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 an 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

[Andrew B]

I does sound like the bias trimmer is "iffy", I would change it and then retake the measurements.

[rjmusto]

Hi Andrew,
Have checked the bias trimmer - a bit high, but not significantly.

I think this diode clamp biasing technique must be highly dependant on the turn-on voltage of the diode, and maybe that has changed with time. This unit is not mine (belongs to a friend), so don't want to interfere with it too much... otherwise I would rework the biasing circuit and add in a transistor.

It has suffered from the dreaded 'twiddler', both the PA and the priamp were well off tune, so now that it is back where it should be, I might just leave it at that.

73,
Ralph