for the SBT-1() RFD amp using the 8295A should not be allowed to go over 5 ma.  Beyond that your taking your chances on how long the tube will last.  This is a KW rated tube, that means 250W am, 1 KW CW or 1 KW PEP SSB.  I pushed mine one day to around 650W out on CW before the breaker kicked out on the power supply.  Probably the tube isn't the best but as long as it supplies 250W on AM, i'm happy.  That's what it's rated to do and 8295A tubes are expensive.  

I have a marked schematic in my PAl-1K manual that shows the changes necessary if you want to convert to a 4CX1500B which are more plentiful and a lot cheaper.  Other than the socket i think there are 3 resistor value changes.  The 1500B is a more linear tube as i understand it so would be really good for SSB.

One could modify the high power amp input, bypassing the 5763/6CL6 and 6146 with a passive input consisting of a couple of power resistors, maybe 20w or so and a series capacitor then driving the amp with your sand state radio at a reduced power of course but then frequency agility would be the big gain.  ALTHOUGH, moving the amp more than 15 or so Kc may require a retuning as they are not a broadband unit.  Mainly a set it on a frequency and leave it there unless a new frequency will be occupied for some time.  Band and frequency hopping are not one of their strong points such as amateur operation might entail.

Be sure you check the bias settings in the PS-4.  There is a capacitor in there on the Screen line i believe that should it go bad, it will turn your 8295A into a good paperweight as well. Perhaps other list members can provide that info.

BTW, on the PAL-500/350 series units, i've yet to find any description for making the overload current adjustment.  Anyone come up with that yet?

Larry
W0OGH
 

Now your as dangerous as i am with this gear.  Always something to learn.
                                                                                                                                                                                                                                 
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