[R-390] C553 on newer EAC IF module

[David Wise]

Or add a Microchip (nee Supertex) LND150.  If enough of us did it, the next generation of owners would have real statistical failure data for C553 instead of one anecdote.

Repost from 2016-11-07:

I put it on a Keystone #847 miniature 3-position terminal strip, which has two terminals and one non-terminal mounting ring.  I mounted it next to the XV501 shield wall using the pre-existing screw which was already securing a ground lug for a bypass cap.  I moved the low end of C553 from the bandwidth switch to the LND150 drain, and connected the LND150 gate to the switch.  The source goes to the gate via a 1K 1/8W resistor.  I did all the terminal strip soldering before mounting it, so as not to melt the plastic insulating strip I placed behind it.

It works fine.  Receiver gain is normal until C553 is shorted, then it drops to nil; but the filter is unharmed, and performance returns to normal when the short is removed.

Why the resistor?  To ensure the short-circuit (current, power) is at most (1mA, 200mW).  The LND150 barely breaks a sweat.

And don't worry about the added resistance, it's in series with V501's plate resistance, which at least an order of magnitude higher.

Dave Wise
________________________________________
From: R-390 <r-390-bounces at mailman.qth.net> on behalf of Larry H <dinlarh at att.net>
Sent: Tuesday, November 29, 2016 3:39 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] C553 on newer EAC IF module

Brad,  After 39 years of use, there's no real way to tell if or when it will fail.  That is a lot of age on any quality of cap.  I would not risk it, if it were mine.  I'd replace it with any very good 600V 'film and foil' (NOT a 'metalized film') cap,  I recommend an SBE 715P orange drop.  Be sure to test any replacement at its rated voltage to make sure it's good.
Regards, Larry

      From: Brad Huff <huffb at southslope.net>
 To: r-390 at mailman.qth.net
 Sent: Monday, November 28, 2016 10:38 PM
 Subject: [R-390] C553 on newer EAC IF module

Gentlemen,
I am going through a 67 contract EAC IF module and was wondering what everyone thought about replacement of the “killer cap” C553. This module does not have any of the bumble bee bypass caps so common to the older contract radios and it appears that many of the caps that were used are all higher quality appearing “West Cap” glass sealed capacitors, including C553. Would you still change it out considering C553 is such a critical item? I realize that the “orange drops” are thought to be top of the line for this particular part of the radio, but I would not wish to change out what might in effect be a better part and replace it with one of lesser quality. Thanks in advance.-Brad
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