I will not let my love/hate relationship with ALS1300 end
The ALS1300 has worked flawlessly for me in two CQWW CW contests from 8P5A and
reinforced why I bought the unit. Unfortunately, multiple design,
manufacturing, and quality issues have repeatedly put the product on the shelf
for 6 other contests. It has been back to MFJ multiple times for repair,
including one set of blown finals. While the contest duty cycles are high, I
do not push the amp over 1KW, the antennas are well matched, and the
bandswitching is automatic so the amp has not been abused. To be fair to MFJ,
they have been mostly accommodating. However, after what looked to be yet
another failure of the finals, I intended to give up.
However, I just could not just let it sit there and had to tinker with it some
more, just to find out what was wrong. Indeed, one of the MRF150's was bad.
As it turns out, I have a set of 4 matched MRF150's from a prior project. I
removed the MRF150's from the prior board and replaced the suspect ALS1300
transistor and its parallel partner. Since replacing the transistors is so
tedious, I just stopped at two just to see if the amp would at least function.
My question is how many of the transistors need to be matched? The design is
two boards, each with 4 transistors in push pull. DO just the two in parallel
need to be matched, all 4 per board need to be matched, or all 8 on both boards
combined. I noticed that all 8 of the original transistors are hand marked
with a "17." I am guessing that is the measured gain and that is how they are
matched.
Also, even if matched, there will be some variation. What is the best way to
adjust bias? Do I just match the quiescent currents? Should I be watching the
waveforms at full power and then adjusting? Should I be adjusting based on a
spectrum analyzer and two tone test?
Right now, the amp appears functional and will make power, so the diagnosis was
correct. However, it is not yet usable as the current draws between modules
is different and there is a big imbalance in output of the two modules. I am
good with replacing the other two transistors on the board, but not too keen on
buying 8 new matched ones. Either way, I will need advice on the biasing
process
Any suggestions appreciated.
tg
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