[BC] Harris FM10 250 driver

[Jerry Mathis]

If we're talking about the FM-10K transmitter from Harris, frankly, I wouldn't put a lot of money into one of them. While the tube cavity and output network are sound, the rest of the transmitter is, shall we say, flaky by modern standards. Relay control and monitoring, and the relays are getting hard to come by. The control circuitry is powered by 240 volts--and only fused on one end! Even the overload control potentiometers are obsolete. And the transmitter metering is absolutely pathetic. For instance, the only monitoring of the problematic 4CX300A tube is cathode current. You can't monitor grid current, screen current, screen voltage, nor plate voltage. Even the final PA tube's screen and grid parameters are not metered. One of the worst transmitter designs I've ever seen. The FM-##H model was the same, BTW.
 
It would be impossible for a SIMPLE retrofit to introduce a SS amplifier into this transmitter, because the IPA output/PA input are directly coupled and not transformed down to 50 ohms in and out. Any such modification would invalidate the transmitters' FCC acceptance, I'm sure.

--
Jerry Mathis

On Mon, Nov 15, 2010 at 8:54 PM, Mike McCarthy <towers at mre.com> wrote:
> I heard a rumor of one TX manufacturer considering working with an
>aftermarket operation to build an interface kit to marry a new 300W
>Exciter/TX to the FM10. Don't know whether that's a real deal or not.
>But someone is thinking about it and would allow THAT TX to be partially
>brought into the 21st century and extend it's useful life another few years.

>MM

>On 11/15/2010 8:31 PM, Jim Offerdahl wrote:
>> I support a Harris FM10 that uses the 250 driver tubes that are no
>> longer being built by eimac. I did get my hands on a couple of chinese
>> tubes, but one is no good and the other is not very good.
>>
>> I believe I recall someone discussing modifying the FM10 to add a solid
>> state IPA. I know this is not a simple mod, but if anyone has done it
>> and could share their story, I will appreciate it.
>>