Gates One Blows PA Fuses w/ Modulation

[mbrockm]

Had an odd problem with our SX5 blowing certain FETS and PA fuses. Turned out to be one of the coax connectors from the divider to the PA stages had become loose. Found it when I re-seated all the connectors to the PA modules. Never worked in a gates one but maybe something similar. Mark Brockmeyer.

[grich]

I would strongly suggest finding a way to do the internal measurement under power without cheating interlocks. There’s often a way of sneaking a couple of test leads out of the cabinet without bypassing interlocks.

Different situation but I once had to measure some voltage in a Continental 315F 10kw that happened to have a window to where I could set up the Fluke so I could read it through the window. Have also run test leads out a door and closed the door on them. Hopefully you can find a way to do it that doesn’t damage your test leads.

Switch to Safety! (apologies to QST)

Per the manual, some troubleshooting must be done with the back door open and the shorting switches blocked open. The switch that tells the controller the back door is open is NOT defeated, so the PA contactor can't pull in.

[Shane]

Roger that. Thank you for the info from the manual.

[mbrockm]

The SX5 has unused thru the bulk head BNCs for making live measurements. Are there any on the Gates one?

[radio_guru]

Let me add one more thing.

Do this work into a dummy load or a very broad load. I know of instances where the load is narrow and the lights will randomly flicker indicating bad FET when there are none. At least this way, you know the problem is in the TX and not being influenced by an antenna which may have gone wonky.

RG

[ChuckG]

I don't recall seeing any on the ones or the fives. I've done the IPA drive measurement many times (scope across a quad resistor, ) and it's safe enough if you are careful. The control and IPA supplies are on but the RF supply should be off.

The SX5 has unused thru the bulk head BNCs for making live measurements. Are there any on the Gates one?

[RodeoJack]

Wondering if engnewbie found the problem and might let us in on the fix....

Over the years, my Gates-X transmitters have presented me a cornucopia of challenges and provided many hours of relaxing solitude, whilst contemplating each one ( ). However, blowing FETs, merely with modulation, has not been one of them.

[wking601]

Sonetimes a faulty fuse holder will cause the fuse to heat and fail.

[RodeoJack]

Sonetimes a faulty fuse holder will cause the fuse to heat and fail.

That would seem a rare occurrence. Matter of fact, in my experience, I recall only one case where I've lost a fuse due to a failed holder. That was the main fuse on a vintage Nautel Ampfet P400. It's still in daily service, btw...

[rfn]

Fuse holders are cheap and usually not at all difficult to change. Nautel had an issue some 10 or so years ago with a bad batch of fuse holders. After a problem with one I replaced all of them on the ND25 just taken out of service this past summer. No fault of Nautel - blame it on the maker of the fuseholders if you can figure out who that might have been. Point is, since Nautel got a bad batch it's not impossible that other manufacturers did as well.

[grich]

Fuse holders are cheap and usually not at all difficult to change. Nautel had an issue some 10 or so years ago with a bad batch of fuse holders. After a problem with one I replaced all of them on the ND25 just taken out of service this past summer. No fault of Nautel - blame it on the maker of the fuseholders if you can figure out who that might have been. Point is, since Nautel got a bad batch it's not impossible that other manufacturers did as well.

Yep...BE had to post a service bulletin about fuse clips in AM transmitter modules a few years ago. It happens.

[RGORJANCE]

Waaaay back when, Harris had a similar fuse holder issue in their DX-10. That case was caused by insufficient tightening of the bolt holding the clip on the circuit board. The clip wasn't tight enough to provide proper tension to the fuse which failed slowly due to heating.

Fossil

[Kent T]

Fuse holders are cheap and usually not at all difficult to change. Nautel had an issue some 10 or so years ago with a bad batch of fuse holders. After a problem with one I replaced all of them on the ND25 just taken out of service this past summer. No fault of Nautel - blame it on the maker of the fuseholders if you can figure out who that might have been. Point is, since Nautel got a bad batch it's not impossible that other manufacturers did as well.

Yep...BE had to post a service bulletin about fuse clips in AM transmitter modules a few years ago. It happens.

Gil recommended replacing those on early BE AM-1A transmitters 2 years ago. We did so, with the oldest of the pair I maintain. The newer of the two we run as Dual Alternate mains had the upgraded fuse clips, cheaply done, one less thing for engineer Kent T to worry about.

[engnewbie]

Boy, do I feel silly. After months of replacing FETs and blown fuses, I found the problem. But first, a bit more background.

One day the manager called me and said the transmitter power was down to 800 watts with no blown fuse indicators, so I scheduled a visit the next day. When I arrived, he told me the transmitter had come on at full power that morning. I looked, and, indeed, it was at full power, with all meters normal. Having learned the hard way not to fix what isn't broken, I made the sign of the cross and went home. The following week, the owner called me and said the transmitter was back down to 800 watts, with the a blown fuse indicator lit.

This time, when I did the in-circuit FET check, two of them had very high readings (previous checks were normal). I pulled the PA board and found that socket pins for two FETs were burned and had fractured solder joints. I replaced them and the transmitter not only makes full power, but I had to dial it back because it was pushing nearly 1,400 watts.

So, fractured solder on the socket pins were causing intermittent power changes and, occasionally, blown fuses. Gates Air was very helpful in suggesting replacement pins, as they no longer carry them, and in suggesting the proper procedure for making the repair. If anyone needs the source for the pins, let me know.

[Deep Thought]

Thanks for the update. There was a similar fuse-blowing issue in another transmitter that turned out to be almost the same thing...bad fuse clips and/or solder joints heating up and literally melting the fuse link.

Glad to hear you found the problem.