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Subject: LX200: 12VDC/18VDC converter solder joints
From: Robert Preston
Reply To: mapug@shore.net
Date: Sun Jul 14 15:25:24 1996

I decided to search out a dark sky site for the second time since buying
my 8" lx200...but first wanted to adjust the voltage of the 12V-18V
converter down to 15V, according to the instructions kindly posted here
on 6/2/96 by Dave Sage (dave.sage@canrem.com). I had not used the converter
in the two years since I bought the 'scope and converter, except for one
time shortly after the purchase, when the converter seemed to be OK since
it ran the scope OK from a car battery at a field site (at the time, I
didn't know that these scopes will run on either 12V or 18V or 20V).

After making the recommended resistor change, I found that the output was
equal to the input 13V (input from a filtered "12V" power supply). I
remembered Dave or someone on the list had mentioned that the solder joints
to the LT1170CT IC on his converter were "cold joints" (i.e., the
fabricators had melted the solder only partially, leaving the connections
to the PC board broken or at best very weak). Mine looked fine by eye,
but then I looked again under a hand lens, while wiggling the chip.
At least three of the five connections were cold joints: the LT1170CT leads
moved in and out of the PC board slightly as I wiggled. After I fixed every
joint on the board (for good measure), the output still equaled the input (even
with the original resistor replaced).

I suppose I'll see if Meade will give a post-warranty replacement, but the IC
in question is also available through mail-order electronics dealers such as
Digi-Key for about ten bucks.

I'd recommend that people who use the converter beware of this potential mode
of failure. If a scope slews more sluggishly with the converter than when
operated from the 120VAC/18VDC supply, the converter output voltage may equal
its input voltage, possibly due to cold-joint-related IC failure.

Rob Preston
Pittsburgh PA
rapr@med.pitt.edu