I got a chance to work on my half-dead fl2100b tonite,
after getting some good advice from people.
A couple of questions at the end.
schematics are here for those playing along at home
ftp://bama.edebris.com/bama/yaesu/2100b/b-sch1.gif
ftp://bama.edebris.com/bama/yaesu/2100b/b-sch2.gif
I had summarized the ills as//
>/ 1) 10 meter band switch connection blown at two places/
>/ 2) T/R relay stays in T, after the first keying with RLY/
>/ 3) bias cutoff doesn't seem to work: amp works on 80/40/20/15 while in/
>/ STBY/
>/ 4) burning on the plate choke coils
/
Interestingly, the T/R relay would flip back after powering off.
This, plus odd voltage readings on the relay coil, made me
think the relay coil was getting a resistive path back to ground
somewhere in the grid bias circuit..getting enough current
to hold the relay in after switching, but not enough to switch it.
So I started probing.
Interestingly, the 2W Carbon resistor suppressor resistors were
in fine shape. Read 20 and 22 ohms. Great.
I probed all the doorknobs around the tubes, and they gave correct
capacitance readings.
So I opened up the metal box under the tube connectors that has
the input network, the alc and the grid resistors and caps.
Things started getting interesting!
There was some black smoke here. Hard to tell which was bad soldering iron
work and which was from burning...But one doorknob looked bad with some
black smoke on it. But it metered out ok. Correct capacitance. A small
wire had some black smoke on it, but no melting...seems to have gotten
it from something else.
Ok...looking at the grid resistors. 1W 33 ohm the spec says.
They're carbon.
One looks a little burned. Measuring them:
59 ohm and 66 ohm.
Markings say 33 ohm...so looks like they've changed with temperature/time.
Now here's the real death blow:
The schematic shows the series grid resistors paralleled to ground with 200pF
dipped mica caps.
(two sets, one for each tube)
The two caps that are there are labelled slightly differently
one is
RMO .02 20% 25V (maybe it says RMC)
the other
RMC .02 20% 50V
When I measure them, I get .015uF and .020uf...so I'm thinking
maybe the .02 above means .02uF???
But they're supposed to be 200pF from the schematic. (first above)
When I ohm them out.......
570 ohms on one, 7.65K ohms on the other.
So this must be my resistive short to ground that's keeping the relay
switched.
I remove the two caps (no tubes in) and try out the relay.
(caveat: the wire to the rest of the alc circuit
is disconnected now too. will make sure that's not a contributor later)
Yup. Relay works perfect now..Doesn't stick in.
So a couple questions:
1) Sure seems like the wrong capacitors were used. Were those caps
20uF? and do they develop shorts? or is it a side effect of the wrong
cap being used (i'm not sure if it's dipped mica or not)
2) Could someone have modified this (different caps) for some bad reason?
Or could it be wrong since manufacture. Or are the schematics wrong?
2) Is it common for the grid resistors to overheat? (there's no airflow
in that metal box they're in)
I have some metal oxide resistors for these and for the suppressor
resistors that I'm going to swap in. And I have some dipped mica
caps (200pF)
thanks for reading this far,
Kevin
KE6RAD
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