TM 11-856, Change 1, page 2 shows the addition of the 8 uF bypass capacitor,
C612
The 8 uF bypass capacitor, C612 is shown in the schematic of TM 11-5820-357-35,
page 179. So I guess the later manuals circa 1962 incorporated all of earlier
the field changes.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Saturday, May 9, 2026 at 01:59:19 PM CDT, Mike Peace via R-390
<[email protected]> wrote:
Hi Tom,
That tantalum cap across R608 in your R-391 AF deck? That's the same 8 ufd
/ 30 WVDC wet tantalum that Collins called out as C609 when they cleaned up
the design for the R-390A. Same part, same job — cathode bypass on the
local output tube.
You won't find it on the R-390 schematic because it came in as a field
change. Early on, they yanked the feedback loop and tacked in that 8 ufd
tantalum from the cathode to ground. Some of these mods hit before the
first production run was even finished, so the prints never caught up.
That's the R-390 for you.
The R-391 is the same radio electrically — it's a non-A with the autotune
bolted on. Same AF module, same tubes, same wiring. If your AF deck got the
field change (and most of them did), there's your tantalum sitting right
across R608 with no callout on the schematic to explain it.
When Collins did the R-390A, they made it official — C609, 8 ufd, 30 WVDC
tantalum electrolytic. The boys on the list call it "the capacitor that
rots off." The wet tantalum seals go bad, sulfuric acid leaks out, and it
eats the board and anything nearby. If yours hasn't let go yet, consider
yourself lucky and get it out of there anyway.
For replacement, a solid dry tantalum at 8 or 10 ufd / 35V drops right in.
Some guys use a small aluminum electrolytic and that works fine too. Watch
your polarity — the negative end points toward the chassis wall. The
original wet slug was spec'd partly because it fit in that tight space
between the board and the frame where a regular electrolytic wouldn't go,
but the modern solid tantalums are small enough that it's a non-issue.
Bottom line: pull it, replace it, don't look back. That acid will do more
damage to the board than the cost of a new cap ten times over.
Mike Peace
VK6ADA
r-390a.net Administrator
On Sat, May 9, 2026 at 1:45 PM Tom via R-390 <[email protected]> wrote:
> The R391 I'm working on has tantalum cap in the AF module like the one in
> the "A" model (that always leaks and needs replacing). Problem is I can not
> find a call out for it on the 390 schematic.
>
>
>
> Anyone have a schematic that shows its usage? It is wired across R608.
>
>
>
> Tom
>
> W3TA
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[email protected]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[email protected]
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
