Electronics > Repair

HP 3400a, LDR's (photo cell) replacement

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--- Quote from: Kleinstein on October 09, 2020, 08:25:26 pm ---AFAIK the HP3400 uses the photo-choppers only internally for the thermocouple converters. So they don't have to be very high impedance. If needed one could likely just replace the whole Chopper amplifier with a modern AZ OP. However this would be quite far from the original.

--- End quote ---

Depends on the SN of the instrument. In late model units from the mid-80s HP switched to the Intersil ICL7650 chopper (non "S" model, as it wasn't available at the time).

I've owned most all of the revisions except units with the old HP logo. It's pretty neat to see the changes made to the instrument over time as new technology became available and old obsoleted. I finally ended up with my coveted 3400B model, but found it to be slower and less accurate than my 3400A that was a few years older than it. Sadly, I sold the 3400A while the 3400B was being shipped out and now have to live with my mistake.

Hello All,

it's a while ago, that I did the last work on my 3400a. I have moved,created a new lab bench, and now its time to continue with this unit.

As garrettm wrote in his last post, HP changes to an ICL7650 in later models, and Kleinstein suggested for a modern AZ OP, this is my idea, to replace the whole board. My 1st Idea is, to use a LT2057 for this job, and design a more modern version of the HP Board with the ICL7650.

I added the schematic of the A6 board with the ICL7650. Interesting for me, that HP generated the positive supply for the ICL7650 out of the 77 / 75 V. Could this newer bord  be a direct replacement for the older A6 boards?


I've got several 3400A of different ages and when diagnosing an older unit, I did use the newer A6 board to confirm the rest of the unit was working.
The newer A2 board has some parts removed & values changed, some parts are not required with the IC based A6 board & some change to update the PSU from Ge to Si transistors, the manual notes that the rear panel in some older units prevents the newer A2 from fitting.


From the HP IO Group.
"A fellow going by the handle br4av01 posted on replacing the photochopper with a 555 and a DG403 quad analog switch. The switch is pretty critical; you need something with low charge injection. Unfortunately the full details seem to have been lost from the HP-Agilent group file and photo archives when they moved from Yahoo to groups.io :-("



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