Electronics > Repair

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

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HalFoster:
Usually there is no issue with using radials instead of axials. At most, a small piece of Kapton tape to cover traces if present and unmasked.

r6502:
Hello All,

long time ago, that I started this thread, and in the mean time I had to do other stuff and the HP3400 was not that mutch in focus.

Now I finished the design of the first prototype board with the already posted schematic, and with the new A6 board I brought my HP3400a back to live. Here are some photos:

Photo 1:
New assembled board, I placed the hole for the stand-off at the wrong position so I had build a mechanic offset. It uses the LTC2057 chopper stabilized op-amp, that is good available and was in stock. For the output I wanted to use the LT1012 but forgot to order it, so I used an AD820 I had in stock, that has a little higher offset but is working well.


Changes to the HP3400a to operate the new A6 board:
To be able, to operate the new A6 board, I removed the resistors connected to the trim-pots R4 "CAL" and R7 "1/10 scale ADJ" (marked red) as this functionality is integrated in the new board. Than I connected the heater of TC402 direct to pin 10 of the edge connector, green line.


Photo2:
View on the HP3400a with removed components and installed new A6 board.


Photo3:
View on component side of installed new A6 board.


My intention was, not to use the older ICL7650, as it is also a very old part. I also used the LTC2057 in an other project to replace replace a chopper stabilized output amplifier in an adjustable voltage reference DSA56A where this chip performs really well. I think the design is cloce to the latest version with the ICL7650 and the nuvistor with som smaller changes ...

I have to do the over all performance check and the RF adjustment, but  I did a rough check between 10Hz and 10MHz and that looked really good, the needle of the instrument was really quiet and no oscillation could be seen. I checked also with different waveforms and found no more issues.

Next step for me is to update the PCB that the hole for the stand-off is at the correct place, and I will also add some test points.

edit:spelling and photos inside of the text

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