Electronics > Repair

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

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Kleinstein:
The chopper part looks similar to the HP419 Null meter or the Fluke 845 Nullmeter.
The H11F.. Photofets are used in the newer Fluke versions. However they do have some offset and may need mode adjustment. There may also be more bias current, as they do show charge injection.

The LDR are usually special types that are relatively fast. So normal cheap ones for normal light detection usually don't work, as they are slow turning all the way off.

Quite often it is the neons or the excitation circuit that go bad, not so much the LDRs. So one could replace the neons with new ones (not so easy) or change to LEDs.

The demodulation part is less critical. Here it would be no big issue using CMOS switches like 4066, 4053 or the like. So to keep the input the same, one only needs 2 working LDRs.

r6502:
Hello All,

meanwhile I did some progress in repair / replacement of the opto chopper assemby of the 3400a.

I assembled the schematic i posted in the first post on a proto board. I removed the opto chopper assembly,  and placed the proto board there and connected the analogue switches to the cosponsoring pads  of the A5 photochopper board.

see 1st attachment.

To operate the new board I needed a positive voltage. Therefore I modified the power supply board. I removed C711, R701 and R702. In the open holes from R701 / R702 connecting to the contacts 17 and 19 i placed a cap 100µF 50V. The transformer output for the 17,5V was used to generate the positive voltage by adding 2 rectifier side by side to CR711 / CR712. the cathodes have been soldered together  and with help an isolated copper wire connected to the positive terminal of the new cap, see schematic / photo.


With this modification the instrument is working very well now without a readjustment. I checked the lower frequency's against my HP34401a and the higher frequency's against the RMS value of my Rigol scope. All measurements are within the specification of the 3400a.

The next step is, to design a small PCB holding the components and that fits perfect to the position where the opto chopper assembly was positioned on the A5 board. When this is ready, I keep you unformed and will upload the design files.

see last 2 attachments.

Have a nice Time ... Guido
Edit: I updated incorrect photos and corrected spelling ...

r6502:
Hello all,

I'm currently exchanging all electrolytic capacitors.

So I have one more question regarding the 3400a, especial the output state of the video amplifier.

The output of the video amplifier is connected with help of C413 / C415 to the thermocouple input. I marked the caps green in the attached section of the schematic. What I'm wondering about is the use of electrolytic capacitors here. The 3400a is specified to work up to to 10MHz and in that range the electrolytic's are really poor.

Do you think, it makes sense, to use ceramic caps here rather than electrolytic caps?

Thank's for your Answers.

Guido

boblevy4321@sbcglobal.net:
Hello All
I just did a retrofit of the Photo chopper in my hp3400.
I used 4 photo fet isolators H11F1 type. The trick is to use a less than 50% square wave to drive the photo fets. My meter uses a 90 -100 hz chop frequency.
I set my oscillator to 95hz and the led on time to 4.8msec .2 msec off time.
I did this because it was fast, cheap (under $10 USA money), and simple.
Because this modification is several magnitudes better than the original , I had to change the 1/10 scale offset voltage to compensate .
I lifted the ground end of R8 and and added a 5500 ohm resistor and connected the other end to the - 17.5 supply.
when you replace the capacitors ,use high quality low leakage  caps in the signal path.

this modification should work in many of the H.P. photo chopper Instruments
hope this helps someone

Kleinstein:
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.

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