Lee -
C90A wasn't used until the late R-4B. It was probably used in the first prototype of the R-4, but
the first R-4 schematic (0001) showed only C90B, C and D. I guess Drake just got a good buy on
four section caps.!
Drake used good quality epoxy 'film' caps, and I don't think I have found a bad one yet. Of course
the tubular electrolytics should be replaced in the BIAS and AF OUTPUT Cathode bypass.
The most likely problem in the AVC circuit is gassy 12BA6 IF tubes. They cause the S-Meter ZERO to
drift among other things.
When you price out a 12BZ6 RF AMP tube you may want to make the mod made by many when the R-4 was
'new', i.e., move a filament wire and use a 6BZ6 instead. The 12BZ6 was expensive and hard to find
even back then.
Beyond that, we're here!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Lee Hiers wrote:
Hi folks,
I picked up an R-4 (no-suffix) about a year ago and listened on it for
a while with just a random wire inside the house. Really like the
radio. Back in the spring, it developed a hum, and sounded like the
radio was zero-beat with a carrier wherever I tuned. I figured filter
caps were bad, and probably something in the AGC circuit. I
subsequently got married, moved, etc. and figured the repair would
have to wait until things settled down a bit.
I'm finally getting to where I can look at the rig. I opened it up,
and it looks like it's never been touched inside. So, I decided to
get a Hayseed can replacement for the filter caps instead of going
with discrete caps. Mine has a three section 100mfd/200v can - not
the 4 section Hayseed sells as a replacement. No problem, just use
the three 100uF sections, ignoring the extra 22uF section.
Anyway, to the point... There are about a half-dozen or so paper caps
in the rig - should I routinely replace them as well? If so, any
particular recommendations?
I'm also planning on replacing a few tubes - in the RF am and AGC
stages for starters. Any pitfalls to watch out for there?
Thanks!
73 de Lee
--
Lee Hiers, AA4GA
www.aa4ga.com
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