[R-390] When good resistors go bad

[Tim Shoppa]

"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.