[R-390] When good resistors go bad

[n4buq at knology.net]

On Fri, 03 Mar 2006 06:38:47 -0500, shoppa_r390a at trailing-edge.com (Tim 
Shoppa) wrote :

> "Barry" <n4buq at knology.net> wrote:
> > All the talk a few weeks ago about certain resistors going high over the
> > years prompted me to start looking at some of the resistors in one of 
my IF
> > decks.  I think the discussion revolved around some 22k plate resistors
> > going high (but I might not be remembering that correctly).
> 
> My complaint was 1/2W 2.2K plate resistors, especially in the RF deck
> and VFO (almost every one was high by over 100%, some 200-300%, many
> showed evidence of past heat damage/charring).
> 
> > My plate resistors have weathered well (at the high end of the 
tolerance or
> > maybe a percent or two above, but not enough to warrant replacement 
IMHO),
> > but I discovered that a couple of the 27k screen grid resistors (2nd 
and 3rd
> > IF amp) have gone to 39k (definitely too high).
> 
> That's not ridiculously high but it can affect gain distribution and
> make it non-optimal. It does affect DC bias and maybe a stage will
> clip a little more easily (but nothing in the IF deck should be clipping
> if AGC is working...)
> 
> > I'm wondering what affect this is currently having on the system.  I 
assume
> > the lower screen grid voltage results in lowered gain at that tube, 
right?
> > I haven't done a voltage measurement on the screens to see how far off 
they
> > are, but I assume they are low.
> 
> Overall there's more than enough gain in the IF stages, alltogether,
> so I don't think you'll see a lot of affect from the screens being
> off a couple of volts. The "GAIN ADJ" pot can be set off a little bit
> to bring the RF/IF gain balance back into alignment.
> 
> > While looking around at the innards of the deck, I did notice one 
particular
> > resistor (I don't remember exactly which one) that is literally buried 
at
> > the first IF amp.  It is a 2.2k (I think) but they installed about a
> > 3-watter down there.  Good thing because it is spot on specification.  
It
> > appears this was the first resistor installed in that area, it would 
require
> > major surgery to replace it, and they must have known it would need to 
be
> > hefty to avoid needing replacing.
> 
> You're talking about the one underneath the bandwidth shaft on the
> front wall of the IF deck? Doesn't look bad at all on my decks, the
> shaft itself is removable.
> 
> Tim.

The resistor I'm talking about is R506 (22k, 2w).  It is underneath the
bandwidth shaft, but there are a lot of other wires in the way.  I'd really
hate to have to replace that one.  Yes, the shaft is removable, but there's
still a lot of other "junk" in the way.

Thanks for the comments.  I plan to replace the 27k resistors, but like you
say, I may not notice a lot of difference in the overall gain as it is
adjustable anyway.  I still need to check the RF deck's resistors, but I'm
dreading pulling that thing...

Barry - N4BUQ