[volt-nuts] Update on 720A

[Charles Steinmetz]

David wrote:

> When I got it, two oil bath resistors were *way* off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration.   There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints.

The resistors in a 720A -- even the pad resistors -- do not drift that 
much by themselves under normal use.  Trim pots do not get used enough 
to wear out under normal operation.  If you have some that are worn, the 
instrument has been abused.  This is further indicated by the broken 
wires.  I have *never* seen a broken wire in a 720A.

Unfortunately, it seems clear that your 720A was traumatized in its 
earlier life, and some of the divider resistors were damaged (in 
addition to other probable damage, based on your reports).  There really 
isn't anything you can do but replace the bad resistors with equal or 
better parts.  Trimming them as you have done is just a temporary 
band-aid.  Resistors that have been traumatized will never allow the 
instrument to meet its time and temperature drift specifications, and 
you will be going back inside to replace (and re-replace) pad resistors 
every time you want to use it.

Worse, the problem goes deeper than that.  Even replacing the resistors 
you know are bad will not restore the instrument to its accuracy and 
stability specs.  The techniques you are using to identify the bad 
resistors will only identify ones that have been grossly damaged -- they 
will not identify all of the resistors that have been damaged and are 
preventing the instrument from meeting its specifications.

Further, the very act of replacing resistors in a 720A is likely to 
cause additional damage -- to the resistors you are soldering, the 
resistors attached to them, and the infrastructure of the instrument. 
Add to this the fact that only a few qualified replacement resistors 
will cost more than a properly-working used 720A, and you can see that 
repairing a traumatized 720A -- if you can do it at all -- is not 
cost-effective for an amateur.

The only 720As I've ever seen that worked correctly after being 
traumatized had been repaired by Fluke at astronomical cost.  I hate to 
say it, but if you expect to use a 720A to calibrate meters of 6-1/2 
digits and up, you will eventually learn that you need to replace the 
one you have.

Best regards,

Charles