November 2012
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Oh, damn: Una Parte

Guess what caught fire this time?  It’s this thing, which has become the newest piece in my burned up shit collection:

BE FM30A IPA regulator board

BE FM30A IPA regulator board

If you give up and are totally flummoxed, this is the IPA power supply regulator for a BE FM35A transmitter.  Here it is in better days, when it was actually working.  The IPAs are in pull out drawers on the right side of the transmitter cabinet, below the FX-30 exciter.

BE FM35A transmitter, on air

BE FM35A transmitter, on air

Said transmitter is aging not so gracefully, as it turns 26 this year.  There does seem to be a finite life to transmitting equipment, something that should be kept in mind when planning out next years capital expense budgets.  Regardless of all that, this event naturally occurred the day after Thanksgiving.

The good news, and there is always good news, we have many spare IPA regulators and PA modules in the shop ready to go.  Upon investigation, there were numerous other problems with this transmitter, which have been or will be addressed.

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6 comments to Oh, damn: Una Parte

  • Bob M.

    I think that IPA reg board definitely falls into the “It’s Dead, Jim” category now; definitely beyond repair. I wonder if it was one of those little tantalum caps that shorted, and the power supply burned everything else up in an attempt to protect the fuse?

  • Paul Thurst

    Bob, the output devices in the IPA amp were slowing failing, the two large resistors below the fuse were over dissipating and one cracked open, the other one caught fire to protect the fuse.

  • Bob M.

    I guess that is why someone invented flame-proof resistors, but that’s an awful lot of carnage just from some lit-up power resistors. Seems like a marginal design at best.

    Maybe it’ll look better after you scrape some of the carbon off, but I think it’s literally and figuratively “toast”. Can you take a photo of the replacement/working module for comparison?

  • mr. mike

    I would have said it was some sort of power supply gadget just based on those large heatsinked transistors and the gauge of the connecting wires.

  • Paul Thurst

    Bob, it’s toast. I will have to find a different regulator and post a picture as an update. Again, this design is almost thirty years old, it is a liner regulator running a transistor power amp. It was the only way do do it back then. Nowadays, one could use a light weight 48 VDC switcher and a pair of MOSFETS, which would require less drive, output more power and have much better efficiency. A correctly designed RF PA circuit could be crowbarred without damage. Solid state RF power amps have come a long way since the mid 1980’s.

  • Lowell

    You are correct that newer RF stages are much improved. However, the linear supply in those drawers is more robust than any switching supply, and usually more repairable. A switching supply would have died a decade ago. It does have current limiting.

    The transformers on those drawers can be tapped to reduce the dissipation of the regulator. BE also has or had a modification to improve cooling with a bigger fan.

    If you really want to make improvements, a new IPA and controller can be installed to convert the rig to a T-series, but I wouldn’t put that much money into a rig that old.

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