[Collins] More 75A-4 Recap

[Thomas Beltran]

Jerry:

> Are you SURE you should be poking your soldering iron into this receiver?


Why not - I've built many projects, heathkits, homebrew, and I completely
rebuilt my Drake R-4C, and when I sent it to Rob Sherwood, he commented me
on what he termed an excellent  job.  But I will admit, that being a builder
of classical guitars, and not an electrical technician, my mechanical/fine
motor skills are far superior to my knowledge of electronics.  I also think
that part of the problem is that the Collins manual (I have the original)
has some errors.  These errors cause me to second guess my initial
impressions.  I don't want to burn or blow up some irreplacable part in my
radio, from using an incorrect replacement part.  I must add that I haven't
seen these mistakes in my Drake manuals.  I've done many many repairs over
the years on my tube equipment since I was in high school, with never a real
problem.  Once I sort out the electrical issues, the rest is pretty much
mechanical (which is the restoration area in which I excel).  So of course I
don't mind poking a soldering iron into my radio that is the easy part - it
is the decision of what parts to put in that I am seeking some guidance.

>
> Generally the electrolytic capacitors are dead from old age and the
> molded oiled paper capacitors have been leaky since they were wired
> into the radio, but mica tend to be fairly reliable.


The only reason that I am replacing the micas, is because I figured that
while I was in the radio, I would replace the capacitors pointed out in
Chuck Rippel's piece, the "seven deadly caps."  In fact, I believe that you,
sir, advised me to read that article.  Should I not bother with the micas?
I have no real reason, other than Mr. Rippel's recommendation, to replace
them. Interestingly, of the original seven (deadly caps), five are micas.
Butch, K0BS added three additional capacitors to the list, two of which are
ceramic, but he also suggests replacing a third, C-71, which he terms as a
"high failure item"........it is a mica.  The 4 pf (C-52) is listed as a
ceramic capacitor, but Mr. Rippel suggests replacing it with a mica.

But I am open to suggestions, and if you now recommend against changing the
micas, that would be fine.  I really want to address as many things as I can
while I have the radio apart.

>
> There is a large size difference between 0.5 PF and 0.5 UF... The 200 volt
> rating hints at 0.5 UF paper, undoubtedly leaky. The 200 volt 0.5 UF
> paper would be between 3/4 and 1" diameter 2 to 2-1/2" long and the
> 0.5 PF ceramic would be on the order of 1/8" diameter and 1/4" long. A
> huge size difference!
>

That was my guess, but because the manual puts the figure in pf (well mmf),
I figured I should consult the collective wisdom of this group - perhaps
someone else has just gone down this road.  My guess was that the figure
should be in uf, and not pf, but I certainly didn't want to just guess.

> Allied Electronics shows a 4 pf dipped silver mica (molded mica haven't
> been made in nigh on 50 years) stock number 862-3103 Cornell Dubilier
> type CD6CD040C03 500 vdc rating, $1.67.
>

Thanks - I checked the two places that were suggested to me, Tubes and More,
and Mouser - neither had it.

> For the 0.5 UF 200 volt use an orange drop, Mouser catalog number 75-
> 225P200V0.47, Vishay part number 225P47492YD3, 0.47 UF 200 volts. $1.82
>

That was my guess, but I had heard that one should go to a higher
capacitance, not a lower one.  I did email Bob Kemp, asking him if I should
use the 0.47.  Tubes and More has a 0.47, but at 400 V.  I have also heard
different opinions about voltages.  Some people have said that the voltage
of the replacement should be close to the original, something about reducing
the effectiveness of the capacitor by using it at a small percentage of its
actual voltage rating - is that true?

> O.5 PF has been available from capacitor makers. It would be tiny.
> What is the function of V-22 pin 6? Is it audio or RF? Audio wouldn't use
> the 0.5 pf capacitor, only VHF RF would. Stray capacity to ground from
> wire and socket pin would be more than 0.5 pf, most tube input and
> output capacitances run more like 5 pf. 0.5 pf would be most useful as a
> coupling capacitor in a 144 MHz multiply tuned circuit filter and if all
else
> fails two 1 pf capacitors in series make a workable solution. 0.5 pf makes
> a nice 50 ohm circuit coupling capacitor at 10 GHz, maybe a bypass for
> solid state circuits a 24 GHz. Part in a tube receiver, nah.
>

Like I said, it didn't make sense to me, I was trying to make sense of the
manual.

Tom W6EIJ