Electronics > Repair

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

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TheDefpom:
I designed a board to replace this, it is on PCBway here: https://www.pcbway.com/project/shareproject/HP_3400A_A6_Chopper_Board_Replacement_06465eb7.html

I did videos on the repair of a 3400A from 1965, I installed the board I designed and it worked fine: https://www.youtube.com/playlist?list=PL2z3V9RkHQE12roTSKiF60icefxviRo72

Kleinstein:
Using a LT1013 for both stages could work, though there may be a little longer term drift. I don't see a need for super low bias for the 2nd OP-amp and the first one does not look like it needs to work with a high voltage. So a complete new design could use a cheap 5 V AZ op-amp (e.g. MCP6V66) and the 2nd OP-amp should be relatively non critical, but could be the same as the 1st. Low power parts could be a good idea. Chances are the LT1012 is choosen for low power precision and not for super low bias.

factory:
I've got several 3400As in my collection of vintage TE, here are couple of the different boards from mine, the only one I haven't got working is the earliest version, seems very hard to find one that hasn't had one of the two newer replacements at some point it's life. Both types of optical chopper are used in some other HP voltmeters & DVMs from the 1960s, with some slight differences.

Oldest version with large optical chopper assembly, 03400-66504 OR.



Two versions of the smaller optical chopper assembly, 03400-66508 Rev B & Rev D.





The newer IC based version, 03400-66152 Rev C.
Microchip seem to still make the 14 pin TC7650CPD (ICL7650 1826-0968), the other is 1826-0310 (might be MC3476G according to a cross ref found on the web, not 100% sure this is correct as it doesn't appear in the original HP cross ref charts).



Attached below is 03400-66512 Rev D, found on the web.

David

r6502:
Hello All,

Thank's for your replay.

Currently, I will go with LTCD2057 / LT1012 solution. Please have a look on the dimension of the board, that I took from my original sample and on the schematic.

For the schematic, I have to say, that I generate a positive supply from the windings that are used to generate the -17.5V rail. therefore I updated my power supply board and it is connected to pin 3, as the voltage for the neon bulbs is no longer used. I will use this rail, generating approx. 35V DC unregulated, to generate the positive rail for my OP-Amps.  I also used film caps instead of the electrolytic caps HP used in the version with the ICL7650 for the 3rd order low pass filter. I use the same  C in different quantity's, as 0.47µF is easier to get than 2.2µ and there is plenty of space on the PCB.

I mainly replaced the ICL7650 by the LTC2057 (in the HP schematic  U601) and the U602 with LT1012, that will do the job not to bad I think.

When I'm finished with the Layout, I will publish it here as well. It is my 1st KiCad Project.

Guido

r6502:

--- Quote from: TheDefpom on August 17, 2023, 03:26:41 am ---I designed a board to replace this, it is on PCBway here: https://www.pcbway.com/project/shareproject/HP_3400A_A6_Chopper_Board_Replacement_06465eb7.html

I did videos on the repair of a 3400A from 1965, I installed the board I designed and it worked fine: https://www.youtube.com/playlist?list=PL2z3V9RkHQE12roTSKiF60icefxviRo72

--- End quote ---

I did not saw your video before I did my work, but very interesting and close to the original solution from HP for the last devices with Nuvistor and ICL7650.

I got my HP 3400a very cheap - I's difficult to get it for a price of less then 200€ here in Germany nowadays, but I'm always looking to get more devices of this type ;D

Guido

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