From: "Ken, WA2LBI" 
Subject: Re: [drakelist] Filter Cap on Drake R4A


Herbert,

Regarding your points 5 through 7, it has been my experience that if any section
of a multi-section cap becomes defective it is only a matter of time before the
remainder of the sections fail, either open or short. The latter condition, of
course, is the worst case.

I only parallel a cap section for testing to determine if it corrects the
problem. If it does the entire cap is replaced. I normally leave the original
can in place for aesthetics and place the new, smaller caps under the chassis as
you suggest.

73,

Ken, WA2LBI
wa2lbi{at}arrl.net

Quoting Herbert Schulz :

> Peter,
>
> i do not own a R-4A, but i may have some practical comments:
>
> 1.) A large ripple voltage combined with excessive trans-
>     former heating may also point to an overload condition,
>     not necessarly may it only be one or more failed cap's.
>     So, before doing any repair, i would suggest to look for
>     that first. The voltage chart usually stated in Drake's
>     manuals could be of some help here.
>
> 2.) It may also be, that one of the two rectifying diodes
>     has failed. This could also lead to transformer heating
>     (dependent on the type of diode failure). Anyhow, in
>     that case the ripple should be asymmetric in the appearance
>     of the waveshape with respect to the positive and negative
>     portion of the mains-sine-waveshape. In that case i would
>     suggest to replace both diodes against 1N4007's.
>
> 3.) A ripple voltage in the range of several [Vpp] seems
>     to be normal direct at the rectifier and the first cap.
>     The ripple voltage of more interest would be the one on
>     the final HV-DC-supply line. In my R-4C, this ripple vol-
>     tage on the +150[V] line is very significantly below 1[Vpp].
>     I do not know, how the power-supply of the R-4A is made up,
>     but i assume, that there are not too much differences.
>
> 4.) As long as one of the cap's has no short circuit (which
>     is usually not the case, they simply dry out), the cap's
>     should not be the reason for your power-transformer to
>     heat more than normal. Maybe this "more than normal"
>     heating has another reason?
>
> 5.) If the cap's only have low capacity and no shorts, i
>     would recommend to parallel them on the bottom of the
>     chassis with the approbriate capacity values. 100[uf]
>     250[V]DC are really small capacitors today compared
>     to the time the R-4A was built. And a 22[uF]/25[V]
>     cap may be so small nowadays, that it may be hard
>     to see. In this way you do not tamper with the origi-
>     nal state of the receiver in a irreversible way. I
>     did exactly this type of repair with my SPR-4.
>
> 6.) As dried-out electrolytics also may exhibit contact pro-
>     blems between the foil and its associated electrode
>     connection to the outside world, i would suggest to
>     parallel each paralleled electrolytic named under topic
>     2.) with an 0.1[uF] ceramic capacitor with approbriate
>     voltage rating. The reason for this is to supress any
>     possible contact noise of this dried-out-electrolytics
>     at the source by shorting it to ground. The new, paral-
>     lelled electrolytics can not do this job because they
>     exhibit significant ESR (equivalent series resistance)
>     at higher frequencies which they have not been made for.
>
> 7.) In case of space problems, i would not see it as a pro-
>     blem, to place the new, paralleled electrolytics at a
>     place under the chassis, where it is approbriate and
>     where the corresponding DC-wiring is runing to. At least
>     in my R-4C the +150[V] DC wiring is distributed nearly all
>     over the bottom of the chassis, so that it should be possible
>     to find an comfortable mounting place. Due to the small
>     currents, that are drawn from the HV-DC-supplies, i would

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