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THE AM BULLETIN BOARD => Technical Forum => Topic started by: Joe Connor on August 02, 2019, 09:17:27 AM

Title: R-388 100 pf caps
Post by: Joe Connor on August 02, 2019, 09:17:27 AM
What's the collective opinion here on replacing the red 100-pf caps in these receivers? Mine is an early one, serial number 583, built on a 1951 contract. Do you replace them as a matter of routine or only if one fails? I've read that Collins had a bad batch of 100-pf caps, and I'm wondering if mine was built before Collins started using the bad caps.

So far, my problem has been the bathtub caps. One (C-214) shorted last night, smoking a resistor (R-142). (Why do these failures always occur when I'm about to call it a night and go to bed?) My next task looks to be replacing all the bathtub caps. This receiver has seen some hard use, but it's got potential and seems to work pretty well. There are some signs of earlier repairs, probably while it was still in military service, but no mods.

Title: Re: R-388 100 pf caps
Post by: WBear2GCR on August 02, 2019, 11:21:35 AM

Which 100pf caps are you speaking of?

Most of the R-388s here have those tubular ceramic caps throughout.
They appear to be very reliable.

In fact I've had zero cap failures in 2 R-388 one over 30+ years and the other
now in over 15 years.

I just acquired 7 total R-388s, tested 3 or 4 and nothing not functional so far.
one has been refurb'd and tested, all 100%.

But caps do fail at this age.
Some folks are in favor of replacing all the caps.
Generally, I'm not, unless there is a performance issue.
Otoh, some folks have said that they get better performance IF the caps
are changed. I don't know if that is so or not.

Electrolytics, I am faster to replace.

The bathtubs can be desoldered and re-stuffed.

Title: Re: R-388 100 pf caps
Post by: Joe Connor on August 02, 2019, 11:33:54 AM
These caps are small square red caps. In some articles, they're called postage-stamp caps. One, C-204, is supposed to be a common source of trouble if it came from the bad batch.

Yes, I changed the electrolytics before firing it up. For the filter cap, I used a really nice plug-in replacement (which is now the only shiny thing in that receiver).  As for restuffing, what kind of gunk is inside those bathtubs? I've always worried about PCBs and other nasty stuff.

This is my first R-388. I have a 51J4, but the R-388s that I ran across at hamfests and swapmeets were always out of my price range.

Title: Re: R-388 100 pf caps
Post by: DMOD on August 02, 2019, 01:36:47 PM
The only caps I have seen go goofy are C113, C143, C184, and C233.

The problem noted was RF and mixer gain changing after warm-up.

If your rig has steady gain after warm-up, I would not be concerned.

Phil - AC0OB

Title: Re: R-388 100 pf caps
Post by: Joe Connor on August 02, 2019, 10:39:42 PM
Thanks, Phil. I'll keep an eye on the caps you listed.

Title: Re: R-388 100 pf caps
Post by: WB6NVH on August 03, 2019, 09:41:09 PM
Assorted contracts have 100 pf Micamold brand postage stamp mica capacitors and they fail in the leaky-shorted mode.  These are green postage stamp style.  The problems ones will be where there is B+ on one side and a tube grid on the other.  Other contracts have reddish ones made by CDE and while I have found them to be far more reliable, they fail too.

These were failing in the 1970's and earlier but the 388 was well past end of life by then and no service bulletins covered it.  I was at a service center then and thought I was shrewd to stock up on replacement 100 pf postage stamp micas from Collins stock on the shelves.  Unfortunately, what did I get but more green Micamolds, just NOS ones. 

I don't know why, but it seems to be just the 100 pf ones that did this.  I don't have my old shop notebook at hand to say what the C-- numbers were, but there are a couple of them which really should be changed as a matter of PM. 

Title: Re: R-388 100 pf caps
Post by: Joe Connor on August 04, 2019, 12:36:02 PM
Thanks, Geoff.

Title: Re: R-388 100 pf caps
Post by: WB6NVH on August 04, 2019, 02:55:12 PM
Curiosity got the better of me and I pulled my old notebook out.  It shows that C136, the injection coupling capacitor to V102, failed in approx. all 150 receivers that were in mobile weather fax shelters used by the Army.  Maybe they were out in the weather too much with the doors and windows open.  This was early 1980. Collins part number was 309100-294 but that was just more of the evil Micamold branded green ones on our shelves, anyway.  C204, in the AVC rectifier stage, was also breaking down and replaced as PM on all units.  I don't know if all the manuals have the same numbering scheme.  These numbers are from military manuals.  The SAMS Photofact numbers are different (51J-3.)  These are 100 pf in value. 

I also had a lot of gassy 6AK5 tubes for some reason.  I replaced them with 5654 tubes intended as spares stock for then defunct radar IF strips.

Everyone's PTO is bad now until they take a turn off the end point coil and replace the "match stick" capacitors inside.  An unpleasant job compared to the R-390A. 

Another thing the weather was doing was rusting the billets that the slugs are glued to on the racks, which swelled them and broke the slugs loose and/or into pieces, where they would be found loose rolling around on the bottom cover.  Not a Collins approved repair, but if you have all the pieces, sand the billet clean and glue them back together with your favorite epoxy.  When dry, sand as necessary until they slide easily in the holes.
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