As summary : After changing the Input Fet (to often ), the LT1012, two MPS-U60, the LM337L (which was 8V instead of 5V), a poti, a broken 100 R Resistor (which was 6.5kOhm) and all Capacitors (one was bursted), my Electrometer works now as expected. Last step is now calibrating the unit.
My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.
@r6502
When my MPS-U60 transistors was on order, i've used for further testing MPSA92 Transistor as a replacement. Maybe they could work also. I orderd my MPS-U60 Transistors via eBay in Italy.
For the OP conversion I am wondering why you need the super high impedance LMC662 to replace both FETs. The feedback side is not very high impedance and could use a lower noise/drift type with no problem.
For the spike like noise, this could be cosmic or radioactive background radiation, hitting something like protecting diodes. Smaller diodes (less sensitive volume) and maybe radiation shielding could help.
Yes the LMC662 and some other very low bias OPs come as dual. Some, like the LMP7721, LMC6001 also come as singles.
Very interesting, so the defect end stage of the input amplifier seems to be a common problem with this instrument?
I'm not really sure, how to proceed - improving the instrument by replacing the defect part wit newer better components or putting it in the original setup and replace the defect parts with the originals.
My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.
My original broken Input-Fet goes to Richis Lab for a nice Die Shot, so in some weeks it should be at his Homepage.
Richi has already done die shots of my leaky JFET, but it is first gen, would be interesting to see if there are differences to newer gen
one question, my 617 seems to be an earlier device - it does not have the input protection Q311 and the Jumper W303 for (better?) offset trimming of the preamp is missing as well.
In this older instrument the offset trimming then is done only with the trim pot R314 correct?
Does somebody have a schematic for the older version of the device?
Difference between REV J and L is at least added fuse on COM, there is a screw on backplate between COM post and BNC connectors for fuse-holder - so watch out for this if you want at least REV L
Edit: See attached pics.
Interesting: between my REV E and Dave's REV J (oldest known) there is maximum only 1 year in between according to highest datecodes on components (23/84 vs. 28/85)
Overview of known Revisions of EM-Board:
MiDi: E
r6502: F
Alex: G
Smith: G
MadTux: L, G
Dave: J
TurboTom: J
HighVoltage: J, L
_Wim_: K
math_indy: L
baltersice (Marco Reps): L
The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.
The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.
The only problem is 0 current reading takes too long time to recover back from ohm mode. Not sure if it's the DA problem of cheap relay.
What do you consider too long? I always wondered what is the "normal" settling time for this...
Fast settling time at low currents and voltages requires a lot of attention to design and it is easily not apparent that settling time is extended unless a detailed analysis is done to predict what it should be. If noise is high enough, then settling time becomes never.
Dielectric absorption, in more than just capacitors, and thermal effects are particularly troublesome.
Noise, especially flicker noise, and linearity are this way also. More than once I have tracked down a problem after noticing that the last digit of a high resolution measurement flickered more than the predicted noise.