Help with Microwave Modules 2m Linear Amp - MML 144/100-S

[rjmusto]

Hi Guys,
Have been handed one of these 1980's (?) 2m amplifiers to check out and have a couple of things I am unsure about, so hoping someone here is familiar with them....

It is basically working; I can set the bias current to be around 1.4A and and, having tuned it up am seeing 12dB of gain.  Nice.

But...setting the bias (and I know the spec says it should be 1A) is very sensitive, the trimmer pot is almost at zero and just a tiny tweak will lift the current from a few hundred milliamps, to over 1A. And it's not very stable. Is this just and ageing transistor maybe?

The other issue, is the 22ohm dropper resistor to the bias circuit gets very hot - not surprising really, as the bias voltage is around 0.7v, so the resistor is having to handle around 8W. So no wonder it gets very hot.  I don't see that as a good long term situation though, and there's no sign of past heating on the pcb. So what am I missing here?

Any thoughts much appreciated.

I've attached the MM info sheet which has the schematic and an image show the innards of this one.

Thanks,
Ralph

[Wallace Gasiewicz]

Can you look for spurious frequencies, i.e. oscillations? Probably on a spectrum analyzer.
If there are any then try to tune them out?

Wally KC9INK

[rjmusto]

Hi Wally,
Haven’t seen any sign of instability.

And having got the amp tuned up properly (phantom twiddlerer had been here) it is producing close to 100W, so happy with that.

I’m just not happy with the stability of the bias circuit. I know these clamp diode types can be improved by adding a transistor, but I’m reluctant to mess with it - it’s not mine, belongs to a friend. “If it aint broke, don’t fix it” and all that…..

Ralph

[Flux-Sucking Shunt]

Declare victory and move on. 

But that rats-nest wiring makes my skin crawl a bit.