[BC] BE FM35A

[John Lyles]

As a former BE design engineer and FM TX design manager during those days, I can speak for this.

There was no Resistor-Transistor Logic (RTL) in it. However:

The BE FM30A was the original box with 8990/4CX20,000A tetrode, introduced about 1981. The first 50 or so had a Motorola CMOS microcontroller, the MC14500, which was a one bit serial CPU, used for stop light controls and things like that. It was a bastard. The I/O was done with comparators through op amps that were called "Norton" current configuration. Definitely nothing real standard there. While it certainly gave BE the first FM rig on the market to have microprocessor controls, it was nothing to smile about. RF would get into the controller, and sometimes smoke would emit from ICs. It was an ugly product, ahead of its time, but not ready for prime time. The RF design was elegant and very strong. After fielding so many of these big rigs, and getting a lot of failures and service trips , we made a retrofit controller, which was completely CMOS 4000 series ICs, all standard off the shelf parts, that did everything the CPU did, but was very reliable. We put the hooks in ther!
 e for a
later optional Z80 CPU, which became the MVDS option. This same controller became the brain for the lower power rigs that I worked on, the FM1.5A, 3.5A, 5A, and later 10A and 20A models. 

The IPA in the original FM30A used (10) CTC BM100-28 bipolar transistors, with one 100 watt board driving four more in a hybrid combiner arrangement. These were somewhat fragile when the grid tuning was run around while at full power, especially having compression mica trimmer capacitors on the boards that could easily get fouled up and mistuned. The thing was not frequency agile. When we introduced the FM3.5A about 1983 at NAB, we made a new IPA that was close to 88-108 without tuning, and used newer Thomson SD1460 or TRW TP9383 bipolar parts. It was a single drawer for 150 watts, with internal power supply. The FM30A then used this new design, combing two drawers and driving with another, along with an improved input circuit that was a lot more like the patented FM3.5A design, with all the impedance transformation done on a microstrip board that had no tuning. This eliminated the input loading control, and a source of narrowbanded input response. 

BE pretty much advanced grid-driven tetrode amplifiers a long way for the time, although Collins/Continental/Harris made them also. The FM30A benefited from the FM3.5A development and vice versa. We were all in one happy family. 

By the time the FM-35A was shipped, a different tube was used (4CX20,000C) along with bigger plate transformer and so forth. The IPA was already the new 'works in a drawer' system, and the controller was all state-of-the-art at the time. There was no RTL nor crappy grid circuit and IPA in that model. 

Thanks for allowing me to set the story straight.  I would not hesititate to take on an FM35A if the price was low, and if it was kept clean and dry. However, that is getting to be an antique transmitter isn't it? 

John Lyles



> Message: 19
> From: "Jim Offerdahl" <offerdahl at gvtel.com>
> 
> We are looking at purchasing a used 1986 vitage BE FM-35A transmitter. We
> have heard good and bad about this one. Does anyone have any experience with
> this one? Any feedback will be much appreciated. I have been told that if it
> has not had the grid tuning circuit upgraded to stay away.
>
> Message: 25
> From: Cowboy <curt at spam-o-matic.net>
>
>  An A series ?
>  Run away, run away.........
> 
>  As I recall, the logic in there is RTL, which hasn't been produced
>  by anyone for years.
>  Sometimes, you can swap chips around to get it back on the air,
>  ( spent the better part of a cold night on Cheyenne Mountain )
>  but sometimes not, and then you're pretty much done. 
>  There are no replacements, unless you buy another A for parts.
> 
>  As I recall........
> 
>