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

[r6502]

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.

Hello Kleinstein,

Singlle supply will be difficult in this design I think, as the output voltage for "Full Scale" is -1V. You could work with + - 2.5V supply, so you will be able to reach the -1V output that is needed to drive the original meter at a current of ca. 3mA.

Guido

[factory]

...snip...

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

The capacitors in the HP IC version of A6 were hermetically sealed wet solid tantalum type Sprague 150D (now Vishay), mine has some Kemet T110 instead. They are not hard to get, but probably more expensive than film caps, or the tantalum beads used in defpom's board.

David

[HalFoster]

To clarify: Sprague 150Ds were tubular solid tantalum capacitors - easily replaced with a modern dipped type with the voltage suitably derated.  True wet electrolyte tantalums (109D/134D/135D/138D etc) are available but a lot more expensive - $50 to over $100 USD each.

For the chopper board here, the capacitors specified by the defpom are pretty much exactly what should be used.

Hal

[HalFoster]

On a related note, one of the biggest errors people new to repairing HP equipment make is to mistake the 150D tantalums for standard aluminum electrolytic capacitors and shotgun replace them along with the aluminum electrolytic capacitors when, in fact, they will pretty much always be good.  The wet electrolyte tantalums can develop leaks over time, with disastrous results, but even that is unusual. The axial aluminum electrolytics (30D, 31D, 39D especially) are almost always going to be dried out and faulty or are heading that way fast - those I automatically replace by default.

Hal

[factory]

Your right I made an error last night, both the Kemet T110 & Sprague/Vishay 150D are hermetically sealed solid tantalum caps.

The 30D is still made by Vishay, check date codes before thinking of replacement, I've had to buy some for another item of HP TE, that I couldn't find an alternate, that wasn't too big to fit in the space available.

David

[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