Electronics > Repair

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

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a few months I've bought a 3400a, that had sometimes the problem, that the indicator needle sows a strange behaviour, but only in the 3 / 1 mV range. Meanwhile, the Instrument show this effect in all ranges, but only after about 30 to 45 minutes after power was turned on.

I did some research in the internal parts, and found out, that the LDR's (photo cells) from the optical chopper amplifier where bad. As soon, as they have been warmed up, this behaviour becomes more visible.
I have locked in the net, but the LDR's you can get today, are much to slow for the application. So I thought about the problem and came to the conclusion,  to replace the LDR's with electronic switches.

Yesterday, I've drawn a schematic, that shows how I'd like to do the replacement of the LDR's from opto chopper assembly.  Please have a look to the schematic  and the original schematic of ths amplifier, that I also uploaded.

In general, I'd like replace the LDR's, but if no more available - what should I do ...

I hope, I can assemble the parts on a bare board tomorrow. Then I will send additional information of how it works.

Kind regards Guido
Edit: with  LDR I mean  the photo cell in the copper amplifier

What do you mean by "LDR"?

There's really only two parts that could go bad in the optical chopper circuit: the photocells or the NE2 neon lamps.

If the cadmium sulfide photocells are bad, you should be able to get replacements. Also, the neon lamps can be subsituted with LEDs which are significantly more reliable.

The newer units eventually used an Intersil chopper stabilized OPA and got rid of the nuvistor input buffer.


With LDR I mean the photo cells inside the chopper amplifier.

LDR -> Light Depending Restor

In my device the Neon bulbs are fine, but not the LDR's / photocells. The nuvistor in the first stage is fine as well, and I got some spare's as well.

It would be interesting, if somebody else has made an update like I planed.


Can you upload some photos of your photocells? I don't think the chopper circuit works at much more than 100Hz. So really any photocell with the correct resistance should work okay.

I attached some photos of an HP photocell I have from a junked piece of gear along with an RCA unit for comparison.

If worst comes to worst, and mine matches your unit's photocells, I could sell you mine for next to nothing. I was going to make a solar tracker with them for fun, but if someone could actually use them to repair some gear, then that's probably a better use.

Or better yet, use Fairchild H11F photo FETs as replacements for both the photocells and neon lamps.


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