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[wxsig] T238+ modem problem update and a question

Ted11 tedlists at hullar.com
Fri Aug 13 02:21:38 UTC 2010

I haven't implemented the fix yet, but thanks for the information!  I found
the posts you referenced by looking through the email archives, and there it
was.  For reference to anyone else:  February 2006 was the (first)
discussion, and the relevant information:

"Change R101 to 4.7 K and the output coupling capacitor from 100 pf to a .1
uf. "

Thanks again for the quick reply, I appreciate it.  


-----Original Message-----
From: wxsig-bounces at tapr.org [mailto:wxsig-bounces at tapr.org] On Behalf Of
Mark Patton
Sent: Thursday, August 12, 2010 5:53 PM
To: TAPR Weather Station SIG Mailing List
Subject: Re: [wxsig] T238+ modem problem update and a question


You were on the right track with the cap idea.  C111 is a DC blocking 
cap on the output side. Yo work with a 100pf it needs to drive a high 
impedance input on the radio.  Most radios are not high impedance 
enough.  I don't have my notes in front of me but try sticking in a .1uf 
(or even larger)  in it's place.  I think you will find this will solve 
your audio problem.

This issue has been covered a few times on the forum so you may want to 
do a search for "low audio" and see what that turns up.  i think the 
resistor issue has also been covered.

Good luck getting it working!  Mine has been working non-stop for at 
least 4 years!


On 8/12/2010 6:03 PM, Ted11 wrote:
> Okay, I think I'm getting this figured out.  This is my first packet
> 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
> is 100K, in the parts list and construction section is 10K.  Similarly,
> 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
> 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
> very faint sound when monitoring my output signal.  So, I bypassed R101
> 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
> 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
> rear panel DB9 input of my Alinco DR-135.  According to the manual, it
> 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
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> wxsig at tapr.org
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