> I also noticed that one of the high output amps seems to have an > issue when feeding the circulator. The output through the circ. (loss > thru the circ is about 1.2 db) starts at 60 watts and then jumps to > 75 or so and the spectrum analyzer light up like a christmas tree. > Turn the power control all the way up, it's OK, turn it down some, > it's OK. I touched up the circ in the chassis and no change.
About the only times I've seen Micor UHF PA's go spurious is due to one of
four reasons:
1. One of the parasitic supressors is bad (the little miniature-domino
looking things that span from base to collector on all but the controlled
stage device). Usually when they go bad, you'll see a crack where one of
the legs attaches to the substrate. Next time you get it to go spurious,
wiggle on each of them with an insulated tool and see if you can change the
spurious response.
2. A cracked board, often not at a point that affects one of the microstrip
lines on the top side of the board. If cracked along an edge or near a
corner, it may "disconnect" the underside's groundplane from the rest of the
RF ground system. Take an excacto knife or a sharp dentist's pick and
gently run it across the boards; sometimes the cracks are hard to see but
usually you can find them that way.
3. A broken ground solder joint either between boards, or more often than
not, up along the top of the board where there are little tin straps that
tie the boards' RF ground to the DC ground bus bar. Usually you can find
these by flexing on the board a little with an insulated tool in the area of
the tin straps.
4. Insufficient drive from the LLA. The spec for the LLA output is 1 to 2
watts. Depending on whether or not you've retuned the helical filter on the
LLA output, it can sometimes behave a little squirrelly when connected to
the PA even though it shows normal/clean output when run into a dummy load.
Sometimes you can confirm or deny this is a problem by adding extending the
cable length a bit, such as with a couple of BNC elbows or male-male +
female-female adapters. If it clears up by changing cable lengths, you need
to retune the filter.
Oh, there's one other possibility, but it's really a longshot. On the
transmitter interconnect board there is a filter cap on the control line (I
think it's around 10 uF; it's visible from the front of the chassis, just a
little to the left of the power control board/chassis frame). If that
opens, you can get oscillations on the control line, which in turn
translates into RF spurs. I've only seen this happen once.
> Any comments on the funky amps would be appreciated.
Well, for the dead ones, first you have to figure out what stage it's dead.
You can sometimes get a rough idea by looking at current draw (i.e. if it's
only drawing maybe 8 amps, chances are it's not in the finals but somewhere
earlier). If it's drawing a lot of current, the problem may be at the PA
output where the RG142 jumper meets the board. The intra-stage coupling on
Micor UHF amps is all 50 ohms, which is nice, so you can open the circuit at
the appropriate point, tack-solder in a little jumper to go to a wattmeter,
and test the stage that way. Look at the schematics to figure out where the
opportune points are to "jump in". Likewise you can open the circuit, say,
before the finals, and transmit about 30 watts into the finals and see if
you get power out. You get the idea.
Another common cause of no power output is one of the DC blocking caps had a
bad solder connection. If memory serves, these are 56 pF ceramic chip caps,
and they are usually at the point where the semi-rigid coax ends (there is
one along the upper-left between the first two stages, and another along the
lower/mid-left between the pre-driver and driver stages). Sorry I don't
have one here to look at it to give you more definitive locations, but I'm
sure you'll find them, especially if you have the schematics.
For the one that drops off in power, usually this is a sign of either an
intermittant solder connection (see 2 through 4 above), or a cap that is
going bad or has a poor solder joint. This happens most often at the four
25.5 pF ceramic chip caps between collector and emitter, and emitter and
base, on each of the transistors. Examine the edges of those caps with a
magnifying glass. If you find any that look the solder didn't wet the face
of the capacitor and flow properly, or in severe cases, if it looks like the
metallized end of the cap has pulled away from the rest of the body of the
capacitor, replace it (don't try to re-solder them - replace with new).
Again, sometimes you can find these by poking at them laterally with an
insulated tool to make/break the connection, or you may even be able to see
them arc-n'-spark a little if you dim the lights in the room while wiggling.
Keep the power control voltage around 7-8 volts when testing; 12 is really
running the thing wide-open.
Do yourself a favor while you have it on the bench - take the lid off the
circulator, mark the locations of the trimmer caps, and drill holes in the
lid so you can access them for tuning. Don't try to tune the circulator
with the lid off - the tuning will change once you re-install the lid.
--- Jeff
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