Hot switching day/night pattern question

[Fran3]

It has been my opinion that it is better to turn the transmitter off, switch the antenna configuration, and then turn the power back on.

But when the transmitter takes 20 to 30 seconds to power the carrier back up everybody complains as most mobile listeners will punch out... even if you make a pre-announcement about a few seconds of silence in order to make a brief technical adjustment.

Another local station has told one of our staff that they switch hot... meaning they just leave the transmitter on and hit the switch that through the big RF Contractors to Day or Night position.

In our case we are switching circuitry in the phasors, in the ATU's, and switching one of three towers in for night and out for day pattern.

So I just wanted to check with you guys and get your opinion's on this topic.

Thanks for any help.

[kkiddkkidd]

Just curious why it takes 20-30 seconds. The last controller that I built, killed the TX, waited 2 seconds, switched the array and powered the tx back on about 2 seconds later. About 5 seconds total. And that was probably being overly conservative on the wait times.

Another on a Harris DX hit the RF mute, simultaneously switched the array and selected the low power preset, waited 2 seconds and then un-muted the tx. A total of 2-3 seconds.

I have heard proper DA changes on stations that didn't really loose a whole word.

But yes you are correct, hot switching an array is problematic on several different levels. Even high power contactors will eventually be damaged by hot switching 1kw.

[Deep Thought]

Hot switching is the fastest route to component failures, contactor spring finger destruction and general transmitter unhappiness. NO station should take more than a few seconds to switch. If yours is taking 20-30 seconds something is seriously screwed up.

[RodeoJack]

What transmitter are you running? Unless you're powering down a tube rig, the only transmitter I'm aware of that has any kind of delay would be the old Nautel Ampfets, and their delay wasn't more than a few seconds from dead cold.

If you have any kind of recent model, it's likely to have some kind of momentary carrier mute that's designed to allow for pattern changes without shutting off the "high voltage" (whatever that is in yours). Some phasor controllers include relays designed to be hooked up to such a mute. I have a couple of Kintronic controllers that run about 1/2 second, which I doubt anyone notices at all.

In any case, it's highly unlikely the transmitter's intent is for you to wait that long... and DT's right. If hotswitching doesn't spike something out in the transmitter, it'll probably burn the fingers off of your contactors, eventually. When that happens, your transmitter might not come back up at all, likely making management even more unhappy! I would definitely not go by the example of your friends down the street. One of these days, they might find themselves off the air while you're still happily chugging along.

As far as overly long mode changes are concerned though, you're not alone. A 2-site station near Seattle drops one transmitter, and I think the other site probably takes something like 10 to 15 seconds to come up. Another station has no mute relays, so they programmed their Burk to do 2 "off" commands (just to be sure), then the pattern and monitor mode changes, and finally the transmitter on. That sequence probably takes them a good 15 seconds. They've been doing it that way for years and nobody complains. But in that case, the owner did the programming, so.....

[ChuckG]

I have heard proper DA changes on stations that didn't really loose a whole word.

Yeah, but you're in the south. They talk faster up here.

I have ours set up to mute the Gates 5, switch the contactors and change power levels simultaneously. As soon as the last contactor microswitch closes the Mute is released. Maybe 2 seconds from start to finish.

I maintained a station in the 80's that did power changes manually. 2-lower, 3 raise, 2-raise. When they'd forget the first step (drop the plates) they'd burn up half the contactor fingers. Of course THEN it would shut down due to VSWR.

I'm interested in what sort of transmitter this is too.

[RodeoJack]

I maintained a station in the 80's that did power changes manually. 2-lower, 3 raise, 2-raise. When they'd forget the first step (drop the plates) they'd burn up half the contactor fingers. Of course THEN it would shut down due to VSWR.

Funny you mention that, Chuck. We had one up here that, because their main transmitter had no power cutback, had to switch to a smaller one at night. Same manual sequence, but the were wired so they had separate position numbers for EVERYTHING... fil on night, plates off day, antenna switch to night, plates on night, fil off day... and reverse the process in the morning. It was one of those Moseleys that had to count up to whatever number you pushed, so every button involved a wait.

The manager believed in the "high school football" rule, which often resulted in the board ops "forgetting" to lower power at night. The program director would run through the high power sequence as soon as he got to work in the morning "so he wouldn't forget". When I took over, I automated the process, which stopped that nonsense. I had expected some resistance to taking it out of their hands, but the staff just took down the power-change reminder signs and promptly forgot they ever had to mess with it.

[kcbooboo]

I wired four DPDT 240VAC relays across the day/night and main/backup contactor coils. All of the normally open relay contacts were wired in parallel and each pole went to the MUTE inputs of the main and backup transmitters. They're activated very briefly, probably 1/2 second, at the same time that the contactors are powered; that duration is a function of the remote control system. This way both transmitters will mute, whether they're on the air or not, when any of the phasor functions are chosen.

I probably could have used one relay and four diodes, but I wanted to keep everything isolated, not knowing what was wired to what.

This was better than the previous setup: no muting at all (i.e. hot switching) which wasn't that bad for a 1kw transmitter but still not good engineering practice.

Bob M.

[kkiddkkidd]

I have heard proper DA changes on stations that didn't really loose a whole word.

Yeah, but you're in the south. They talk faster up here.

People down here listen slower too so it all equals out... <g>...

[Fran3]

Showing my flat spots... what do does "mute" transmitter mean?

- turn off the transmitter
- kill the audio
- or what?

Thanks.

[Kelly]

Sounds like someone accidentally wired the "carrier mute" from the phasor controller to the "filament off" RC terminals. That would sure take 20-30 seconds, depending on the transmitter.

[RGORJANCE]

The term "mute" means that the xmttr is turned off before the pattern switches in order to remove RF from the system, eliminating arcing in the contactors.

In modern xmttrs, like PDM rigs, the mute (usually a relay system included in the phasor controller) merely shuts off the PDM drive, turning off the RF, while leaving the HV on. Upon successful completion of the pattern change, with all contactors providing proof of switching, the xmttr is turned back on by the controller.

As for older tube transmitters, the mute would kill the plate voltage, then after pattern change, the plate voltage is turned back on.

Hope this helps.

Fossil

[kkiddkkidd]

To go a little further on Fossil's comment, RF MUTE supposedly stops rf generation INSTANTLY while simply hitting PA Off (tube or SS) may allow a few more cycles to be made at high power as the PS fades down. High enough power to damage contacts or SS devices if switched simultaneously.

And I agree that someone probably has wired the controller to the one button Start & Stop rather than PA On/Off. Or possibly has the macro set for that amount of delay. Just changing it to PAE On/Off and/or reducing the off time to a couple seconds will save HOURS of intentional off air time per year.

The math didn't really hit me until I was writing this but 30 seconds per day for a year is over 3hrs total. Or for some of my clients... $365 at a dollar-a-hollar.

[Shane]

You could put a J-38 across the mute terminals of an AM1-A and send CW. It's very quick.

No I haven't tried it!