[ARC5] SCR-183 Cheap TUbes

[AKLDGUY]

Neutralization is a critical factor in getting these transmitters going.
The neutralizing capacitor (referred to in the manual as the "Balancing
condenser") is a simple compression trimmer made of a fixed plate and a
springy plate separated by a sheet of mica. Through the center is a stud
with threads, on which is fitted a 1/4 inch nut with a nylon washer.
Tightening of the nut moves the springy plate closer to the fixed plate.

I have high quality photos of the partially removed capacitor if anyone
wants them forwarded.

The capacitor will probably still be appropriately set for the original
VT-25 tube, but if you change the tubes to something like 6AQ5s or 5763s or
similar, it is essential that you re-neutralize the PA. Bad FM, instability
and lousy AM modulation will result from an incorrect setting. This is why
the manual says that only Western Electric VT-25s must be used as
replacements. Evidently the W.E. ones were held to close tolerance in the
grid-plate capacitance so that they could be replaced without
re-neutralization.

To neutralize, tune the transmitter as normal into a dummy load or antenna.
Stop the PA from operating. Easiest way is to plug a phone plug into the
bottom jack labelled RF Amp at the rear. Turn down your mic gain so any
audio does not ruin the open-circuited modulation transformer.

Make a detector: connect one end of a germanium diode or 1N914 or similar
to a .001 or .002 uF capacitor. Connect the free end of the diode to the
Antenna terminal and the free end of the capacitor to the Ground terminal.
Connect your voltmeter (5 or 10 volt scale) across the capacitor. Key the
transmitter on. The meter will probably indicate a volt or more, which is
VFO signal being coupled via the grid-plate capacitance of the
non-functioning PA tube.

Check that no B+ voltage appears at the Balancing condenser nut. It is
grounded through the coil, but check to make sure. Using a 1/4 inch socket
on a short socket extension, adjust the nut from above chassis until you
get a minimum. The metal socket interferes a bit with the reading, but you
should get a good null by trial and error. When satisfied, the process is
complete.

Neil ZL1ANM


On Sunday, April 7, 2019, David Stinson <arc5 at ix.netcom.com> wrote:

> Thank you to everyone for the encouraging words on
> the "get the SCR-183 Transmitter (BC-230) going
> with cheap tubes" project.  Work has been too
> pressing
> for any work the past week, but going to try to
> fiddle some more this weekend.  The 6S4 works well
> in the RF, but still having trouble getting them
> to modulate the PA more than about 70%.
> More work to be done.
>
> TNX OM ES 73 DE Dave AB5S
>
>
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