Keithley 230 repair help - Page 1
Forum section: electronics / repair

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: September 11, 2020, 04:43:06 pm
content: The problem is that some positive +1.7V offset is constantly present at the output. For example when I set +1V in the output it is actually a +2.7V, same for 100V range. When output is enabled power supply lines -+15 are also shifted to +16.7 and -12.8V. Same happening on negative range. After measuring some voltages I suspect that problem is in U309 op amp which feeds U313 LM339 sense amplifier. It should be 5V in inputs 2,3 but it is +16 and -12V instead. So far do not know what next step should be performed.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2020, 07:52:39 pm
content: U309 should not be a LM339 (4 x comparator), but an OP in a 8 pin case.It could still be worth looking at the outputs of U313 (LM339) that is used to control the JFET switches. There my be one stuck on.The circuit uses several different grounds. One has to be careful which ground is used when measuring voltages.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 21, 2020, 11:25:33 pm
content: check the back side,there is a switch for remote/internal sensecycle it couple times, if still present, try to move it to internal sense and move the toggle switch handle slightly back and forth to see if you can get normal reading.

author: ZhuraYuk
author_rank: Regular Contributor
timestamp: September 26, 2020, 05:36:47 pm
content: Quote from: opa627bm on September 21, 2020, 11:25:33 pmThank you mate! That fixed the problem, after pressing switch a little to the bottom unit works flawlessly.

author: opa627bm
author_rank: Regular Contributor
timestamp: November 03, 2020, 07:15:23 am
content: No Problem, I had to replace mine :DIGI:450-1316-ND

author: DavidAlfa
author_rank: Super Contributor
timestamp: February 28, 2025, 02:18:06 pm
content: Sorry for the necro, I have what I suspect to be a very different problem, but I didn't check the switch yet.- The output has a 17V offset for 20mA limit, 30V for 100mA.- Current limit blinks all the time (No load applied).- Else than the huge drift, the voltage regulation works, setting 0V gives 30V, 100V outputs 70V, -100V does -128V or so.Sometimes I can hear a ~1KHz oscillation somewhere, I didn't identify the signal, I had to stop diagnosing to attend more important tasks.

author: MathWizard
author_rank: Super Contributor
timestamp: March 03, 2025, 05:17:49 am
content: I'm made a model of the 2nd pic in LTSpice, so the main output section and op-amps. And I have the main GND for the low voltage rails, that's clamped to plus or minus 2 diodes drops away from the output. And I have HV rails feed the push-pull, and return current back through the load, to the common node between +/-127V.But so far, I can't get it to do anything useful in LTSpice. I've tried feeding big and small currents into op-amp U315, and I've tried different currents for the special diode Q311, biasing the pull/pull. But so far it's not working.What is the JFET supposed to do ? C315 is pretty small, and so is the gate capacitance. I get what the comparators are doing, and basically what U315 is doing. I'm not sure about U316 yet. But yeah for AC analysis of this stuff, it must be a lot of work.But again, I can;t get the DC model working yet. Have you tried one ? My LTSpice is using non-stock parts, like the LM339, so IDK if you want that or not.Ok I have the model working, IDK what a good choice would be yet, but a 10k or 20k and a diode does fine for the 'special' diode.And about +/-600uA into the diff amp can get about +/-120V across a 10k ohm load for 12mA, which is in the ballpark of it's current rating.I have an old TV transformer, I might someday use for a low power +/-60V HV supply. So I might use something like this design.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 10, 2025, 03:15:26 pm
content: Fixed!Someone plugged it to 220V with the internal switch set to 110V.Else than adamaged SRAM, all the damage went into Q309 and Q312.Working great now