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.

I had some hair-brained idea (this was when I was in primary school) to
hook the thing up to the PC-speaker output of the computer (since I knew
how to make tones) and then use some circuit interfaced to the game port
(since I knew how to read the switches on those) and try to send data
using AFSK.

Exactly what data rate I'd achieve, given the whole lot would be
implemented in QBasic I have no idea. I doubt it'd outpace PSK31.

Not that I knew what AFSK was back then. Or that to do what I wanted to
do, I really should have a radio license which I didn't back then.

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.

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...

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...

On 03/08/14 09:11, Stuart Longland wrote:
In the interest of science, I gave it a shot just then. With a 100nF
capacitor in series to block the DC, I wired it to a DIN5 plug (all my
radios have been set up with adaptors to DIN5 headset jacks) and tried it.

It did work, but without any amplification or impedance matching, the
modulation is well down. I might try winding a small transformer and
see what that does.

Well, I did some math, to transform a 32ohm load to ~600 ohms, I need a
turns ratio of ~4.3:1.

math.sqrt(600.0/32.0)
4.330127018922194

I played with this a little, and so I thought I'd add a couple more,
make it a nice round number. 4.5:1 would give me a 648ohm load on the
HV side.

I had a few L8 toroid ferrite cores laying around, these to be precise:
http://www.jaycar.com.au/productView.asp?ID=LO1230

Not being sure about the number of turns, I made a guess at 45:10, and
started winding.

Just tried it and now the audio is completely dead. Transformer loss is
too high for the feeble signal out of the "microphone".

My guess is that the turns count is waay too small, and that maybe
450:100 might be closer to the mark, but I really don't feel like
winding that many turns on a toroid. 45 felt like a marathon.

So for this to work, I need an in-line amplifier of some sort. My
challenge is to RF-harden it, and have it compatible with both dynamic
microphone inputs as well as electret: as the same headset will probably
be used with both. (If the phone rings and I'm on the bike, I will
sometimes plug the helmet into the phone to answer it. Right now all
headsets are interchangeable with all devices, a feature I'd like to keep.)

I'm thinking possibly a FET, since that's what's embedded in the
electret capsules, however I suspect this will be prone to the same problem.

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.

Stuart Longland wrote in news:kea1bb-
:

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.


Did my post earlier not help? If your equipment has a frame ground and a
signal ground, or perhaps an analog plus a digital ground, that might help.
Your CAT5 has no screen so far as I know, it's just 4 twisted pairs. But if
you handle the twisted pair as usual, referenced to signal ground, catch the
RF on the screen and send it to the other ground. if you already tried this,
what happened?

Stuart Longland wrote:
My guess is that the turns count is waay too small, and that maybe
450:100 might be closer to the mark, but I really don't feel like
winding that many turns on a toroid. 45 felt like a marathon.

Don't you have an ancient defective portable AM radio (you know, the
kind that proudly mentioned "6 transistors" on the case)?

The have those small E-I core transformers (soaked in wax) that are
suitable for the job.

Lostgallifreyan wrote in
:

But if
you handle the twisted pair as usual, referenced to signal ground, catch
the RF on the screen

Meaning a screened mic cable. I said it badly there, thinking of the context
in my earlier post which did mention it directly..

Stuart Longland wrote in news:kea1bb-
:

Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.


Did you try ferrite beads? To stop propagation along a screen? No-one seems
to have mentioned them yet so I thought I should...

On 03/08/14 19:12, Lostgallifreyan wrote:
Looks like I might be getting out the oscilloscope and making some
measurements with the PTT down, try to figure out where the RF is
getting in.

Did my post earlier not help? If your equipment has a frame ground and a
signal ground, or perhaps an analog plus a digital ground, that might help.

I did see your post… basically the frame is purely for the antenna
counterpoise, and nothing else.

Yaesu do connect their negative supply to the antenna ground, which
yields a negative earth, however I do not use this for any kind of DC
path. Not intentionally anyway. (It did happen by accident one day:
+12v came in contact and nearly started a fire. Oopsie!)

Unfortunately, I do not get provided with separate analogue and digital
grounds. I suspect in the set they are one in the same.

Yaesu do supply a separation kit: it basically consists of a mounting
bracket for the head unit, a RJ11-RJ11 round cable for the head unit, a
RJ45-RJ45 flat cable for the microphone and a RJ45-RJ45 adaptor so you
can plug the handmic in.

For the RJ11, I have no idea what the exact pinout is. On that cable
would be serial data (tx/rx), power (probably 5V), speaker output, the
"power/fast tune" button and a signal ground. It'd be nice to know
which one is which but I'm guessing Yaesu probably want to keep that secret.

For the RJ45, it's documented in the handbook. There is +5V, signal
ground, open-collector inputs for PTT, Up, Down and Power/Fast Tune,
then microphone + and -. The microphone input is a nominal 600ohm
impedance, intended for a dynamic microphone.

For convenience, I run 3 lengths of CAT5 with DB25 connectors at each
end. At the ends I then plug in a suitable break-out cable which maps
pins on the RJ11/RJ45 connectors to pins on the DB25.

This is because the cables are frequently connected and disconnected,
particularly on the front basket as I park my bike outside. I found
RJ11 and RJ45 connectors became unreliable when they were used in this
manner. DB25s have been good and reliable however.

I did try wrapping this triple-CAT5 cable in adhesive aluminium tape and
grounding that to the frame, but this did not help. I also have fairly
sizeable ferrite cores clipped on.

I could try running a separate screened cable just for the microphone,
I'd probably have to have a separate connector too. I'll have to
procure parts to do this.

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

"Michael Black" wrote in message
xample.org...

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.

Nowadays when transistors are almost ten-a-penny, it is the switching
that is expensive, so otherwise than as a novelty, there's not much
to be said for single transistor rigs.

I have in my museum pieces a boxed PM2A valve and the conditions
of sale printed on the bottom say that it must not be sold to the public
for less than 8 shillings and 9 pence, which pre-war was about 10%
of the weekly take-home pay, but imagine paying £$40 today for
each active device !