Welcome to forums. K2002 is my second favourite meter .
Today I checked the base level noise and got 4uV. This did not improve after an hour of a warm up.
I used a copper strand with Q-tip to short the inputs. This seems too high.
I checked the noise and so far it seems around +/- .5uV and the offset is around -4.2mV even after warmup.
I am still trying to troubleshoot 1.98uA source and currently suspecting problems with either Q244, Q242 or CR217. Do you know what parts they are? Here is a photo of one of them.
Also here is a thermal shot of the meter after warmup. Surprisingly ADC is very hot as well as area around 3 electrolytics by display board (they themselves are cold - cold horizontal line in the lower right corner)
Have you checked input leakage / bias in voltage mode ?
A jumpy test current could also be due to leakage at the input. The ohms current source is reasonably well protected and thus not that likely to show a defect. Input protection and switching is much more exposed to things like ESD.
did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
are the current jumps still noticeable when measuring a resistor in non-auto-ranging mode?
it's a siliconix J2611 JFET.
note colour dots on top. these are factory-selected and marked, so it might be better to not try and change/remove them, until you are absolutely sure that they've gone south.
Measuring leakage / input current is relatively easy. There are two options:
1) use a 10 M or similar resistor across the inputs in the volts (e.g. 2 V) range.
2) have a low leakage capacitor (e.g. 1 nF) across the inputs an watch is charging / discharging.
did you try to slightly(!) knock with a popsickle stick on the board? can that provoke a jumpy ohms reading?
The 9.6 µA and 2 µA current source seem to use many of the same parts. So one might exclude those used for the 9.6 µA current source. The change from 9.6 µA to 2 µA seems to be changing the reference voltages - thus not in the super higher impedance part. My suspect would be more like U232.
hmm, after another look at your photos, i now see that it is not the transformer that caused the brown spot on the pcb, it's one of the two push-pull transistors on the primary side.
given the age of the instrument and the fact that the supply of the display is still giving the right voltages for filament and anode, you probably shouldn't be too worried about it.
the only thing that you could do is to exchange the transistors with some better ones. don't know from top of my head whether these are BJTs or MOSFETs, have a look at the KEI2001 schematics (VFD supply should be same as in KEI2002, but is located on digital board) on TiN's site to find out.
Now that I am on the way of fixing the 2MOhm range and waiting for parts, I am looking for the reason for overheating front panel.
I took an infrared image of the front panel after the meter was on for a while and it shows very noticeable overheating in the same area where the board is discolored. See attached IR photo.
I do not have a high resolution thermal imager, so the photos are somewhat blurry. Nonetheless I can see that the VFD transformer gets hot, but only to 60C; At the same time 5 transistors in the vicinity and 4 somewhat large 1/4W resistors get to about 80C. This is with the panel open i.e. some ventilation. The temperature is probably much worse when panel is closed. In overall thermal shot of the front panel I can see the temperature bleeding into area of banana terminals and can attribute to EMF voltages.
Can someone please take a thermal photo of the front panel of K2002 after it had been running for a whole? No opening of the unit required. Thanks.
Here it is, after approx 1 hour of warm up.
Ambient temp 24C
Just cut legs off. No heat, no contamination.