From: "Steven Weber" <[snipped]>
To: "Monty" <>
Subject: Important AT Sprint mod
Date: Mon, 2 Jun 2003 20:58:01 -0400
Organization: Steve's Electronics

The problem:

I've had two cases of DDS chip failure.
Apparently, running the chip with two different
supply voltages wasn't a good idea. On close
inspection of the data sheet, the absolute max
ratings specify Vdd and Vaa should be no greater
than 300 mv difference. This is different from
other DDS chips like the 9850, which can use
different supply voltages.

I called ADI and talked to an applications
engineer, who's only comment when asked about
using different supply voltages on the 9835 was "I
won't recommend it seeing that absolute spec". I
asked if he could check with someone who knew the
chip better and he said he would, but of course,
never got back to me.

Anyway, it seems the problem is a potentual
latch-up problem and doesn't actually damage the
chip. At least with the one I got back here to
look at. I put the suspect chip onto a stand alone
VFO board with 5V supplies and it still worked.

In the first case which was reported, it seems the
cpu ports driving two of the DDS data lines were
also damaged, so I suspect maybe something else
happened here. In the second case, which I was
able to get the rig back and look at, the cpu
ports were fine, but the chip was not responding
to data.

The Fix:

The fix is to run both the analog and digital
sides of the chip at 3.5 volts. This is done by
cutting the 5V track between C29 and C27. There's
just enough space there to get an Xacto knife
between the chip caps and make the cut. Then run a
short jumper between the top of C29 and the top of
C28. If you have a little piece of the #28 magnet
wire still kicking around, that works well.

I would recommend everyone doing this mod, even if
your rig is already up and running and working
well. Even though this problem didn't show up
sooner and many of the rigs are working okay this
way, no sense taking a chance it could fail.

Also, if you haven't done the 10 K pull up
resistor mod, that is strongly recommended too.
Monty has documented this mod on his web site, Thanks

Steve, KD1JV
White mountains of New Hampshire
"melt Solder"