Products > Test Equipment

HP 735A DC Transfer Standard

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Vgkid:
interesting that your 735 uses a lm723, I believe that the 10544 series oscillators used one in their oven controller. here is the schematic for the 105 oven.

LazyJack:
Let me resurrect this thread a bit.
I recently scored a nice 735A, and the only problem was the oven lamp bulb was burnt out. Since i could not really find a good replacement, I went the LED way. However, simply a LED with a series resistor won't work, as the LED has a much smaller current and the oven activity is not really noticeable. So the solution was a bright orange LED, a 13V Zener and a 10k resistor. The bulb was smashed, and all these were soldered into the original lamp head. So, no changes to the instrument itself. The lamp nicely fits the holder and the orange LED gives a really fancy glow. With the above component values oven status is very well visible.

pelule:
I got my HP735A in 2015 for just 15 EUR + 6 EUR shipping as defect/for parts only. I thought, for that price give em a try.
When it arrived, it was in a near excellent condition, just need some repair in the power supply (elocetrolyte), a new oven lamp and some calibration.

Mine was build in the 70s and the drift chart shows a much lower drift, seems HP improved the design.

In my experiance, it was quite easy to get all required parts from Farnell (see order codes on pictures).
As I was afraid regarding a potential lift-of of the solder pads on such an old PCB.
So I cut the wire and soldered the new part at the cutted.

I measuring the drift rate since that time, it is arround -2.2ppm/y (+/-1ppm with calibrated HP3458A)

Meanwhile I have added a 10V range (using a switch at the rear to switch between normal voltage output and 10V output.
Its actual in burn in - see chart

LazyJack:
Yes, they have likely improved the design. Here is the chart of my instrument from 1974.

blackdog:
Hi!

My HP735A is still going strong  :)

The drift with "normal" LAB temperature changes is less than 1PPM.
More than a week now the display shows this...


Its a nice little tranfer standard.

Kind regards,
Blackdog

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