[R-390] HP410B Intermittent(s)

[Jon Schlegel]

Hi All,

The recent thread on the AC probe for the HP410C prompted me to look 
at a nagging problem on my 410B.  On Rx1 scale with leads open 
circuited and meter indicating full scale, an intermittent existed 
somewhere as the full scale adjustment was not stable.  To make a 
long story short, the problem looked like a solderability problem AT 
THE TIME OF MANUFACTURE.  The problem area was the smaller of the two 
resistor boards located on the left side of the instrument, same side 
as the function switch.

The specific problem seems to be poor solderability of the foil on 
the circuit board.  They used rivets in the through-holes of the 
board to keep the foil anchored and the solderability problem of the 
foil looked like it prevented a good joint between the two from the 
get go.  Removing the old gucky solder then liquid flux and heat 
seemed to give a ok joint.  It was surprising to see that kind of 
problem inside of an HP instrument.

It may be worthwhile to check for problems like this on any thing 
that has intermittent problems that uses this construction 
technique.  As an aside, it looks like the schematic symbol  for CR1 
("metallic" rectifier) is reversed unless they were using some other 
current-convention back then.

Regards,

Jon Schlegel
WA3MVM