[volt-nuts] VS 330 circuit board oddity

[r j]

You could maybe search YT for "krohn hite". In one of the videos (the
Australian teardown-man) you can see the schematic for a similar unit. My
guess is that they are all built on the same topology. You have little need
to worry - the box in the video also has this DIY-look. The whole unit is
made from a current source, an 1N829 precision Zener, a (custom "DIY"-board
in the video) chopper amp and a precision divider. Very easy to calibrate
and very easy to repair/change for more modern components. What you have
bought is a fairly simple but good small-scale "lab-bench"-production
instrument with a lot of expensive precision resistors. The handwritten
label is for the precision diode. Normally it runs at 7.5 mA, but yours
seems to be fed 8.5. Imo you should have this box checked for possibly
sloppy repair and then calibrated. This unit is magnitude(s) more stable
then your MAX chip and the modern DMM (as I recall a Fluke?).  You have
paid quite a lot, but checked and calibrated this box will be just as
precise and stable for another 20 or more years.

Roy


On Sat, Aug 3, 2013 at 9:20 AM, Joseph Gray <jgray at zianet.com> wrote:

> Of course, before powering up the VS 330, I had to remove the covers and
> take a look inside. Everything looked intact and reasonably clean.
>
> You can tell that someone re-painted the top cover. It is glossy and
> smooth. The bottom cover is matte finish and textured.
>
> I did see one oddity on the circuit board. There is a broken nylon wire tie
> attached to the underside of the main circuit board. You can see it in the
> first picture here:
>
>
> https://dl.dropboxusercontent.com/u/19599147/VS%20330%20Board%20-%20Bottom.jpg
>
> Here is a picture of the top side. You can see that the holes straddle the
> 801 amplifier module.
>
> https://dl.dropboxusercontent.com/u/19599147/VS%20330%20Board%20-%20Top.jpg
>
> You might think that perhaps the nylon tie was there to hold the amplifier
> module onto the board, but the module's pins are soldered. The way the tie
> is hanging, it looks to me that perhaps something was attached to the
> bottom side of the board, underneath the amplifier module.
>
> I'll have to email Krohn-Hite and request a manual. In the mean time, any
> guesses?
>
> Also, I am normally inclined to replace old electrolytic caps. Opinions on
> whether I should or shouldn't?
>
> Joe Gray
> W5JG
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