Discussion of KOB Dial-up Circuit and 4 Ohm Sounders
On Oct 20, 12:42, W K Dunbar wrote:
> Subject: Dial-Up Morse with 4 ohm Sounders?
>
>
> Greg:
>
> An afterthought - - many KOB sets have 4 ohm sounders. Due to the
> current requirement (240 mils) I avoid using 4 ohm units with
> Dial-Up Morse.
There is no reason for general discrimination against 4 ohm sounders.
I originally believed 4 ohm sounders were used only for "toy" practice
sets, but I later discovered they were quite common in commercial use.
I've got a few in my collection.
> On the other hand, 30 Main Line sounders (60 mils) and 400 ohm
> sounders wired for either 100 ohms (60 mils) or 400 ohms (30
> mils) should work nicely. The value R-3 and R-4 may have to be
> juggled to get the desired loop current.
I've wired a set for a 20 ohm KOB. It is powered by the DCM-6. The sounder
has to be adjusted quite carefully for good copy. 20 ohms is as low as I
would recommend if you steal the loop power from the DCM-6; that small wall
transformer is at the edge of its capability.
100 ohm sounders should work just fine with the resistor values I showed in
the schematic.
400 ohms, well I can not remember what the current requirement is. Assuming
it is about 15 mil, the resistor values would be 35 Volts/(.015A X 2)=1167
ohms. Make that that a nice round 1000 ohms. Never tried a 400 ohms sounder;
it should work fine as long as the loop voltage of 35 Volts is high enough for
good "snappy" operation (as Ed Trump would say).
>
> Would you want to change the note in the schematic to read
> something like:
>
> "Values shown for R3 and R4 set loop current to 50 ma.
> Sounders of different resistance may require different
> values for R3 and R4. Do not use 4 ohm sounders."
>
> Or am I wrong to shun 4-ohm sounders in this application?
Perhaps it would be correct to say "Do not use 4 ohm sounders with this power
supply". Also, the transistor I put in the schematic is a small switching
device that is only good for about 400 mA. I would recommend a heftier device
for 4 ohm sounder applications. The device shown in Ed's original schematic
would handle the current just fine. I changed to the smaller device since it
worked with the 120 and 20 ohm sounders, and it is a little easier to wire
into a breadboard circuit.
I have thought about using a single D cell battery to power a 4 ohm dial-up
set. The battery holder would be bolted to the top of the DCM. The DCM would
supply the voltages for the remainder of the circuits. I just don't know how
long the D cell would last. If it works, this would be a good option for
people who have a nice 4 ohm sounder or KOB set they would like to use. Of
course you could always use a 1.5 volt power supply, but this would be pretty
large and bulky due to the current requirement.
So, to sum it up, your revised note is appropriate. The schematic shown will
not support 4 ohm sounders due to limitations of the power supply and
switching transistor. We need to determine the easiest way to support 4 ohm
sounder usage. Maybe Ed Trump will have some ideas.
--
Regards,
Greg Raven
Greg... I've found that if you set the set up for a 120 ohm
sounder/50 ma loop, you
can substitute a 400 ohm sounder directly. the added coil
resistance in the 400 ohm
sounder nicely reduces the loop current to a value that the 400
ohm sounder "likes"
and it works just fine. I use 400 ohm sounders with all my
portable sets because they
are plentiful, and smaller than a 15B ML sounder.
The 20 ohm sounders like about 110 to 120 ma or more to be
"happy".
4 ohm sounders will work fine with the s/s circuit design, just
change the loop resistors
to get a 250 ma loop, and make sure you power it with something
hefty enough to put
out a volt or two at 250 ma. Actually you can use a 6 or 10 volt
supply, and appropriate
dropping resistance, if it will meet the current requirements.
73 ED FB
From: wkdunbar@heartland.bradley.edu (W K Dunbar)
To: ravengre@comm.mot.com
Subject: Re: 4 Ohm
Hi, Greg:
Thanks for the relay from Ed Trump. Regarding 400 ohm sounders,
I put one in one of the portable sets I use for Dial-Up Morse
work. When I checked loop current, it was still higher than
necessary, so I put a 1/2 watt resistor between sounder
terminals 2 & 3 in place of the jumper wire, and that dropped
the loop current to about 35 mils.
Hold everything - I found an error in my rendering of your
circuit. The 1000 mfd filter capacitor in the negative side of
the power supply was shown backwards. I've corrected it.
Faxed a copy of the circuit to Ed Berryere, in Saskatoon, Sask.
as he E-mailed me for a modem to fix up for someone in Kamloops.
Also am sending a copy to Arlan Tate, in Fort Worth, who also
contacted me for a modem.
Incidentally, Tate makes circuit boards for the TU, and said
when he gets the diagram, will make a board for it and send me
about a dozen of them for distribution. So, we might have a
source for the boards. He will try to build the circuit into the
DCM-6 case, less the power supply, of course, so I'll have to
wait until he experiments some to find out whether the board
will be for external or internal use.
73,
Bill Dunbar
To: Bill Dunbar and Ed Trump
GM-
Just wanted to let you know I put together a dial-up set designed for use with
4 ohm sounders. It works great ! Here are the changes required to make the
KOB circuit work:
1. The loop resistors must be in the range of 10 ohms. I was using an
external variable power supply. With two 10 ohms in the loop, I had to adjust
the supply to 7.5 volts to get good "snappy" sounder response. This is
somewhere in the range of 250 to 300 mA (need to get an exact measure). I am
going to add another 10 ohm in parallel to make a 5 resistor, since the goal
was to operate the loop from a 5 volt source.
2. The loop keying transistor must be something heftier than a 2N3904 ! The
collector to emitter voltage rose to almost 2 volts with a loop current of 250
mA, which causes overdissipation in the transistor. A 2N3055 or similar like
used in the original dial-up circuit is the correct choice. I used a power
NPN from the junkbox that worked just fine.
3. The 1N914 in the loop (connected to the collector of the keying
transistor) must be replaced with a 1N4000 series diode due to the higher
current in the loop. A 1N914 is only good for 75 mA or so. The only thing I
had in the junkbox was a 1.4KV 1.5 Amp diode. It worked AOK.
I was using a MESCO sounder. It certainly seemed to be harder to copy than
the Western Electric 120 ohm mainline sounder I normally use for dial-up.
During the 4 pm session on Saturday, I was adjusting the sounder like mad
trying to improve my ability to copy it. I finally put it in a spare
resonator hood, which improved copy considerably. This sounder has a pretty
strong "ring" to it. I have other 4 ohm sounders which I will try with this
new dial-up.
73
--
Regards,
Greg Raven
Subject: Re: KOB Circuit with 4 Ohm Sounder
Reply-To: wkdunbar@heartland.bradley.edu
Hi, Greg:
Thanks for the info on using 4 ohm sounders for Dial-Up work.
I've saved it in the archives and will work up a schematic using
the modified values, so that anyone wanting to use KOB learner's
sets or other 4-ohm sounders will have a plan to use.
73,
Bill Dunbar
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