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

[thoyer1 at verizon.net]

Perfect, thanks Mike! Now I can stop looking at the schematic 😊

 

Yes, I’ve been through several “A” models and they have all exposed their insides……. I have a stock of 8uf electrolytics here that I use for that purpose. I’m not a fan of tantalums.

 

Tom

W3TA

 

From: Mike Peace <mike.peace at gmail.com> 
Sent: Saturday, May 9, 2026 2:58 PM
To: thoyer1 at verizon.net
Cc: R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] R390/391 AF Module Tantalum Cap

 

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 <http://r-390a.net>  Administrator

 

On Sat, May 9, 2026 at 1:45 PM Tom via R-390 <r-390 at mailman.qth.net <mailto: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

______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net <mailto:R-390 at mailman.qth.net> 

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html