Electronics Evaluation, Testing & Restoration
Dated: July 20,
Redesigned PTT Circuit
I would like to describe the reworking I have done
to the the PTT circuit for this radio. As previously mentioned the radio
orginally only had the capability of switching the internal antenna
changover relay by turning on or off the high voltage switch on the
front panel. The microphone switch only makes or breaks the audio signal
coming from the mic. A previous owner of this radio added an adhoc full
PTT function. This included some wiring, a relay and powering
transformer, and replacing the two contact mic input connector (signal
& ground) with a three contact mic connector (signal, PTT &
It appears many of these components were taken from a "junk box". The
wiring was also done in a somewhat messy manner, not taking into
consideration the original wiring type and layout. It also did not
utilize the existing terminal strips to maintain the modularity inherant
in the Power Supply/Modulator design. This made separating the Modulator
module from the Power Supply module problematic and creates a
need to cut wires in order to do so.
I had to make a decision as to whether or not to return the radio
back to its original function, in effect maintaining its historical
significance. Or making the decision to bring the radio into a more
modern operational status for its continued use. I chose the
latter. So, I have redesigned a PTT circuit and utilized more
modern components, built up a small DC power supply and designed the
wiring to be integrated into the original wiring harness and used
the terminal strips for maintaining the modularity of the deck. It was
also important to design and install this circuit in such a way as to
not have it appear to have modified the radio in any way and maintain
the function of how the original limited PTT functioned as much as
This is an image of the adhoc PTT circuit that existed when I
received this radio:
The redesigned circuit needed to be installed in the
same location under the chassis as this previous circuit, but also it
had to contain the parts for a full wave DC power supply. I did not want
to use any of the DC from the radio itself in an attempt to not take
away from the tranmitters CCS rating. I chose a small 12V sealed
relay, a 12VCT transformer of an appropriate current rating for the
coil, and small terminal strip to mount the rectifiers and filter cap
for the power supply.
Several functions had to be accomplished in the relay.
On PTT, the relay must be energized. This engages the contacts
to the plate transformer center tap to be connected to ground
and the "Plate On" indicator lamp to be connected to the 6.3V filament
voltage of the radio. Energizing the relay is accomplished by the mic
switch connecting the PTT relay transformers center tap to ground. This
keeps any microphone used at ground potential.
Fortunately the simplicity of this transmitters design
allowed for an easy addition of true PTT function to be added. This
circuit is obviously not complicated. What was more complicated was
connecting everything up in a way that addressed the modularity and
adding this wiring to the existing harnesses.
This is an image of the new PPT circuit installed into
the radio in the old location.
As you can see the entire circuit is located underneath
inside the chassis directly below the plate transformer. Only two
additional holes were needed to be drilled since existing hardware used
for mounting the transformer were utilized. The two new holes are
located underneath the transformer and were countersunk to accomodate
stainless steel Philips flat head hardware making them flush mount. This
way they do not interfere with the transformer mounting.
This also allows these additional holes to be unseen in the
final assembled radio.
There is a full-wave rectifier and 220uf of filtering on
the PSU for this design. It is built up using a standard terminal strip
and using 1N4007 diodes and a 220uf 50V electrolytic capacitor. This may
be more then what is needed, but I fiqured why not. The parts are
inexpensive and small enough. The transformer is Hammond Mfg. 187B12
transformer, 12.6VCT @476mA which is more than enough current needed for
this relay coil.
The wiring for the circuit was added and strapped
separately and attached to follow the exitsting wiring harness. All
connections were made to the solder terminals on the underside of the
terminal strip. A single wire was added from the microphone connector
added to the wiring harness on the modulator module. It was terminated
with spade connector and was run in the modulator harness to the
terminal strip. For the PTT line coming from the mic, the unused
terminal 9 of the terminal strip was utilized for interconnection
between the modulator and the PSU modules.
The relay chosen has contacts that exceed the maximum
possible current draw (even at low AC line voltage) by three times. The
transformer for the relay has a CCS current rating twice that of the
draw of the supply and the relay coil. The cloth covered stranded wiring
is also overated for current. These design features are inline with
the CCS rating nature of this transmitter.
More to come as time permits ...