[volt-nuts] plastic caps on 3458A reference board

[Frank Stellmach]

Hello Joe,

yes the cap consists of two parts.
The upper one serves for thermal isolation of the LTZ1000A TO99 case 
against the environment and therefore reduces the power dissipation of 
the oven.

The part on the solder side is much more important, as it covers the 
solder joints and avoids air draught over these pins. That avoids these 
low frequency voltage variations, which are mentioned in the LTZ data sheet.

Anyhow, there are no further shieldings around the PCB, so the solder 
joints of the OP Amp and the precision resistors are exposed to that air 
draught, maybe from the fan.
That's a further engineering fault they made on this reference.

(The other faults are the 95°C oven temperature, the use of the A 
version instead of the non A, and the use of R417,200k temperature 
compensation resistor, which is necessary for the non A version only.)

The hat is not included in the BOM inside the CLIP, therefore can not be 
ordered from HP, obviously.

It is a smooth, shiny plastic, resembles the one used around the LM399H.
Wasn't latter one something like polysulphone?

Maybe suitable pieces of polystyrol foam will do the job also.

Frank