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 Post subject: Why is this transformer configuration used here?
PostPosted: Dec Mon 02, 2019 7:57 pm 
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Location: Leesburg, Virginia
Power transformer in a General Resistance Inc., 'Dial-A-Volt' model DAV46E. Instrument supposed to dial down to uV. What characteristics made for the selection of this open frame with primary and secondary separated desirable?

The complete unit is housed in the shell on the left. Thermal stability? Primary on the left 120V in via white/black pair. Secondary on the right the two red leads feed AC via the 150 ohm R to a Motorola rectifier chip (can use see the edge). The secondary has a pigtail sticking out; it is a solder connection between a yellow and green leads. Temp sensor? Output is 60V dc on the orange/black leads.

Must be a reason to pick this configuration?


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Mon 02, 2019 10:36 pm 
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W0MPM wrote:
Power transformer in a General Resistance Inc., 'Dial-A-Volt' model DAV46E. Instrument supposed to dial down to uV. What characteristics made for the selection of this open frame with primary and secondary separated desirable?

That transformer has very low capacitive coupling between the primary and the secondary. Why that's important for the instrument I don't know. Perhaps the idea is to have the output floating, to avoid ground loops that might introduce enough 60 Hz hum to prevent accurate measurement of the very small DC voltage differences provided by the instrument?

I guess it's an ovenized zener in that silver box. That's a nice instrument, does it use a Kelvin Varley divider? Are the resistors in an oil-bath? Six digits of accuracy is not easy to get.

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 2:43 am 
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Location: Leesburg, Virginia
Thanks Steve. In fact the 1N3029B 24V zener lives on the pcb next to the transformer enclosure. But they did go to a lot of effort (well about 4 extra .005% resistors) to elevate the 20V supply to the Kelvin Varley dividers above ground. So your explanation makes sense. Noise. Everything is shielded. There is a shield box goes over the Kelvin Varley set.

I have it doing good down to the 4th digit. Simply an exercise in learning. Not inclined to do more. Replacing all the caps both raised the output closer to the thumbwheel indication and reduced noise. But there is still too much noise/drift to justify expecting the 6th digit. Used an borrowed HP-34465A with data collection graphing all the good stuff to watch the output. Makes me want to have a spare $1000 to spend. ;-)

Stumbled on something else too. After taking the pictures and blowing them up I saw on the power supply a jumper from the top post to the fourth and it was right up against the second. It's an insulated jumper. But the insulation was hard cracked and the usual corrosion permeated between the second post and the wire. Stretched that wire away from the second post and then scraped all the rosin residue off that and some other places. Measured voltages on the power supply all shifted about 1 1/2 volt! Reduced the error from dial reading about 50uV.


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 2:16 pm 
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Interesting stuff. At 6 digits, even dirt on wires can throw things off. I wonder if it was ever actually that good when it was new? Do you know what the actual +/- accuracy spec was?

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 7:12 pm 
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Barry H Bennett wrote:
Interesting stuff. At 6 digits, even dirt on wires can throw things off. I wonder if it was ever actually that good when it was new? Do you know what the actual +/- accuracy spec was?


Nope. This is one of those boxes which even the vast reservoir of useless internet information fails to explain. I see some second hand listings but no data. General Resistance Inc seems to have built lots of these kinds of boxes. Some of the later versions show up but the single schematic I found was a much different newer topology.

Like you I'm somewhat skeptical it was ever useful at 1uV.

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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Tue 03, 2019 10:35 pm 
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Joined: Feb Sun 17, 2008 11:36 pm
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Location: Dayton, Ohio
It is not unusual to find such devices that have resolution that exceeds their accuracy. Sometimes for nulling purposes.

For devices that are expected to be accurate down in the micro-volt region, will have special switches, contacts, terminals and circuit insulation.

Charlie


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 Post subject: Re: Why is this transformer configuration used here?
PostPosted: Dec Wed 04, 2019 7:08 pm 
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In other words, it has precision, but not accuracy. :wink:

-Steve

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