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.