PS: How can I quote from different replies in the same answer? Sorry for double posting but I don't know how to quote without using the "Quote" button that selects all the message.
Changing the relays has been a nerve wrecking experience. I don't know why, but solder almost doesn't "stick" to this board. Solder wick didn't work either so in the end I had to "pull" the relay out... The results are NOT pretty and my mental well being was seriously compromised . Any tips as to why this might be would be very much appreciated since eventually I will have to put the relay back!!! Is this common on old boards like this? Specially the shield pin seemed like it would just not fit. Or I just doing something wrong? The "black" traces are cleaning residue that were properly taken care of after taking the photo.
EDIT: I forgot to mention I've done my first "low current" measurement! Just connecting a 1G resistor to a power supply was able to check the nano amp ranges and the first of the pA ranges. It worked without any issues!! Now I'm looking for a proper triaxial cable and after reading the forums it seems that in order to get a "safe" option paying 120 euros for a Pomona 4725 is the best idea.
Fiddling with the current adjustment can get you down to 0.004pA approximately but after that it gets really fiddly and unstable.
I've taken apart the leaky relay and attached in this post. What exactly is the broken brown epoxy and it's ends?
Ok, so I had same issues with my board, thought it was due to the incontinent cap and the electrolyte residues all over the board.
I used good portion of flux and first put good amount of fresh solder on the joints, that helped to clean the board.
The PCB quality is not at a level I know from other instruments of that era, it is quite sensitive to heat.
Which revision of EM board do you have?
The Pomona are not suitable for sensitive measurements as they are not low (tribuelectric) noise.
I recommend to do the 3 lug mod or just file one pin off the 2 lug.
You can build your own triax cables as I described or just get a good deal on used Keitleys or take new Keysight with alligator clips.
You could measure the resistance of stripped relay, if you are lucky its the sleeve that is responsible for leakage.
If it is still bad, a thorough cleaning and baking could heal it, but chances are quite low.
There could be another leaky relay, which was masked by the really bad one.
You should check for noise, manual states 1.5fA is normal.
For brown expoxy have no clue, it could just be to hold the relay in place, but I guess it serves a bit more than that.
For the read contacts the distance between the coil and the glass does not matter much. The imporant thing is the number of turns and more distance to the glass only makes the turns longer and thus more resistance or a thicker wire needed.
As the coil is still there, one could in theory look for a new contact. Some types are avialable as separate contacts for a relatively low price.
This includes the 1A66 contacts from Meder/Standex (though with not so great leakage specs for the single contracts. Actual performance could still be OK.
TME.eu also has a small coto contact (RI80) with 1 Tohms isolation specs. Reusing the coil one would still have the shield. One would "only" need a kind of low leakage glue.
The Coto 9000 series would be an option too - though a different form factor, but also available with shield.
I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.
I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.
The relay coil looks rather large from the outside diameter, but hard to tell how much of this is isolation / shield.
Separate reed contacts are not very expensive (the coto RI80 is a kind of exception) so a test both for leakage and maybe in the coil would not hurt that much.
I found the coto 9002 with shield at TME for a reasonable price. The datasheet says 1 Tohms limit and 10 Tohms typical, which could be OK.I have a new COTO 9002 here that I need to do a leakage check on anyway - would it be useful to bump it up my to-do queue? Would be using a Keithley 237 so I can go up to 1100V bias if needed.
So I went and checked the COTO 9002 I have, at 100V (same as datasheet resistance spec condition):
Either contact to any other pin (other contact, coil or shield) - all measured <100fA @ 100V
Coil to shield - <200fA @ 100V (pic below is of this measurement and shows 150fA, but the offset drifted a few 10s of fA when i checked before and after, so I'd say <200 to be safe)
Pics show the setup inside and outside the shielded test box. The connection to the SMU (a 236 upgraded to 237) is via a DIY breakout box I made up on the right- this is shielded, connected via triax and maintains the guard all the way to the jacks so should not affect the result (and I checked the zero offset current without the DUT before and after the tests to make sure - i did use the suppression function to null out the initial offset but it was only 2 counts, i.e. -20fA). Note that the triax connection from the breakout box to the test box is in parallel with the jacks on the breakout box - I'll try and write up what I built at some point.
All the measurements suggest an insulation resistance WAY higher than datasheet spec - on the order of a peta-ohm!
With the meter partially working, one could test a few low leakage "Diodes". Besides expensive diodes tested for pA range leakage the candidates are:
1) small BJTs like 2N3904 , BC548 using the BE or BC junction
2) small JFETs (e.g. 2N4117/8 , could also be SMD version)
3) low leakage diodes. A can't remember a THT version, but just a common SMD one: BAV199: usually very low leakage, but not tested for low leakage.
The protection diodes are used with low voltage ( 10 mV range) across, while the usual leakage specs are for a significant voltage (like 20 V or even 100 V). So no need test at such a high votlage, more like some 10 to 100 mV and even than the leakage will likely go down from this.