Kevin Normoyle writes:
>
> 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.
Or the amplifier has been badly overdriven/drawn excessive grid
current for a long time. Perhaps, there has also been DC current
through those resistors due to the capacitor problem below.
> Now here's the real death blow:
>
> The schematic shows the series grid resistors paralleled to
> ground with 200pF dipped mica caps.
>
> 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)
It' possible that someone has replaced with .02uF for better RF
grounding of the grids. That's not a bad idea.
> 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.
The cap that measures 570 Ohms is most definitely causing your
relay problem. Try putting a 560 Ohm resistor across the PTT
line ... the relay will stay open until you apply a contact
closure across the 560 Ohm. However, open the contact closure
and the relay will hold in. That's normal, a relay needs much
less current to hold in than it takes to pull in.
> 2) Could someone have modified this (different caps) for some
> bad reason? Or could it be wrong since manufacture. Or are
> the schematics wrong?
Would have been a good reason (better grid grounding) or could
have been manufacturing change.
> 2) Is it common for the grid resistors to overheat? (there's
> no airflow in that metal box they're in)
Not necessarily - there should be relatively little current
through them. Since the FL-2100B lacks grid current metering
the 33 Ohm values are not that critical.
> 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)
I'd replace the grid resistors, get some mica or good quality
ceramic .01 uF caps for the grid, clean up the carbon/smoke
from the other components (cosmetic only). You'll probably
find the amp runs fine after that if you take care of it and
don't push it too hard.
BTW, while I was in the amplifier, I would make the bias change
recommended by Svetlana even if you don't use Svetlana 572Bs
(http://216.87.144.102/svetlana/TechBulletins/appnoteNO.15.asp)
and I would add a "soft key" circuit to protect the PTT line
from modern rigs.
73,
... Joe, K4IK
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