Kevin: To start with , those grid caps are supposed to be 200 PF
each. and you need to replace them with a total of about 1000 pf on each
tube. The fact that the relay works ( with the tubes out) makes me think one
( or both) tubes might have a grid/gnd short, but only when hot. The grid
resistors may have changed from grid current overload due to a parasitic
oscillation.!! There are also two other caps you need to look at, C208 and
C209, .01 and 2mfd , respectively .! The grid to gnd caps. should be at
least 500 v to be safe, and altho mica is best , I have used ceramic (0 temp
coeff.) with no problems.
That's all for now . you seem to be on the right track, so keep at it and
keep us all informed !!!!
the above info is from my experiences with the SB200 amps, which use the
same tubes with only slight differences in bias and such.!!!
Have a good day.!!
carl / kz5ca
----- Original Message -----
From: "Kevin Normoyle" <knormoyle@comcast.net>
To: <AMPS@contesting.com>
Sent: Sunday, March 06, 2005 4:15 AM
Subject: Re: [Amps] Yaesu FL-2100B failure (grid resistor caps)
> 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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