GM3SEK Triode board modification


What follows here is the modification that will allow the GM3SEK triode board to work with a toroid type transformer that supplies the high volts for the anode, and, the supply for the tube heaters. (this might also include lows volts for control, such as 18vac for 12vDC relays)

The following information comes from Ian GM3SEK himself.

Look at the "TRIODE BOARD" printing along the bottom edge of the board. Immediately above the "R" in "BOARD" is a plated-through hole - this is the warm-up control signal coming from U3 (LM555CN).

Using a sharp 3mm drill held in your fingertips, countersink this hole from the top, just enough to remove the top copper and break the connection. Check with an ohmmeter that there is no continuity.

One side of this broken connection goes to pin 11 of U5. Underneath the board, link pin 11 to pin 14 (+12V). Double-check that you have identified the correct pins!

Remove the HV_OK input pin and remove D6, C11, R11 and R12. That leaves nothing connected to U5 pin 3. Instead, link pin 3 to the bottom side of the connection that you drilled out in step 2. The warm-up timer will now block the PTT, using what used to be the HV_OK input.

Re-check everything and re-insert the ICs. You should now find that HV_CTRL comes up immediately, but the PTT is still blocked for the usual interval.




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