[Amps] Yaesu FL-2100B failure (grid resistor caps)

[Joe Subich, K4IK]

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