GenRad 1403 Standard Air Capacitors

[dzseki]

The other day I was up to my elbows in the company’s e-waste bin, and found these bad guys.

Apparently they are all out of tolerance, measured with a Wayne-Kerr 6505P (1kHz @ 1V):
1pF: 0.997 pF
10pF (#1): 9.97 pF
10pF (#2): 9.96 pF
100pF: 99.86 pF

I have opened up these, in principle they look very simple. Is it possible to adjust these simply by adjusting the plate distance? Also I have noted that in some of these the insulating white plastic is shattered. Are these really sentenced to be thrown away?

[branadic]

Who cares if they don't meet their initial tolerance, unless their true value is known and written on the units?

-branadic-

[jonpaul]

Hello: Guarded three term or just 2 term measurement? Is the Wayne Kerr in CAL?

These are very well designed classics and can be repaired.

A CAL lab can CL them.

If you want to throw them I will take them for the cost of shipping !


PM me if you like

Kind Regards,

Jon

[dzseki]

Hello: Guarded three term or just 2 term measurement? Is the Wayne Kerr in CAL?

2 terminal (floating) measurement were done, the Wayne-Kerr is in CAL, infact we also have a classic HP 4280A (also in CAL), which reads these sligthly beyond limit. The HP is fixed at 1MHz and I know the accuracy has a function with the measurement frequency, but still too high.

At the moment I have no plans to throw them away

[Henrik_V]

While 'stored' outside... these caps are free air caps (not sealed cans like the 1404) .. maybe they need some drying?

Nice found.. Some time ago I harvested some 1404 1nF cans (only the cans) build by Quadtech .. some failed due to whisker .... 

Can you send a pic of the 1pF open? 

[dzseki]

Here are the pictures. From left to right: 1pF, 10pF, 100pF
The 100pF one has the worst plastic spacer, and probably not a coincidence that the CAL rejection sticker also tells "Drifts"
The spacer is cracked also on the 10pF capacitor, but no chip is missing.
I am fairly certain that the 1pF one is in perfect shape, and can be adjusted up to specs by the small screw.

[Conrad Hoffman]

I'd be really curious what you think the materials are. I know the 1404 uses invar for low thermal expansion but no idea about these. There are also thermally reentrant designs where you choose the materials so the thermal effects cancel out but no idea if they did that here. I've thought about building my own, but stamping the sheets is beyond me and individually cutting and drilling is too much work!

[Henrik_V]

I have seen enough 1404 open .. (genuine GR to recent), just by the posted pictures I would say same construction, same material, just the can is missing..

[dzseki]

I'd be really curious what you think the materials are.

Well I am not a materials expert, but the plates look like simple aluminum, and the supporting screws look just like any other galvanized bolts of similar size. The plastic sheet is a little bit strange, because it looks like Teflon, but is much stiffer and certainly much more brittle too. This plate is the base for all the support screws so having this sheet broken will certainly affect the accuracy / stability of the capacitors.

[Vtile]

By colors I suspect the plates are silver plated (polished silver have many times this red/brown tint on it) and bolts again by color and shine do look more inox/stainless.

Edit. By looking again the picture, it seems the reddish tint is propably a sunset or sunshine or equivalent light not metal reflection itself.

[Henrik_V]

the 1pF look like Aluminium, but I guess invar too, the round plates are invar steel. And the genuine GR are polished .. not to mirror but much better than the new ones

[dzseki]

Yes, plates are all aluminum, polished on the higher pF ones and raw on the 1pF one. Note the 1pF cap is the oldest as it has a marking of 1uuF (micro-micro).

The main spacers on the 1pF unit is some sort of (white) plastic.
On the 10pF unit there is none, as you can see the plates are confined between two nuts on the support screws, whether the nuts or the screws have any special behaviour I sure can't tell by the look.
On the 100pF unit there are metal spacer rings, these look dark grey and very well possible they are made of invar.

[PartialDischarge]

So in the 1pF capacitor the direct capacitance comes from the fringe field lines?
Cause all I see is a big grounded plate in between the input plates.

[dzseki]

So in the 1pF capacitor the direct capacitance comes from the fringe field lines?
Cause all I see in a big grounded plate in between the input plates.

Not exactly: there are three plates: the outer two are connected to the coaxial connector hot pole, the middle plate is indeed grounded, but there is a round hole in it too, about 2" dia. So the outer plates see eachother too, the screw is for fine adjustment.

[PartialDischarge]

Not exactly: there are three plates: the outer two are connected to the coaxial connector hot pole, the middle plate is indeed grounded, but there is a round hole in it too, about 2" dia. So the outer plates see eachother too, the screw is for fine adjustment.

Ok, that hole I don't see in the pics. So it is capacitance by straight field lines and the fringe fields is what they are trying to avoid by the bendings in the grounded plate.