[R-390] R390/391 AF Module Tantalum Cap

Sat May 9 15:45:57 EDT 2026

Thanks Jim - I see that now, should have looked there once I didn't see it on the schematic.

That being said, I typically have either an original manual or a printed version for every radio I have here. The printed manual I have to the 391 I'm working on does not show the cap. So I naturally grabbed the 390 binder and didn't see it there either........But........... I don't have the 1962 version in that binder, it is the 1955 version. 

Looking on my computer, I found the '62 version, guess I need to print and add it to the 390 binder.

Thanks for opening my eyes!

Tom
W3TA

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of Jim Whartenby via R-390
Sent: Saturday, May 9, 2026 3:28 PM
Cc: R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] R390/391 AF Module Tantalum Cap

 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 <r-390 at mailman.qth.net> 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 <r-390 at mailman.qth.net> 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
>
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