[Milsurplus] add an amplified speaker to Racal TRA-967

[Patrick Jankowiak]

I just added a amplified speaker to my TRA-967. For those who do not
know, the Racal TRA-967 is a backpack type radio about 1/3 the size
(and weight!) of the PRC-77. It is interoperable, but has less
deviation. The PRC-77 will be loud in your handset and the Racal will
be soft in the PRC-77's handset. They unsquelch each other fine. 

Since I mentioned handsets, you know you can never hear those things
if they are not at your ear. The TRA-967 and PRC-77 radios have very
small audio amps, and won't drive a speaker properly. 

I discovered that the TRA-967's handset connector has +15VDC power
switched to it when the radio is on. 

My radio came not with the British handset, but with an American
military one, and an appropriate connector. I took apart the handset's
connector and found the audio output, power, and ground connections. I
added three #18 wires, and brought them out the cord end of the
handset's connector. I bought a small audio amp from a kit made by
Velleman Kits, and placed this inside a small 3" square extension
speaker. I installed a metal plug assembly to the three wires from the
hanset's connector, and to the speaker/amp.

This way I can unplug the speaker/amp from the handset when I don't
need it. I realize I could have used the second handset connector for
this, but the carry case has only one handset plug hole, and the
connector is hard to find!

I would like to mention that the wires inside some of the U.S handset
coiled cords can be, for lack of a better term, "some funky old kind
of rubber-covered wire that's crappy and almost impossible to solder
to, and flux does not help".

I found that these wires can be 'tinned' if you heat them with the
soldering iron to about 850 degrees. Even after this, they won't take
solder well and will not want to play nice, especially when trying to
attach one of those, along with one of the new extra wires, to the
handset connector. The person who originally made up this handset must
have known this because the handset cable wires ends were  snipped off
while still in the original lugs of the original U.S connector. These
remining brass 'nibs' were then tacked to the lugs of the connector to
fit the Racal. I decided this was 'wrong' and prone to fail. 

I cut these nibs off, and stripped the hanset cord wires to 1/8". I
then tinned them at 870 degrees. In order to make them stick to the
ends of the new wires which I was putting in, I laid the 1/8" of
tinned handset cord wire alongside the first 1/8" of a 1/4" stripped
section of the new (extension speaker) wire, and then took 4" of #26
stranded wire, and using one strand as a wire rope, wrapped the two
wires together. I was then able to solidly solder them together as a
joint at a normal temperature of about 650 degrees. This left 1/8" of
tinned new wire sticking out. This was inserted into the
freshly-cleaned hollow contact of the handset cord connector. After
completing this work, the connections were very solid. The remaining
original handset wires, once tinned, soldered well to the connector
pins.

I thought I would share this method, since it resulted in a good job.
It would have been much more frustrating and time consuming and sloppy
otherwise, trying to stick those old wires and new, to each pin of the
connector.

I then discovered that too much gain in the speaker amp causes
motorboating or howling as the volume is increased, and even with the
radio squelched, there is some noise. The problem was caused partially
by using the same wire for power and signal ground. With the cable to
the extension speaker carrying power, audio, and all of it returning
through one ground, it is a concern. The amp needs only 300mV to reach
full output of 3 watts, but the radio can put out about 12V. The amp
has a 5600 ohm resistor in series with its input. I added a 100 ohm
resistor to ground right after this resistor. I think this lower
impedance to ground had more to do with killing the ground loop
oscillation than simply the attenuation itself, which could have been
done by adding more series resistance. I still get 2 watts of audio,
and the radio now reaches clipping before the external amp.

And if someone calls me on Six, I will at least hear them!

On a similar note, Steve Haney sells an excellent amplified speaker
for the PRC-25/77. It's made in the case of an LS-166 or -454, and
connects to 12-14 VDC to supply the radio including the 3VDC heater
for the -25 as well. But I'm taling about the Racal job here. If  y
PRC has an external loudspeaker, I wanted the Racal to have one also.

SGT PJ
KD5OEI