[volt-nuts] Update on 720A

[Charles Steinmetz]

Chuck wrote:

> I wouldn't expect physical trauma to be the issue, more something
> like a metal flash due to an arc, or tin whiskers... Something
> like that.

The damage to the resistors in the unit was almost certainly caused by 
an electrical overload (the "trauma" to which I referred).  Or, more 
likely, to a number of overloads perpetrated by one or more clueless 
previous owners on more than one occasion.

The other damage (disconnected wires, etc.) was caused by someone 
hamhandedly attempting to diagnose or repair the original overload 
damage, or contemplating harvesting parts from a unit they knew was 
damaged beyond any practical repair.

The only cure is to replace all damaged resistors with new resistors of 
equal or better quality.  If they are in the oil bath, the complexity of 
any repair is raised exponentially.  Note that there are almost 
certainly other damaged resistors in addition to the ones that the OP 
has identified.

As a practical matter, an instrument with damaged (overloaded) resistors 
will never work properly again.  The only 720As I have ever seen that 
worked properly after damage of this sort were repaired by Fluke at 
astronomical cost.

Best regards,

Charles