RCA killing the Softrock

[K6JEK]

My buddy W6THW has been blowing up his Softrock receivers with his BTA 1R3.  It's not a fair fight so he's been using the relay sequencer we were all talking about awhile ago.  He ended up with a two step sequencer based on Don's design from ER.  There is a full 430 milliseconds after B+ off until he flops the antenna over.  Still he killed the Softrock.

This transmitter doesn't have a separate bias supply.  It goes off by killing the B+

Isn't 430 milliseconds plenty?
Are there other precautions he might take?

[W1VD]

What type of T/R relay is he using? Measured isolation? Timing is only part of the deal...

[k4kyv]

The antenna relay should be switching the antenna from the transmitter to the receiver.  The output of the transmitter cannot find its way into the receiver, unless there is enough RF still present from the transmitter to carry an arc as the mobile contact on the relay moves over to engage the receiver.

When going from transmit to  receive, he should kill the B+ to the transmitter and the rf from the VFO or xtal oscillator on step 1, and change over the antenna relay on step two.  When going from receive to transmit, the antenna relay should switch over in step one, and everything else should change over in step two.  That way, there is no danger of hot-switching at the antenna relay.

The only thing I can figure out is that it is taking some time for the HV filter caps in the transmitter to discharge, and after the T/R switching has occurred the transmitter is still putting out rf, either because the rf excitor is still supplying rf to the transmitter, or some stage in the transmitter is going into self oscillation as soon as its excitation is removed.

He might try a larger capacitor in the sequencer  circuit to increase the time delay between steps, or use a heavier bleeder in the transmitter power supply to kill the B+ more quickly.  The transmitter should not be putting out any rf by the time the antenna changeover relay begins to move.  Maybe he needs to  check the PA and low-level rf stages for a tendency to self-oscillate.

[flintstone mop]

hmmmm that's pretty strange.
I'm using a B'cast TX 250Watts and no issues here killing my SoftRock board. Are you using them as I.F.'s or as actual receivers?? You may have a point about residual RF left over during the switchover and may need a delay.

Fred

[Steve - WB3HUZ]

Heh.  I told you SDRs were crap. 

[KD6VXI]

My buddy W6THW has been blowing up his Softrock receivers with his BTA 1R3.  It's not a fair fight so he's been using the relay sequencer we were all talking about awhile ago.  He ended up with a two step sequencer based on Don's design from ER.  There is a full 430 milliseconds after B+ off until he flops the antenna over.  Still he killed the Softrock.

This transmitter doesn't have a separate bias supply.  It goes off by killing the B+

Isn't 430 milliseconds plenty?
Are there other precautions he might take?

Is it killing the B+ via interrupting the B+ before or after the cap?

Might be, if it's AFTER the filter cap, the cap isn't discharged, which would cause the tx to still be active..

Just a thought...  Might just need to move the contactor that interrupts the TX /  B+

--Shane

[K6JEK]

The drive stays on after the B+ is cut and is shut off in step two along with the antenna change over.  We thought that was the right thing to do -- we were leary of having B+ with no drive.  We turn it on before the B+ going the other way.   The only thing on the last on, first off part of the sequence is the B+.

I don't know if it's cutting the AC mains to the B+ supply or flipping the B+ itself. I also don't know what the antenna relay is. I'll find out tomorrow.

Thanks for the suggestions.

Jon

[k4kyv]

Try connecting it so that the antenna relay is first on, last off.  Put everything else last on, first off.

For the split second that the capacitors are discharging, if there is no self oscillation in the final, it won't hurt the tubes to not have any excitation to the final.  If anything, they will bleed down the capacitors faster.  Or else, add a clamp tube or some protective bias to the final.

One thing you want to avoid is self oscillation in the final near the operating frequency as soon as excitation is removed, as the capacitor discharges, since a short pulse of spurious signal would be transmitted, probably as a nasty sounding click whenever you release the T/R switch.  That shouldn't occur if the PA is properly neutralised, particularly since the transmitter still has a load until step two.  If there is no self-oscillation in the final, the RF should disappear the instant the first set of relays opens at step one. There shouldn't even be a trace of a spark at the antenna relay contacts when switching from T to R and vice versa.

I protect my receiver with a small SPDT coaxial relay that resembles a miniature Dow-key, but uses BNC connectors instead of SO-239's - a hamfest find.  It switches, simultaneously with the receiver mute, the rf input jack on the receiver from the antenna to ground.  I put it there to protect the receiver from possible rf damage when I use a separate receiving antenna, but it also adds double protection in case something in the T/R relay system fouls up.

[Steve - WB3HUZ]

Are the terminals to the RX shorted?

[K6JEK]

Are the terminals to the RX shorted?

I'm told they are.  I'm going to encourage Mike, W6THW, to jump in here and answer the questions himself since I'm just passing on what he told me over the air.

Don.  Thanks for the explanation of the key off events.  I use those little BNC relays myself.