On Sun, 3 Aug 2014, Rob wrote:

Stuart Longland wrote:
On 03/08/14 11:10, Michael Black wrote:
The circuits of such devices were often very cleverly designed, re-using
many components between receive and transmit (using a multipole switch).

They were really complicated switches, for the sake of a few transistors.

Yep. This one I gutted, I recall de-soldering the switch and then
reverse-engineering the pinout so I could replace it with a relay, which
I did. A 4-pole double-throw relay IIRC.

Not only that the switch has many poles, the circuit is often very tricky.
It is not a receiver and a transmitter with a switch to toggle the
power, antenna and speaker/mike to connect to one of them, no it is
a blob of electronics that morphs between being a transmitter and being
a receiver when the PTT switch is switched over.

Yes, the switch would be so much simpler if they were just switching audio
and power.

I get the feeling these were the solid state equivalent of the one tube
transcievers used to homestead the higher bands. They were a modulated
oscillator on transmit, a superregenerative receiver on receive, and a
common audio amplifier. There the space and cost of a tube meant they
switch it between the two functions, but since it was a modulated
oscillator, it was a simpler arrangement than switching between a
superregen and a crystal controlled transmitter.

Those single tube transceivers were certainly simple, and got people onto
the higher bands. ONce a band got busy, there'd be a rule put in that you
had to use crystal control (or have equivalent stability) on that band.
So these rigs would start off at the "UHF" 10metre band, then move to
5metres, then up to 2.5Metres. Even fifty years ago, they were being used
on the 420MHz band. SImple and cheap, you didn't get much range, but they
helped get people on the band.

In those days I sometimes tried drawing the schematic by looking at the
PCB traces and components, and it is very difficult to draw a schematic
that makes any sense...

Expecially when you were a kid without much ability to figure out what the
switch contacts were doing. All these circuit board traces would go into
what amounted to a black box switch, crtainly beyond my skill at the time
to trace out.

It is completely contrary to the electronics world today, where one would
prefer having a thousand extra transistors to save a single mechanical
component (like an extra pole on the switch). The times have changed...

I guess it makes sense at the beginning, but transistor prices dropped
fast, yet the same scheme was used into the seventies. I assume when
cheap walkie talkies moved to 49MHz, they didn't add transistors but still
used that complicated switch (but I've never looked at a superregen 49MHz
walkie talkie). Considering that "transistor radios" at the time were
certain to tell you that they had "X transistors" you'd think the cost of
adding a transistor for transmit (and thus be able to say "four
transistors" or whatever) would increase sales enough that it would offset
th cost of the extra transistor.

It is a lesson worth repeating, adding transistors may nominall make the
circuit more complicated (and expensive), but often results in the overall
design being simpler.

Of course, once ICs came along, that took the idea to the extreme, endless
transistors in the IC, but you never see them.

Michael