[R-390] Capacitor replacement when restoring

Thu, 10 Jan 2002 10:46:36 -0500

Hi Greg:

I second Bill's observations. I just finished restoring a Collins KWM-1 of
1958 vintage, and one of the pink block-style mica capacitors in the set's
product detector was operational, but noisy as all get-out.

In this circuit, in Standby mode, audio from the product detector is still
fed into the set's AF amplifier string, along with sidetone when on CW, and
I could hear that puppy crackling away like mad in the loudspeaker when I
was transmitting. I zeroed in on it using a signal tracer and scope. It was
a satisfying feeling to clip it out and see all the noise go away. Finding
it was relatively easy; replacing it was tricky (very tight quarters), but
now all's well in KWM-1 land.

Good luck, and go easy with your "wholesale" component replacement plans. My
feeling is if it ain't broke, don't (necessarily) fix it.  Critical caps
(such as the plate blocking cap that could short and feed DC to the
mechanical filter coils), are another story.

Vy 73, AI2Q, Alex in Kennebunk, Maine   .-.-.

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Bill Smith
Sent: Wednesday, January 09, 2002 10:32 PM
To: Gregory W. Moore; [email protected]
Subject: Re: [R-390] Capacitor replacement when restoring

My understanding is it varies with manufacture of the unit.  Others can give
you better details.  For example, my R-390 is filled with "Vitamin Q" caps,
which are reputed to be of excellent quality and generally do not need
replacement.  I have tested a few and have found this to be the case.  Thus
most of the caps in this receiver are original.

In the R-390 (non-a) here, which is apparently a late Motorola model, S/N
4700, I have had problems with mica capacitors.  Micas are generally very
reliable, but the "pink, square" style in this receiver look like they are
candidates for further examination.  I have replaced three, and believe
there are others needing replacement in the RF stages.

The AVC caps, C546 and C547 (1 mfd, located in the IF section near V511 on
the schematic) have a reputation for leakage.   I disconnected one (they are
in parallel) and gutted other case and replaced the insides with a 2-Mfd.
cap.

Generally, many capacitors in the .0005-.5 MFd range are constructed of
paper and aluminum foil.  They become leaky and/or open with age.  Bath-tub
capacitors (not in an R-390 as far as I know, but popular in other
receivers) are now showing up leaky.  Performance really depends upon
manufacture, with Sprague paper "Black Beauties" notorious as requiring
replacement.   Any wax covered capacitors are known offenders as are many
molded plastic capacitors

As another general statement, one should NEVER turn on any equipment that
has been on the shelf for 5 or more years without bringing up the set using
a variac.  Hitting the set with 120 VAC is a good way to guarantee that
electrolytic capacitors in the power supply will need replacement.

In short, capacitors can be generally considered suspect.  In most receivers
replacement of all cylindrical capacitors is appropriate, with a watch on
micas.  Filter electroytics may often be recovered by reforming using a
variac.  While you are at it, make sure to test resistors.  Typically, 10%
or more of the carbon types in the set will have drifted 20% or more higher
in value and should be replaced.  I found several open resistors in the
R-390 power supply circuit.  The power supply was non-operational until the
resistors were replaced.

73 de Bill, AB6MT
[email protected]

----- Original Message -----
From: "Gregory W. Moore" <[email protected]>
To: "Boatanchors Reflector" <[email protected]>; "Eico Reflector"
<[email protected]>; "R390 reflector" <[email protected]>
Sent: Wednesday, January 09, 2002 3:38 PM
Subject: [R-390] Capacitor replacement when restoring

> Ok, I realize that this is a simple question, and I should already know
> this, but WHICH
> caps beside the electrolytics should be replaced when doing a total
> rebuild/restoration?
> I am now doing an Eico 753 but will be following shortly thereafter with
> my R390A.
>
> Do I have to worry about the ceramic disks? They all test good with the
> Fluke DVM , and otherwise look ok. What I sm conserned about, besides
> the electrolytics are the silver mica, the molded silver mica, and the
> mylar ones.
>
> Any info here is appreciated...also, if someone has a male Amphenol MC2M
> plug, or prefably 2 of them...I wouldnt complain if they had the
> matching receptacle as well, let me know.... I do want to get this thing
> back on the air ASAP. Yes I do have a second, and works reasonably well,
> but I am doing a complete teardown and restore on the first.
> 73 es tnx
> Greg WA3IVX
>
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> R-390 mailing list
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>

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