[arm-allstar] Important information about the current DMK-URI and the CM119B

[Doug Crompton]

DMK-URI changed to the CM119B chip sometime around the beginning of the
year. I don't know for sure exactly when it could have been prior to that.
The problem is we do not have a schematic for the new board as their site
although saying they use the B chip only shows the CM119A design.

The CM119B is a good chip and works very well but it has different
characteristics that have to be taken into account in the design. It
appears that when installing the CM119B DMK did not make the adjustments
necessary given the differing input and output levels of the B chip. Since
we do not have a schematic nor a sample to look at this is an assumption
but measuring facts by others bear this out. In the last several days at
least two people have been burnt by this in trying to interface to a UV82
and not getting enough input (RX) level (item 2) to the FOB even in boost
mode with a setting of 999.

As I mentioned in an earlier email the price of the DMK URI has gone up
significantly and with this problem it would probably be a good idea to
hold off on buying one at least until the hardware problems are fixed.
There are many other lower price choices that work well. If you did buy a
DMK-URI in the last several months and you experience this problem there is
a work around. The DB25 has two mic inputs - AC coupled and direct. The AC
coupled input on pin 21 has an internal resistive divider that has too much
attenuation for the B chip. The alternative is to use the pin 9 direct
input. Be careful though as this pin goes directly to the CM119 input. You
need to use a 4-10uf 16V or more capacitor in series with pin 9 with the
positive lead going towards the chip. Then a series resistor if needed. We
generally use a 1K as this offers some protection and little attenuation.
You may need to use a resistive divider there if your incoming audio is at
a higher level.

In addition the last update of the HAMVOIP code corrected a long standing
bug in simpleusb that prevented all of the ouptut (TX) steps from working.
So you will want to make sure you are up to date.  This is the output item
3 and is not related to the above problem BUT it seems DMK also did not
take into account the reduced output levels either with the B chip. This
could be somewhat corrected by using the internal 2X opamp and running it
on 5V. This requires a few jumpers on the DB25 connector.

The ARN-1 and 1+ FOBS sold so far do not have the input (RX) problem but
they do have the reduced output problem. Fortunately most radios work OK
just by bringing the item 3 level up. The REV 2  ARN boards due out in a
week or so will have an X7 rail to rail opamp that more than corrects for
the output level difference giving a full 4.9VPP into 100 ohms if needed.

If you have any questions about this I would be glad to answer them.


*73 Doug*

*WA3DSP*

*http://www.crompton.com/hamradio <http://www.crompton.com/hamradio>*