FM3.5 odd problem

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REW
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FM3.5 odd problem

Post by REW » Mon Apr 09, 2018 12:34 pm

I am working on an old Harris FM3.5 transmitter that is going to be used as a backup transmitter. I turn it on and It comes up to full power with no problems. Meter readings are very close to the original checkout sheets. After it has been on for awhile it will make a noise that seems to vibrate the cabinet, makes me think something has put a huge load on the HV supply as you can hear what I think is the transformer really humming, then the building lights flicker and the plate and main power breaker trip. All of this happens in less than a second. Watching the meters as this happens the HV seems to dip some and the plate current also dips. Not what I would expect if it were a short somewhere. A different tube and rectifiers didn't make any difference. Also when it trips out there is still some HV showing on the meter till the bleeder resistors pull it down. I would think a short in the HV would kill the HV totally. I think I ran into this several years ago but just cant remember what I found. Any Ideas.

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RGORJANCE
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Re: FM3.5 odd problem

Post by RGORJANCE » Mon Apr 09, 2018 1:10 pm

This is a real big SWAG!

Take a board, a real dry one, and lift the HV filter choke completely off ground to see if it will stay on. Totally insulate it from ground. It could also be something breaking din in the plate xfmr. But definitely do the filter choke test first.

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REW
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Re: FM3.5 odd problem

Post by REW » Mon Apr 09, 2018 2:40 pm

I took the plate xfmr out of a HT-3.5 and tried it. Did the same thing so that eliminated the transformer. Also no sparks or smoke in the back when it happens. I will try getting the chokes off ground tomorrow afternoon. Thanks

ChuckG
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Re: FM3.5 odd problem

Post by ChuckG » Mon Apr 09, 2018 7:58 pm

It's quicker to lift them all up off ground at once and put a small fuse between transformer/choke chassis and ground. The blown fuse will tell you which iron has the problem.
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Chuck Gennaro
Central Wisconsin

kcbooboo
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Re: FM3.5 odd problem

Post by kcbooboo » Tue Apr 10, 2018 2:46 am

I'm not familiar with the FM3.5, but if it's running on 3-phase AC power, perhaps one of the phases is dropping out. I had a similar problem with a Continental 20kw 816R2 that uses SCRs in the xfmr primary to regulate the HV. Turns out a 3PDT 24V relay was bad and it was intermittently dropping the signal going to the phase control board, which would then drop one SCR out of the circuit, causing an imbalance, and tripping the 75A plate breaker. Same humming then winking of the building power. The relay itself was the problem; swapping them around caused vastly different issues. Eventually we took the clear relay cover off and saw one of the wires from one movable contact to the connector tab had never been soldered. The wire was nicely wrapped through the hole in the tab and left there. Took 10 seconds to fix, but that was after the plate breaker, all three SCR modules, and a control transformer had been replaced. This relay was in the right xmtr bay that has the big rotary fan and plate xfmr in it, so the frame is constantly vibrating.

Bob M.

REW
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Re: FM3.5 odd problem

Post by REW » Tue Apr 10, 2018 1:45 pm

Thanks for all the help. At about 1am I found the problem. The FM3.5 uses a solid state relay in the primary of the plate xfmr. It seems this relay would start conducting on just half of the AC cycle so it was acting as a rectifier and for just an instance it was putting DC on the transformer primary. Replaced the SSR and transmitter has been on all day with no problems. I ran into this problem several years ago at another site. I just couldn't remember what the problem was. When I remembered what station it was I got the manager there to read the maintenance log to me. Another good reason to keep a logbook on things.

ChuckG
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Re: FM3.5 odd problem

Post by ChuckG » Tue Apr 10, 2018 7:17 pm

REW wrote:
Tue Apr 10, 2018 1:45 pm
The FM3.5 uses a solid state relay in the primary of the plate xfmr.
Nice work finding that. Would never have guessed to look there.

Harris had a field upgrade for that, adding a magnetic contactor. The contactor went in the plate primary circuit,the SSR then got rewired to operate just the contactor coil. FIxed a lot of oddball problems.
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Chuck Gennaro
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RGORJANCE
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Re: FM3.5 odd problem

Post by RGORJANCE » Wed Apr 11, 2018 6:30 am

Well done. That one goes in the notes for the one I work on.

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grich
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Re: FM3.5 odd problem

Post by grich » Wed Apr 11, 2018 10:33 am

ChuckG wrote:
Tue Apr 10, 2018 7:17 pm
REW wrote:
Tue Apr 10, 2018 1:45 pm
The FM3.5 uses a solid state relay in the primary of the plate xfmr.
Nice work finding that. Would never have guessed to look there.

Harris had a field upgrade for that, adding a magnetic contactor. The contactor went in the plate primary circuit,the SSR then got rewired to operate just the contactor coil. FIxed a lot of oddball problems.
Yep...that was the case on the very early HT-10 I was blessed with in 1988(?). Also blessed with the step-start resistors burning through the main wiring harness. And the filament rheostat overheating due to lack of airflow. Oh baby...

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RGORJANCE
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Re: FM3.5 odd problem

Post by RGORJANCE » Thu Apr 12, 2018 6:24 am

It's "day juh view all over again"!!!!

Our FM-3.5K just blew a solid state relay on the plate ckt. This rig is a later version with a mechanical contactor.

We got hit two days ago with a weird power surge that somehow affected the plate current metering circuit. While two other guys were working on that mtrg problem, one of them decided to "chicxen stick" the innards. (He neglected to turn off the main breaker). With the shorted SSA, I heard a "tick, tick, KA-BOOOOOOM, flash of light and sudden quiet and total darkness. Followed by words not to be used in polite company. One AC leg thru the bad SSA put 240 onto one side of the plate xfmr.

First you say it, then you do it!

A learning experience for the younger guy. Run to electrical supply store scheduled this morning after breakfast and coffee.

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