Do I need a fourth sequencer? No. However, the three I have are all based on the common approach of grounding the PTT line to the linear to change to transmit. Enter a Microset SR-500 144MHz linear amplifier which has an unusual way of activating transmit mode.
The PTT line from the SR500 comes out via a 3.5mm stereo socket, with a ground contact and a pair of wires which the manual explains cause the linear to go into PTT when connected together. However, grounding the lines just turns the linear off. All my existing sequencers use the standard grounding method for triggering the PTT and thus the Microset just turned off when it should be transmitting. So I needed a sequencer which is isolated from ground.
The need for a sequencer for the 2m set-up was explained on this blog back in 2021 here. The basic diea is to turn off the preamp before activating the linear PTT, and do the reverse on returning to receive. The diagram from that posting is reproduced below:-
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| Sequencing set-up for transmitting |
Compared to the diagram, the problem with the SR500 is that the "Delayed PTT (grounded on TX)" is that the SR500 doesn't accept grounding of its PTT line.
As the first step I decided to try to cobble something temporary up with my current Down East Microwave sequencers. This would have to allow for a PTT switching output isolated from ground. This did not seem practical from the PTT outputs themselves, so then I thought of using a relay. I bodged up a relay in the preamp supply and this worked. It worked, but it was a mess. As the relay was powered during receive I worried that it would overheat. As it happens the Microset has a very noisy set of fans so I turn it off when not in use. I wired the sequencer to the same supply. It and thus the relay is off when I am not using 2m and so heating of the relay is limited.
However, even though I used this lash-up for eight months it was not ideal. I did a lot of tests with the timing. My concern was that by feeding the relay from the preamp supply meant that the usual timing was reversed and the timing might be wrong causing either hot switching by the amplifier or causing damage to the preamp. Also this bodge only worked because I did not need a preamp supply as I was using my IC-9700 and it sequences the preamp supply itself. I would like to try using my IC-7100 for 2m, or at least have the option of using it.
The relay I was using was one from my parts box, and no doubt it is slow acting and liable to overheat if left energised. Perhaps I could source something better. Surely there was a better way to solve this problem.
I dithered about how to do this. I thought about building something from scratch but then I discovered that Hamradioparts make a sequencer board with isolated outputs. The details are here:-
https://www.hamradioparts.eu/4-port-isolated-universal-sequencer-board?Product=606679942&Lng=en
At €50 it is about half the price of the Down East ones I have been using and it has four isolated outputs which are all configurable for "normally open" or "normally closed" (in other words, useful for linear, preamp or fan use). Their non-isolated sequencers are even cheaper. The delay time is also adjustable. By comparison the Down East ones only have three outputs preset for purpose and the timing is also fixed. The big plus that made me decide to purchase the Down East ones was that they were solid state and therefore potentially faster acting and with no mechanical parts to wear out. Still, I thought this one would be worth a try for this specific purpose.
The ordering process was easy and the board arrived quickly. The instructions were clear and it took a short time to box it up with various sockets and LEDs coming from my parts box.
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| The boxed 4 port sequencer at GM4FVM |
When the photo was taken I had not connected up the PTT line (yellow wires). After experimenting I connected it to the third relay which seems to give good timing between it and the preamp supply which was already connected to relay one. The four relays can be seen in the photo alongside their connector blocks which have both NO and NC connections. I fused it and added some LEDs to show that it is working.
So far it is working well. I did eventually put the lid on the box, whereas the old "temporary" relay system never got under cover (to allow for more cooling that old relay?) before I replaced it with this device. It seems very old tech compared with the solid state Down East ones, but it is cheaper and with more potential. The sound of four relays clicking in succession as I got to PTT is a bit odd, with four more when I go back to receive. Still, I don't have to switch it off to prevent against my old relay getting too hot. And I should be able to bring the IC-7100 into use with the Microset, even if just as a backup.
Hamradioparts also have an configurable amplifier controller which looks interesting but which is out of stock right as I write.
So the question remains - why does the Microset SR500 not work with the standard PTT to ground system? I have had two other Microset amps and both of these used the standard system.
73 Jim
GM4FVM
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