Power Amplifier Dressler D200 (144MHz)

Faster R/T Switching

The original R/T switching of the PA is damn slow. I have no means to measure it but I estimate that the overall time to switch to TX is about 500 mS, maybe more. It is mainly annoying for CW. Using WinKeyer 3 it is possible to insert spaces (or pipes) in front of the string that is sent to the keyer to introduce an extra delay. However, for hand-keyed CW there is no other choice than to control PTT manually.

The controlling logic of the PA works in the following manner. When PTT is depressed, two relays that toggle the signal path are switched first. Then, after some protective period of time, another relay switches. This relay puts the cathode of the tube to ground to enable a high current through it. When PTT is released all repeats in reverse order. This way the tube is protected during the transient time of the R/T switching.

My goal was to squeeze the R/T switching time to about 50 mS in order to avoid the need of controlling PTT manually. If the PA switches that fast all you need is to set the proper lead-in time for WinKeyer (see its manual) and learn the keying with that delay.

I have developed the following mods. Now the switching happens very fast. Yet, one can hear that the relays don't switch at the same time and that there is a little time space in between. This way the original protective idea is kept. To make it more robust I have also disabled the tricky VOX functionality.




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