Products > Test Equipment
Krohn-Hite 522 DC calibrator.
grumpydoc:
--- Quote from: Vgkid ---I would love to see the schematic for the 100v range.
--- End quote ---
There isn't anything special - the output buffer amp is fed by ±150V and gain adjusted as appropriate
Having found a few old ebay auctions with better pics, and having had chance to look through the schematics for the 521 I can see a couple of differences between it and the 522
- The 521 has a 7-segment display, the 522 actually has an alphanumeric display
- The 522 doesn't seem to have provision for the 1000V option.
A few teardown pics:
Inside the unit (click on picture for larger version).
Generally it's pretty tidy inside. There is evidence of repair to one of the voltage regulators but otherwise it looks factory fresh. Date codes on ICs suggest a 1999 or 2000 build date so it's actually not that old.
The unit is built around the same voltage reference as Dave's MV106J but instead of the range switches forming a voltage divider the reference voltage is fed into a 24-bit DAC.
The DAC architecture is a bit odd in that is seems to be built as a decimal rather than binary circuit. First of all there is a 10-tap voltage divider formed from 10x precision 1k 0.01% resistors, these can be seen in a row under a set of trimmers in the centre of the picture. Each voltage tap can be tweaked with the trimmer. Then pairs of 4051 analogue multiplexers, which can be seen above the row of trimmers, are used to form 1-of-16 switches (of which 10 inputs are used). Finally the voltages from the 10 taps are summed via a six input decade weighted mixer and then passed to the buffer amp and then to the outputs.
The whole thing is controlled by a 6502 microprocessor.
It really looks like they took the basic design of the older units and just built a digital version of the front panel switches.
This is the voltage reference.
As with Dave's unit there is a little sticker with the diode parameters. Again a 1N829A is used for a 0.0005% per oC temperature coefficient - presumably aged and hand picked. What I can't quite figure out is why the temperature coefficients of the divider chain aren't thought to worsen the diode figure. The 1K resistors are probably low tempco ones but even if they are measured and matched they aren't exactly taking care to ensure a consistent thermal environment. Trimmers have, of course, notoriously bad thermal coefficients but I suspect the saving grace with them is that they are being used as voltage dividers so any change in absolute resistance will affect the ratio of resistance between the wiper and each end much less.
Unlike Dave's the diode is just soldered into the board
I imagine the bent leads are for strain relief.
The CPU sits on a long plug in board at the front of the unit - very old school 65C02 with external ROM and RAM.
the reverse of the main PCB is not especially interesting, top right are the links which select front or rear panel output.
On the whole the design appears to be much more straightforward than the Fluke 343A, for much the same spec
codeboy2k:
--- Quote from: saturation on March 01, 2015, 05:56:06 pm ---You can email KH for full schematics:
UM:
http://www.krohn-hite.com/htm/ServiceSupport/PDF/Manuals/522%20Manual.pdf
--- End quote ---
Even that manual is incomplete, and has a pop-up notice in the PDF that requests to contact KH for a manual. Maybe that's what you meant by email for full schematics?
In any case, when someone has them, please post :) thanks!
dacman:
--- Quote from: grumpydoc on March 01, 2015, 02:38:58 pm ---
--- Quote from: JohnnyBerg on March 01, 2015, 12:11:52 pm ---Nice score :-+
--- End quote ---
Better than the Fluke 343A which was supposed to be working and cost me twice as much.
It is still, somewhat optimistically, in the repair pile. If I were honest with myself it would be in the "too shagged to be fixable" pile but I live in hope :)
--- End quote ---
Your 343A may be 45 years old and it has some capacitors that tend to go bad. Check the silver looking TE series capacitors (or all the aluminum electrolytic capacitors). There are two or three under the shield in the right front, in addition to the ones that are in plain site.
grumpydoc:
--- Quote ---Your 343A may be 45 years old and it has some capacitors that tend to go bad. Check the silver looking TE series capacitors (or all the aluminum electrolytic capacitors). There are two or three under the shield in the right front, in addition to the ones that are in plain site.
--- End quote ---
A good tip but before I get that far there are a couple of things I need to sort out including some mechanical damage, broken and shorted out indicator bulbs and several chewed up screws to extract.
grumpydoc:
--- Quote from: codeboy2k on March 02, 2015, 01:36:37 am ---
Even that manual is incomplete, and has a pop-up notice in the PDF that requests to contact KH for a manual. Maybe that's what you meant by email for full schematics?
In any case, when someone has them, please post :) thanks!
--- End quote ---
Krohn-Hite were very helpful, but they did point out that they normally charge for the schematics so I do not think that they would want me to post them.
However the 521 manual is available with a schematic. The quality isn't great but it is readable and appears to be substantially the same as far as the reference, DAC and output is concerned. There is a link in my first post.
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