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[wxsig] T238+ modem problem update and a question

Ted11 tedlists at hullar.com
Fri Aug 13 00:03:31 UTC 2010


Okay, I think I'm getting this figured out.  This is my first packet attempt
and my first kit in a long time, so it's been a slow but very interesting
process.  

First, some typos in the T238 modem assembly manual (Rev 8, March 2006).
Top of page 6, the table listing voltage checks:  "C8" should be "C108".
Second, and more importantly, the values for R100 and R101 are switched in
the assembly instructions relative to the schematic.  R100 in the schematic
is 100K, in the parts list and construction section is 10K.  Similarly, R101
is 10K in the schematic, and 100K in the instructions.  I think the
schematic is correct, as it matches the resistor values in another similar
MX614 project I found on the web.  

Since my problem was no signal out of pin 8 of the DB9 connector, I had high
hopes reducing the resistance from 100K to 10K would fix everything.
Unfortunately, no.  But, when I turned the trimpot down all the way, I got a
very faint sound when monitoring my output signal.  So, I bypassed R101 and
used only the trimpot for resistance control.  I slowly turned down the
resistance, and the signal kept getting louder, until finally I was at 0
ohms.  However, even with no resistance between pin 7 of the MX614 and pin 8
of the DB9, my signal was still considerably fainter than others I
monitored.  Encouragingly, I've seen some packets make it to the internet
(http://aprs.fi/?call=KE6ROS-2), but not as many as I would like.  

Which leads me to another thought:  I'm running the output of modem into the
rear panel DB9 input of my Alinco DR-135.  According to the manual, it wants
100mV/600 ohms in.  I'm guessing the modem was designed to hook up to a
microphone input, which I would think would be a lower input.  Does this
theory make sense?  Anybody hook their T238 modem up to this radio, or a
similar one with a DB9 jack?  I suppose I can hook the T238 using the
microphone and rear speaker jacks, but was hoping to use the DB9 for a
tidier install.  

Thanks for any help!

Ted
KE6ROS




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