[volt-nuts] Dekavider DV411 repair and question

[Dave M]

From: Hendrik Dietrich <don_hendi at gmx.de>
> Subject: [volt-nuts] Dekavider DV411 repair and question
>
> Hi List,
>
> I partially fixed a "lowly" DV411 Dekavider from the
> "Electro-Measurements" epoque and wanted to report what worked out and
> what still is and have a general question about such devices.
> The part in question (just look what the cheapest one on ebay was, one
> made a power supply out of it) had a working first and second decade,
> but the can with the 3rd switch and last decade potentiometer required
> some work.
>
> The switch freewheeled, it didn't lock onto the right on-digit
> position. That was due to the slotted wheel slipping on the drive
> shaft out of position so it was no longer pressed against the
> spring-loaded ball. I was surprised that this wheel was just soldered
> to the drive shaft, so it was enough to push it back into the right
> position and solder it down with some very agressive flux (Soldeen-1).
> http://dg3hda.primeintrag.org/lib/exe/fetch.php?media=locking_cogwheel.jpg
>
>
> The 4th(5th) decade potentiometer seems to be electrically okay, but
> it turns 360? instead of 270?, I consider that it had some delimiters
> but these are scraped off. However, careful operation will be
> possible or replacement by a potentiometer and a adapter between the
> thin driveshaft of the switch  to the axle of a new potentiometer.
> (Hoped to keep it original.)
>  http://dg3hda.primeintrag.org/lib/exe/fetch.php?media=potentiometer.jpg
>
> For Pronographic reasons, a overview is available too.
> http://dg3hda.primeintrag.org/lib/exe/fetch.php?media=overview.jpg
>
>
> Besides hints for further repair, does someone know which alloy is
> used for wiring by ESI? Seems a bit hard for plain cooper.
>
> Best regards,
>
> Hendrik
>

Hendrik,
It's been more than a few years since I had my hands on one of these boxes, 
but I aeem to remember that the vernier potentiometers in these instruments 
are designed to rotate a full 360 deg.  I don't know why; seems illogical, 
but that's the way they are.
If the original pot  works, I wouldn't bother changing it.  It will be 
extremely hard to find a replacement pot with the low tempco that the 
original pot has.

The wire used to make the resistors in these units is probably Manganin, an 
extremely low tempco compound. Manganin wire has been used by instrument 
manufacturers for many years for temperature stable resistors.  I think that 
most lab standard resistors are made with Manganin wire, and thermally 
insulated from the outside world to maintain their accuracy.

Dave M