Hi guys, sorry in advance if this post becomes a bit too implicit and detailed. I am addressing people who knows about these meters in detail and the questions will be quite specific. I will be making references to specific ICs as seen in the following schematic:
https://xdevs.com/doc/Keithley/2000/K2000.pdfAs some of you know, I am trying to repair a Keithley 2000. Currently, I have the following problem: The meter never initializes. There is never any measurement carried out. The meter just shows "---.---- mVDC" and freezes if I try to change range. Relays are not set to the correct setting and so on. There is something that isn't initialized. I am however able to run the self-test, which fails at 100.1. Luckily I have borrowed a working unit, which I am able to use as a sort of cheat sheet to see what should be going on.
Probing around inside reveals that the ADC circuit doesn't function. In a working unit, the ADC output(pin 6 on U137, page 7 in the schematic) is a triangular wave, which is then zero-cross detected. The zero-cross detection pulses are then fed to the digital circuit and the density(i guess) of these pulses are what produces the actual digital value. At the same time the data bus should constantly be sending out different commands to switch various voltages to the ADC circuit, along with configuration of relays and dividers when changing range and/or measurement. This doesn't happen in my unit, but happens in the working unit. In the broken unit, the data bus basically sends out the same command all the time, at regular intervals, as if it is asking for something but never getting a reply. The initial commands are identical on both units, but the working unit "moves on" and starts setting up the relays and so on. I suspect it is somehow waiting for an ADC result before being able to proceed with actual setup, but I can't verify this. I do however think it sounds plausible given the "repeating the same command" scenario.
From the schematic(page 7) it can be seen that the triangle is generated with two currents, being switch in and out to the ADC. Those voltages are ICB+ and ICB-. My theory is that they flow into(and out of, depending which is engaged) the ADC, causing charge to added to or removed from C171. After all, thats the usual method by which an opamp integrator works.
Looking at the very first, initial data pulses, I get the following result on the working unit:
Ch2 is the data bus, CH3 is pin 1 on U148-A, which is responsible for switching in ICB+ to the ADC, and Ch4 is pin 6 of U137, which is the ADC output to be zero-cross detected. It is clearly seen that the ADC triangular waveform follows pin 1 and produces the correct pattern.
However, on my non-working unit the result is a bit different:
Channels are as above. While the ADC output(Ch4) on the working unit starts dropping at a fixed rate, it can be seen that the faulty ADC output immediately drops to its negative supply rail, but I am unable to pinpoint the cause of this. The data bus(Ch2) is identical in both cases and so are the initial patterns on pin 1(Ch3). However, the duty cycle ends up being different. I am sort of thinking that the control system is limiting the duty cycle in order to try and get the ADC level back up around zero, but I don't know that for sure. One thing to notice, though: Whenever Ch3 is not high, the opamp does start to ramp up. This can be seen by the two small sawtooth-like waveforms right after Ch4 goes to -15V. It is hard to see from the picture due to the limited resolution, but I have verified that this is the case. It also fits with the idea from the working unit, that whenever Ch3 is low, Ch4 raises(this corresponds to ICB- being coupled to the ADC).
This behavior is very weird to me. I was thinking that the cause could be the magnitude of the currents flowing into the ADC(ICB+ and ICB-), so I tested the custom resistor network R271, which is generating the currents(its a voltage applied to a resistor). The custom resistor network works well and nothing is shorted or out of value. Due to the negative feedback at pin 2 of U138 I can not verify ICB+ and ICB- directly, but I do believe that the switching action performed by the XOR gates(U148-A and -B and the U149-A, -B, -C and -D) is correct.
I am puzzled by this and I have no idea where to go next.. I could start looking at the opamps of the ADC, but I don't think that they are to blame. Negative feedback keeps pin 2 of U138 at zero, which sort-of indicates working performance. The two small sawtooth pulses mentioned previously supports this. I have ordered replacements of PA177GS, but it will take some time before they arrive. And, like I said, I don't think that either U138 or U137 are to blame. I have tested other circuits(Q132 and Q133), and I guess they operate as they should, in terms of when they are switched on and off. They work much less frequent than the ICB+ and ICB- switching, so they should have no effect on the generation of the triangular waveform on the adc output.
I have checked all the digital logic from the controller(U165) through to the switches(U149), and the signals are consistent all the way. The thin duty cycle clearly emanates from the controller itself. It is not due to a defect gate or anything. The four switches work inversely of each other, like one would expect, so that isn't the issue.
I welcome all answers and ideas and will happily try whatever I can to try to fix this issue. I can't borrow components from the working unit, though.
Thank you for the read!
Edit: for some reason I wrote Keysight instead of Keithley in the subject. Sorry about that.