TX/RX sequencing

[kb3ouk]

How exactly would you do on/off sequencing of a transmitter, without using one of those sequencer boards. Like in this order:
Antenna changeover
Key exciter/RF drive
Turn on high voltage
Now I figure that for receive to transmit it would be easy, use a dual pole relay, one side turns on what is needed, the other side keys the next relay in the sequence. how it would work is the transmit switch would energize the first relay, which uses one set of contacts to switch the antenna relay, and the other side keys the second relay. that relay keys the exciter with one set of contacts and keys the third relay with the other set. the third relay turns on all the volages to the tubes. Now the problem is how do you get sequence reversed, so that the B+ is off, then the exciter, then finally the antenna is switched to the receiver last? Or, a better question is, with a tube transmitter how critical is it for the switching to be that precise, or can it pretty much be done all at once.

[k4kyv]

Mine uses two separate pilot relays to activate the antenna changeover, receiver mute and transmitter plate relays.  A third relay employs a latching circuit to switch a timing capacitor from one pilot relay coil to the other, to reverse the sequence going from T to R, vs going from R to T.

Two steps should be sufficient. You want the antenna changed over, the receiver muted and the VFO switched on BEFORE +HV is applied to the transmitter stages, and to turn off all the transmitter stages HV before killing the VFO, unmuting the receiver and switching the antenna back over to the receiver. It doesn't hurt to key the exciter/RF drive/VFO and change over the antenna in one step; you want to sequence turning on the transmitter high voltage to avoid hot-switching the antenna relay.  I use about a 250 millisecond delay between the two steps. Before building the sequencer, all my station relays were switched at the same time in one single step.  I always noticed sparking at the antenna changeover contacts with on open frame antenna relay and never paid a lot of attention to it, but when the Gates broadcast transmitter turned a Dow Key coaxial relay to a solid block of charcoal, I decided I needed to sequence the relays.

The circuit is described in the November, 2005 issue of Electric Radio.

[K5UJ]

How exactly would you do on/off sequencing of a transmitter, ....

Brother, let me tell you, I went through exactly the same thought process last year so you are not the only one 

Back when I ran separate tx and rx years ago I got by with separate antennas but I ran only 40 watts or so on CW.  I got into dealing with this seriously when I started operating AM and figured it would be trivial.  That was until I started thinking about reversing everything hi hi.   I am building the circuit Don described, that is, when I don't get distracted with other projects. 

[VA3AEX]

A great question.  Add in a modulator to the transmitter, and are switching same on at the same time as the HV+?

[W4NEQ]

I thought about this for a while, and never came up with a simple way to reverse the process. So I bought the Advanced Receiver Research board, looked at the schematic, and thought "why didn't I think of that?"  There IS a simple and elegant solution.  I will not reveal his design here, as the low price is probably less than you can make PC boards for.

Chris

EDIT: spelling errors

[k4kyv]

Mine is completely passive, with no transistors or tubes, other than a few protective diodes across relay coils.  It functioned flawlessly for years, until it became flaky a couple of years ago.  I found that one of the relays had developed intermittent contacts, so I replaced the relay and it returned to flawless operation.  Don't know if it was hit with a lightning surge or if normal use wore out the relay contacts, but I have a box full of spare relays identical to the ones I used in the circuit, so that should keep me going for the rest of my life even if I make to 150.

[kb3ouk]

ok, so how does the third relay switch the capacitor between the other two relays?

[ke7trp]

W0VMC has one he makes.  Its a simple box with the TR switch and a row of RCA/phone jacks for Normaly open and normaly closed contacts. There are also receiver mute and TX jacks. Mine is a bit more complicated since I have seperate MOD and TX power supplys.  SO I also have to key on the Mod PS and its important to have that carrier up and running before that happens.

C

[k4kyv]

ok, so how does the third relay switch the capacitor between the other two relays?

When the T/R switch is switched to transmit, it applies DC voltage to the relays.  The  relay that controls the antenna change-over turns on immediately.  The other set of contacts on the same  relay makes the DC voltage available to the latching relay, but doesn't turn it on.

The relay that activates the +HV to the transmitter turns on after a delay, due to the RC delay circuit in series with the coil. The other set  of contacts completes the circuit and turns on the latching relay, which transfers the timing capacitor over to the other relay, the one that controls the antenna change-over.

When switching back to receive mode, the DC is taken off all the relay coils.  The antenna change-over control relay remains engaged for a split second due to the charge on the timing capacitor, but the one controlling the +HV turns off immediately since the timing capacitor is no longer connected across  the coil of that relay. When  the DC voltage is removed from the relay coils, the latching relay unlatches and returns to the "off" position, resetting the timing capacitor back to the +HV control relay, ready to repeat the sequence next time the T/R switch is activated.

[W2XR]

Hi Shelby,

I posted the following response back in November of 2009, in reply to a similar thread.

Here is how I do it in my homebrew 2x 4-400A modulated by 2x 833A rig.

Here goes:

A) For transmit start-up, the following events occur serially:

1) Plate contactor closes 240 VAC circuit to HV/plate power supply

2) 240 VAC plate xfmr primary soft-start limit resistor is shorted out approx. 700ms. later.

3) Apply RF PA screen voltage

4) T/R relay contacts transfer to "Transmit"

5) Apply RF drive to RF PA

6) 1250 VDC 2x 845 audio driver plate supply contactor closes, enabling audio to be applied to the modulator stage.

B) For transmit shut-down to receive mode, the following events occur serially:

1) 1250 VDC audio driver plate supply contactor opens, effectively removing audio drive to the modulator stage, eliminating the possibility of the modulator looking into an unloaded condition.

2) RF drive is removed from the RF PA

3) RF PA screen voltage is removed

4) 240 VAC plate supply contactor opens, removing the plate voltage from both the 4-400As and the 833As.

5) T/R relay contacts transfer to "receive".

Note that all of these critical switching functions are interlocked for fail-safe protection of the transmitter and its associated equipment, so that the next serial switching function cannot occur unless the previous one was executed successfully. This rig is far from being the quickest finger on the trigger in terms of fast break-in operation, but it has proven to be extremely reliable since I first built it (in it's first iteration at least) in 1979, and with no major failures. The total tranmitter start-up time takes, by my guess, about 1 second or so, from the time I throw the "transmit" switch.

The time delay relays I use are made by a company named NCC (National Controls Corp.). They are microprocessor controlled and programmable by virtue of DIP switch selection, and plug directly into a standard octal socket. Hell, you can even DIN-rail mount them if you want for housekeeping purposes, or if chassis real estate is at a premium. I bought mine years ago, but both Allied and Newark carry them, and they are reasonably priced, and I have seen them for sale on eBay. I bypass the 115 VAC input to each TD relay with a couple of .001 uf caps to ground, and never have any issues with RFI affecting their operation, etc.

Forget about those adjustable RC time constant-controlled TD relays; the ones with the control knob on the top. They are fine for use in a soft-start circuit for power tube filaments, etc., but not in those applications where you want precise and repeatable time delays. This is why I used the NCC relays.

Let me know if you need more help with this!

73,

Bruce

[k4kyv]

Here is the sequencer schematic from my original notes.  It contains one ERROR; D4 IS DRAWN BACKWARDS. The cathode of D4 goes to R1 and the T/R switch.

The DC voltage source must be higher than the actual relay coil voltage, since the dropping resistors are part of the RC time delay circuit.

[kb3ouk]

OK, now that I can actually see it drawn out, its not that hard to understand.

[ke7trp]

That is very very close to my sequencer.

C

[VA3AEX]

Not sure if my question on when the Modulator gets switched on in the sequence was highjacking the thread -- my apologies if it did.  I understand I shouldn't be applying HV+ to the Plate Modulator until the TX HV+ has been switched on otherwise you run the risk of zorching the plate transformer?  73  Alex

[Steve - K4HX]

Jay is a smart cookie.

I thought about this for a while, and never came up with a simple way to reverse the process. So I bought the Advanced Receiver Research board, looked at the schematic, and thought "why didn't I think of that?"  There IS a simple and elegant solution.  I will not reveal his design here, as the low price is probably less than you can make PC boards for.

Chris

EDIT: spelling errors

[kb3ouk]

Yes, it is possible to blow the mod transformer if you key the modulator before the HV on the final is turned on, becuase there isn't any load on the mod iron. But, if you are using a mod reactor along with the transformer, there is an easy fix where if you loose your final's HV, it won't hurt the iron if the modulator's HV stays on. Simply put a cap off of each lead of the mod iron secondary between the transformer and the reactor, like in this post: http://amfone.net/Amforum/index.php?topic=30357.msg236509#msg236509

[k4kyv]

If you are using a common power supply for modulator and final, there is little risk of operating the modulator without the final load, since both are powered by the same +HV. I say little risk, because there is always the possibility of component failure, such as the rf plate choke opening up, a wire lead becoming disconnected, an open filament in the final amp tube or a filament transformer failure.  For that reason, it is always advisable to install a spark gap across the primary of the modulation transformer, and adjust the spacing just beyond the point where it sparks over on modulation peaks. This spark gap should be across the primary, not the secondary, since a spark discharge on the secondary side, through  the transformer,  could generate an inductive kick and generate a dangerous spike, the very thing you are trying to avoid with the gap.  The gap may be a single gap across the entire primary winding, or better still, a couple of gaps, one each from mid-tap (+HV tap) of the primary to the plate tap connection for each modulator tube, as RCA does it.

Losing +HV to the final does not leave the modulator completely unprotected, since the PA tube will still conduct over the positive half of the audio cycle, but it runs unloaded over the negative half of  the cycle.  Exactly the same thing happens whenever the final is overmodulated in the negative direction, but only on negative peaks that exceed the DC plate voltage, not over the entire negative half-cycle.

[VA3AEX]

This is good info..  I'm taking my first foray into scratch building an AM transmitter and will incorporate the ideas from this thread.  Starting out PW using 6V6's to modulate a 6L6G final.  As it's intended to be easy to debug, I'm building the modulator, osc/exciter, and common power supply separately.  Finished and tested the power supply yesterday.  Modulator is next based on a multi-match Thordarson rated at 75W -- overkill as max there will 25W into the final, but its the only one I have on hand.  Osc will based on another 6V6 (a common theme: I've got lots of 6V6's from building guitar amps!).

Alex