HT-5 plate off problem

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fm-engineer
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HT-5 plate off problem

Post by fm-engineer » Wed Mar 14, 2018 8:14 pm

Here's an odd one for the braintrust: Have an HT-5 that likes to stay on, and I mean on. The plate off button results in a clunking /shuttering plate contactor, where its trying to stay on. HV and current jumps up and down with the clunking. Remote or local commands make no difference. Anyone ever experience this coffee break puzzle? A bad or weak coil on a normally open contactor would result in the opposite condition of not staying on, so I'm thinking its something in the primary logic.

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Deep Thought
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Re: HT-5 plate off problem

Post by Deep Thought » Wed Mar 14, 2018 8:38 pm

Start with the easy stuff: Is the plate on button stuck on?
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Re: HT-5 plate off problem

Post by fm-engineer » Wed Mar 14, 2018 8:54 pm

True words there. Did that first thing. Its not the panel button, but I didn't have time to dig into the remote control interface board. I do think the local switch bypasses the remote command strip, and the problem exists in local also. Headed back there in a few days to examine the schematics in depth.

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Re: HT-5 plate off problem

Post by TPT » Thu Mar 15, 2018 6:54 am

Here's where you need to start (from an HT10 manual):
"The FM Transmitter Controller provides the following functions:
Transmitter On/Off. Raise/Lower Power. Automatic VSWR Foldback. Automatic Power Control. AC Restart. Remote Control Interface. Overload Detection, Protection, and Display."

Bet it is a bad IC, shorted cap or other component on that board.

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Dale H. Cook
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Re: HT-5 plate off problem

Post by Dale H. Cook » Thu Mar 15, 2018 8:56 am

fm-engineer wrote:
Wed Mar 14, 2018 8:54 pm
I do think the local switch bypasses the remote command strip ...
I think it may have been a requirement for type acceptance but AFAIK every transmitter I have worked with works that way. Even in those with a local/remote switch the plate off button always works.
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RGORJANCE
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Re: HT-5 plate off problem

Post by RGORJANCE » Thu Mar 15, 2018 9:06 am

I second the suggestion of the controller board, Got an FM3.5K that had a plate off inop. On that rig there is a board vertically mounted at the back of the controller area. Replaced it and it's happy. BTW, the 3.5/5k is the early predecessor to the HT series. Quite a few common parts. I think the HT did away with the little interface board I mentioned earlier.

If you carefully trace thru the controller board for the plate off string, you should be easily able to identify those components.

Fossil

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Shane
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Re: HT-5 plate off problem

Post by Shane » Thu Mar 15, 2018 4:55 pm

Isn’t there also a backup controller one could work from while working on the main controller? There is on the higher powered versions. Maybe that would be a way to rule in or rule out the controller.
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Re: HT-5 plate off problem

Post by TPT » Fri Mar 16, 2018 6:11 am

The HT10 I've worked on only had the one board. Suspect the same for his 5 kw.

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Re: HT-5 plate off problem

Post by kkiddkkidd » Fri Mar 16, 2018 6:38 am

The HT30/35's that I have maintained over the years had a CPU type controller with a rudimentary backup controller. They had an alphanumeric display for status and most of the multimeter readings.

I don't recall ever seeing an HT-3 or 5 with the alpha display.

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Re: HT-5 plate off problem

Post by fm-engineer » Fri Mar 16, 2018 10:36 am

Correct,

All analog metering and I think there's just one board on HT5. I appreciate everyone's input.

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Re: HT-5 plate off problem

Post by ChuckG » Sat Mar 17, 2018 1:21 am

The controller board sends a +V signal to a solid state relay, which in turn operates the plate contactor. I've had those solid state relays fail in all sorts of ways, most resulting in the plates not staying off. Remove the input signal to the SSR, if the plates stay off then it's the controller. If they stay on, it's the SSR.
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