What R U using for T/R and TX delay power switching? New relay tricks!

[VE7 Kilohertz]

I stayed up until 11:00pm last night, working on my Bauer TX/RX switching arrangement as I discovered now that I have changed the AGC in my RA6790, the RX doesn't recover in time to hear the first 8 or 10 words of the next person, which in quick break QSO's doesn't work. BTW, trying to think and work around HV after 8pm is not a good idea. 

What I have is an HP 8640B as my VFO, the output goes thru a Dow Key relay right at the generator and I switch this on and off to keep from swamping the RX while listening. Works great. Inside the Bauer is one honkin 8 contact relay that was there when I bought it, actually more of a contactor as the pads are 3/8" wide. This thing makes a gawd awefull clunk when turning on and has been my hallmark signature sound for years. I use an extra set of contacts to short out the RX when TXing but still have a lot of leakage getting into the RX, almost full scale on the dB meter, so I installed another realy about 10' down line to short the coax line going to the RX again, and this gets the level down to about S9 +20, which is fine and makes for good local monitoring. What I should do eventually is relocate this contactor out of the TX onto a wall near the antenna entry. Mmmm. Good idea. Should have done this last night. Rats. Well, later.  I also have a big time delay relay for the HV xfmr and delay HV turn on by about 1/2 second.
All of this switching action is triggered by a DPDT switch at the operating position, but as mentioned, I still am getting a big pulse when going from TX to RX which nukes the RX for a time and probably isn't good for the front end. It is either the HV collapsing causing a spike or the RF remnants getting into the RX before the relays do their thing. Either way, I needed to fix it.

So, to the point of my article..finally you say?  I discovered thru much experimenting that you can time delay standard 120VAC coil relays quite easily. There isn't really any 12VDC or 24VDC in most bcast TXs and most relays in them are 120VAC. I didn't want to build a power supply and wanted this to be simple and use parts I had in stock. You can go out and buy TD relays for around $80 new or find them surplus for but this scheme I came up with seems to work well. I will report back if I find any problem.

I used a standard 120VAC octal relay, half wave recified the power to the coil which makes it pulsed DC, then played with different capacitor values directly accross the coil. This worked great but with the right delay off, the on delay was a bit too much as it took time to charge the cap so I ended up placing a resistor in series with the cap to limit the inrush current when charging and this allows the relay to turn on normally and the then when the power is removed, the cap discharges back thru the R and gives an easy 1/4 - 1 or 2 second delay, depending on C and R values. I found for a 1/2 second delay with a standard octal P&B KCBP14 relay that a 100uF @ 250V cap  and a 560 ohm R in series, produced perfect results. Play with the C and R values to get the timing results you need. To get a turn on delay, swap the position of the diode and R, restricting the charge rate and thus effect a ramping up voltage on the relay coil. It will shut off instantly as the diode will block the current trying to get out of the C.

I'll try to get a drawing posted here somehow to show you the two circuits. It really works very well and is dirt cheap to build with parts at hand.

Now when I switch on, the VFO Dow Key and the antenna relays close immediately, then 1/2 - 3/4 second later, the HV kicks on. When switching off, the HV drops immediately, then 3/4 second later, the antenna and VFO drop. The reason I delayed the VFO going off is so the tank cct is never without drive when powered up. All of this with simple R's, C's and diodes. Used 1N5408 diodes just to be safe but you could probably get away with 1N4004 or similar.

Look forward to hearing how you do your switching.

Cheers

Paul
VE7KHz

[KL7OF]

Paul....Can't the racal 6790 be muted?

[WA1GFZ]

Just open the speaker line. put a 100 ohm resistor across the radio if you feel it needs a load.

[K1JJ]

The solution, as suggested by Steve, is to mute the receiver first.  Over here I key a relay that FIRST immediately mutes all receivers in the shack including disconnecting the speaker system.. THEN  a second set of contacts on this first relay keys another relay which does the rest of the shack duties...ants, T/Ring, etc.  Muting the RX might mean providing a cutoff bias somewhere in the RX system, as long as it doesn't trigger the AGC to become delayed.

The idea is to get those receiver front ends killed first, then the mechanical delay of keying the next relay in line is all that's needed to isolate the receiver AGC pop.  Be sure that the RX antenna gets disconnected before the TX comes on. If you use the technique above, the RX is off before the exciter comes on.   One relay keying another is a form of sequencing if you can use the real short delay. Same for antenna relays.

Yes, Paul, that steering diode idea is the key to getting those R/C relay delays to work FB. I've used it here too.  Slow on, fast off and it doesn't affect the other relays on the same line/power supply.

T

[Steve W8TOW]

Lots of different stuff,
On station combos that I intend to use on cw, I use a "Radio Adventures"
delay TR control board. (Bought a bunch of these a few years ago)
They control the PTT circuit thru a time delay keying circuit activated by the
key contacts.
I parallel antenna relay control with that...
On rigs I intend to use only on AM,
I control the B+ transformer by simply controlling the primay of the trany thru
a control relay...sometimes, I include a sequencing circuit, but plate transformers
are easy to find, MOD iron isn't...so....
I want the B+ to always be there, right went I hit the switch....
If you ever have a protective relay on your mod iron stick when there
is HV B+ for the Class C missing, boy can ya ever get some lightning!
I have had some experience with the solid state time delay cubes, but
if you got any RF floating around, they mis-behave too....
KISS....Throw the switch, activate the relay off ya go.....
73 steve

[VE7 Kilohertz]

The solution, as suggested by Steve, is to mute the receiver first.  Over here I key a relay that FIRST immediately mutes all receivers in the shack including disconnecting the speaker system.. THEN  a second set of contacts on this first relay keys another relay which does the rest of the shack duties...ants, T/Ring, etc.  Muting the RX might mean providing a cutoff bias somewhere in the RX system, as long as it doesn't trigger the AGC to become delayed.

The idea is to get those receiver front ends killed first, then the mechanical delay of keying the next relay in line is all that's needed to isolate the receiver AGC pop.  Be sure that the RX antenna gets disconnected before the TX comes on. If you use the technique above, the RX is off before the exciter comes on.   One relay keying another is a form of sequencing if you can use the real short delay. Same for antenna relays.

Yes, Paul, that steering diode idea is the key to getting those R/C relay delays to work FB. I've used it here too.  Slow on, fast off and it doesn't affect the other relays on the same line/power supply.

T

Thanks for the thoughts guys. What I am really trying to do, is make sure the transmitter is going to be happy, as well as the RX. I don't think the tank cct would like to be unloaded with B+ on the tube plates of the finals. I guess I should have specified I am working on broadcast rigs, which aren't designed PTT and have special switching needs.

I don't know if the 6790 can be muted. Good thought. I will have to read the manual again. Yes, disconnecting the speaker is on the agenda but I am firstly concerned about not knocking out the front end and ensuring the TX is happy. With my current switching scheme, it is, and all is well, except I made reduce the delay times as I am missing a few words on RX from TX.

Cheers

Paul

PS Steve how do you switch the Gates and Collins rigs?

[W3CRR]

Here's the very simple, electromechanical, and darned near bulletproof method I use to switch my Collins 32V-2 and R-388 combo:

I employ a Heil double switch footswitch -- a new product from Bob's company.  The two switches on the device are staggered slightly.  In my setup, the first switch to trip activates the receiver's muting relay (which is powered internally thanks to a smple mod).  The second switch, which is tripped a moment afterwards, activates the transmitter's HV and PTT circuits which, in turn, trip the antenna switching Dow Key installed on the transmitter. 

It ain't sophisticated but it's dead simple to install and foolproof in operation.

73,
Craig
W3CRR

[KL7OF]

Paul.... I use a dow key for antenna switching......The rest is manual...I reach over and mute the receiver and then key the transmitter......

[WA1GFZ]

Paul,
A low frequency pin diode across the antenna terminal is a good limiter. I can give you a schematic of the 6830 limiter if you want sometinng that handles 20 watts. Also consider shorting the input while in TX so you don't hammer the AGC.  The 6830 has AGC control coming out the DB 25 audio for diversity connections Maybe the 6790 has similar stuff. I usually run med agc so no problem with delay. Long does sound nice when conditions are good. There is also an AGC dump signal used for bite. You might be able to fake it to dump when tx goes to rx. This signal discharges the agc cap quickly. 

[VE7 Kilohertz]

Paul,
A low frequency pin diode across the antenna terminal is a good limiter. I can give you a schematic of the 6830 limiter if you want sometinng that handles 20 watts. Also consider shorting the input while in TX so you don't hammer the AGC.  The 6830 has AGC control coming out the DB 25 audio for diversity connections Maybe the 6790 has similar stuff. I usually run med agc so no problem with delay. Long does sound nice when conditions are good. There is also an AGC dump signal used for bite. You might be able to fake it to dump when tx goes to rx. This signal discharges the agc cap quickly. 

Hi Frank. Ya baby. I missed that AGC dump. I looked to see if it had mute per Steve's suggestion but it doesn't. That AGC dump should do it as I have made Dallas Lankfords AGC mod to this RX, which changes from hang AGC to RC only, and extends the TC to around .75 seconds in MED.

I have been shorting the RX antenna line, actually in 2 different places but the RF still gets in.

Yes, please email me the limiter schematic with PIN diode.

thanks

Paul
VE7KHz

[WA1GFZ]

search the web on pin diodes. I found just hanging a pin diode across an input will give limiter action.  More elaborate configurations drive the diode. The 6830 set up does not use pin diodes it uses high speed diodes. positive bias on one bank and a negative on the other. I'm considering changing to pins in the future to see if I can bring up the dynamic range. 
The AGC dump is the signal to yank if you want the radio to recover faster. You might consider some real SMA RF relays at the RX to provide better isolation.
Also RG58 is crap coax use RG224. Crap coax shields don't give good isolation. Maybe you need to mount a SMA relay right off the back of the radio and short it close to the input. A SPDT SMA coax relay with a short across the NO contact ant on the NC input. Common to RX.