Products > Test Equipment

HP 735A DC Transfer Standard

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ArthurDent:
Looks like I have the oldest 735A shown so far. Going by the serial numbers you can estimate how many of these HP units were made per year.  You can also see changes in the power supply board, the counter dial, and the oven board. It looks like it was a work in progress.

LazyJack:

--- Quote from: jasonbrent on February 14, 2018, 05:56:30 pm ---These must be reasonably rare on fleabay. I didn't know they existed... would be a cool addition.

--- End quote ---
Yeah, they seem to be quite rare. Ever since I got mine on a local auction site (for an embarrassingly low price), I was looking for an other one to have a comparison unit, I have seen maybe a couple for sale, either non working or for some crazy price.
 

LazyJack:
Interesting to see that the chart scale was halved during the years. Shows that as they achieved better stability, it was not expected to go offscale on a tighter tolerance.

However, I do not understand how your unit can start from -21ppm deviation. Don't they set the deviation to 0 initially? Or was it not possible on the older models to set to this precision?

ArthurDent:
LazyJack, I'm not sure if HP made great improvements in the 735A over the years to make the aging chart look better by half, or they just got better at selecting the zener so the spread was tighter, or marketing thought the +/-25 ppm chart looked better.  I assume they assembled the reference ovens to output about 5.9 VDC and if this output value was within that +/-50ppm range they could use the adjustments to trim the outputs; outside that range and you wouldn't have enough range on the adjustments to bring the outputs in. Other than selecting 2 series resistors before the oven was assembled, there are no adjustments on the oven. The main concern with the oven assembly was stability. The output series resistors in my oven are 200 ohms and 75 ohms.

My 735A oven output may have been 21ppm under the 5.9 VDC and they graphed the stability that indicated initially it was 21ppm low but still very stable around that value and within the range they could correct for with the adjustments. The later charts possibly ignored the initial absolute ppm value from the reference and just plotted the ppm difference from the initial value. Obviously the absolute value of the oven output is of no concern as long as it can be corrected for with the adjustments or my 735A would have never made it out the door.

One problem I had with my older oven and they may have made changes in the newer ovens was that there appeared to be corrosion where the leads were soldered into the holes on the oven board and the slightest touch would cause them to break off. I ended cutting a 1/4" off the ends of the teflon wires and stripping them to get to solid metal and resoldered all the oven leads. Here is a photo of my oven board that looks different than a previous posted photo of another oven board.   

LazyJack:
Yeah, your explanation on the initial deviation does make sense indeed.

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