Electronics > Repair
HP 3400a, LDR's (photo cell) replacement
<< < (4/7) > >>
garrettm:

--- 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.
r6502:
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?

Guido

factory:
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.

David
Qmavam:
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 :-("

                       Mikek
r6502:
Hello all,

meanwhile I've changed all caps in the unit and had to change a transistor in the power supply, and the readings are not so stable, as expected. It depends on the signal and ...

I decided to put in a new A6 board, with more modern stuff.  The last version of this Meter had a integrated chopper ICL7650 instead of the photo-chopper, that has very little drift, but it is an old design and it is getting obsolete.

I'd like to go to more modern components.

1st thought was to use a LTC2057, followed by a LT1012 for driving the meter.

2nd thought was, using a double OP with low drift and Vos so the LT1013 as replacement for both OP's. I think, the specifications of the LT1013 are good enough, to be used in both stages - what do you think?

I attached the circuit diagram of the HP 3400a with ICL7650 as chopper amplifier.

Guido

Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod