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
not expect the wiring resistances to generate
any serious
problem.
Much success in repairing your R-4A, kind regards and 73
Herbert, DG7MCC
Peter Hoon schrieb:
R4A: It appears that one or more sections of the Sangamo power supply filter
cap are defective, since I measure 5.2 volts p - p with a scope at the
junction of diodes D4 and D5 (156 VDC). And my power transformer gets very
hot.The cap is in a single aluminum Sangamo chassis mount with:
100 MFD 200 VDC
100 MFD 200 VDC
100 MFD 200 VDC
20 MFD 20 VDC1) Anyone have a spare from an R4A they are parting out?
2) Suggestions for a source for this filter cap from others who have gone
before me.3) Have improvisors found discrete caps from a source (pse recommend one),
left the chassis mount unit alone, and mounted these discrete caps under the
chassis?4) Has anyone actually drilled out this filter cap shell, and mounted
discrete components inside?Any other ideas or comments appreciated.
Peter, VE1CHS
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