[NJARC] Help with RCA

[Philip Harris]

Nick:
>From a little past experience with RCA "Electric Tuning" push button
type tuning the usual problem can be traced to dirty switches on the
push button contacts themselves.  The inactive push buttons (the ones
that are not pushed) contacts also must be cleaned for the set to work
properly.  Use a high quality non residue contact cleaner for this
purpose.  Same for the band switch.

In the 6SA7 oscillator circuit, there may be a fixed "padder" capacitor
that takes place of the variable cap when push buttons are being used.
I would be sure that all of the paper type capacitors in the 6SA7
oscillator/mixer  are changed, the resistors are all with in 20% of the
printed value on the schematic.  Resistors almost always go high in
value in older sets. Generally, the mica type capacitors will be okay
and should not be changed unless physically damaged.

Obviously, the tube is good and has been checked, but are the pin
voltages as marked?  After the resistors are checked and replaced as
necessary, check the voltages on the osc./mixer stage  Today's
multimeters read a little higher in high impedance circuit locations
than the one used back then, so some voltages will be higher than the
schematic shows.  Particularly noticeable when reading grid voltages.
Some meters will stop the oscillator when touched to the grid of the
oscillator section.  If the variable capacitor is isolated from ground
be sure the rubber isolators have not rotted out, thus grounding the
frame of the variable capacitor.

This set did not employ a loop antenna.  You must connect an external
long wire antenna to the terminals so marked.   The input has a tuned
RF input coil in place of a loop.  Usually a few feet of wire is okay
for local stations.

I have not looked at the schematic of this particular set but will
shortly.

Phil

----- Original Message -----
From: "Nicholas Senker" <n.senker@worldnet.att.net>
To: <njarc@mailman.qth.net>
Sent: Saturday, March 29, 2003 11:53 AM
Subject: [NJARC] Help with RCA


> I would appreciate any help or suggestions from the membership
regarding an
> early
> RCA AC-DC table radio I am working on.  It is circa 1940 and has a
wafer
> selector
> switch for AM, short-wave, and pushbuttons.  It has five push buttons
which
> are wound
> coils and trimmers for preset stations.  The set works although the
back and
> loop
> antenna are missing.  When I connect a jumper lead to the antenna
connection
> for the
> loop antenna, the reception improves but is otherwise poor.  When in
the
> push button
> mode however, the reception is much better.  For example, WOR comes in
loud
> and
> clear when the push button is in but when I switch to the AM tuning
mode, I
> can't get it
> in, only a few stations on the high end of the dial come in.  There is
a lot
> of static when
> I tune as if I am passing a station but can't really get it in.  My
problem
> therefore seems
> to be in the tuning stage (tuning capacitor) as the rest of the set
seems to
> work with the
> pushbuttons.  It seems the tuning capacitor is bypassed when in the
push
> button mode.
> The tuning capacitor was very dusty and I vacuumed it and brushed it
as
> clean as I
> could.  Any thoughts would be appreciated.  Nick Senker  It is a Mod
96T5
>
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