[Heathkit] Heathkit DX-100 question

[Ron Youvan]

Y S Lee wrote:

>   I am studying DX-100 schematic to acquire knowledge to build an AM-CW
> transmitter of my own design. Looking through it, I could not understand how to synchronize the
> TX frequency with a matching RX. Usually TX has a SPOT function to monitor
> the TX freq with an RX. DX-100 does not seem to have that.
> Another question is how to switch between SEND- RECEIVE. I could not find this
> switch at a DX-100 photo.

   Two excellent questions.
   First, you can add a "spot" switch (a button that operates just the oscillator tube and possibly 
the next tube, you will need a diode here) as many have, internal or external.
The receiver can hear the transmitter when the transmitter is feeding the station test load without 
doubt.  (even when there is NO high Voltage feeding the 6146s)
   Second, with a separate transmitter and receiver there has to be a "change over" switch or relay 
of some kind, some where.
   I did it in 1973 by building and installing a "TR switch" constructed out of the ARRL handbook, 
mounted on the back side of the plate tuning capacitor in a little homemade box made of sheet 
copper, with one JFET in it.*  (and other parts)  You can also feed the 120 Volts AC from the [PLATE 
ON] switch (out through the 11 pin connector on the back panel (or whatever it is) to an external 
power relay that changes the antenna between the receiver and transmitter.  (or a coaxial relay if 
you have or can find one)
   You can also use an antenna switch (backwards) to connect the antenna to the transmitter or the 
receiver, but you can easily bump the key with no load on the transmitter this way.

   If I were to build a transmitter it would be a QRO type with one or two 813 tubes, grid driven 
with everything else solid state.  (with 1,500 to 1,700 Volts on the plate/s and a clamp tube or 
transistor)

* if you use a TR switch you must bias the 6146s plate current completely OFF with the clamp tube 
adjustment or you will hear "shot noise" in the receiver.  (drowning out the received signals)
-- 
    Ron  KA4INM - rule of thumb:
                  If it's winders it needs rebooted weekly and will always benefit from more memory!