As we have had many issues with our 220 PLL exciter with microphonics and
other noise, most of this was traced down to the final amp stage. This stage
uses a Phillips to-38 style transistor which has the collector commen to the
case, and Hamtronics puts a big heatsink on the case! Needless to say this
creates a bunch of noise on the final signal due to RF being on the heatsink.
We have been able to get about 3.0W out of it starting out ice cold, but
after it heats up, it goes as low as 900mW.
I replaced it with a NTE341 (MRF237 equiv), which has the emitter commen to
the case and is designed to have the case soldered to the ground plane of the
board for heatsinking. To make it fit it needs to be mounted "backwards" so
i had to drill a hole for the base pin to go through an remove the green
solder mask on the ground plane. I did not do any math on the circuits so my
matching is probably not optimal, the only thing done was to change out the
input and out put matching cap's with variable units that we had on hand.
After this was done, I was able to get 2.30W out (@13.8V)of the exciter with
no drop off due to heat, and 95% of all microphonics were eliminated!
The output was checked after 1 hour and it was still at 2.28 watts output, so
it looks like it was a success.
I would think we could get more output if the matching networks were redone,
but I did not feel like screwing with the coils for the backup repeater.
I may do that in the future, and also try the real Motorola mrf237 in the
circuit and see how it compairs to the NTE part.
--
Bryan Fields, KB9MCI
____________________
22:58:17 up 15 min, 1 user, load average: 1.31, 0.93, 0.73
You will be surprised by a loud noise.
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