As an addition to the information already shared here on the forum, here my $0.20.
Bought an defect 2400, bought as broken/for parts. Unit came from a automated test set-up so visually in good condition. Symptoms:
- -240V on banana jacks when output was disabled
- When enabled, the current reading was always off by 9.8mA. When sourcing 9.8mA, the current measured with the output shorted with a DMM in current range was zero. The leaking current was independent of the selected current range.
- The voltage reading was always wrong. Also, when sourcing 50V, the output went up to +240V.
There are quitte some usable test pads on the board. My 2400 differs quite a bit from the photo's from TiN, but here are some hints based on the testpoint labels on TiNs 2400:
- TP201 'NVDAC' and TP203 'NIDAC' referenced against FCOM: respectively DAC voltage and current setpoint. Range dependent, but should linear change when giving a new setpoint.
- TP214 ' EAMP' referenced against FCOM: difference of voltage between pin 2 and 3 of U500. Zero volt means that the setpoint voltage matches the feedback voltage, indicating that the amplifier stage is probably working OK. When reading other than zero volt: output stage clips or is defect.
- TP221 '+si' referenced to TP222 '-si': feedback current. Range dependent, should linear rise with measured output current.
- TP227 '+sv' referenced to TP228 '-sv' feedback voltage. Rang dependent, should linear rise with measured output voltage.
I started with reverse-engineering the output stage, as they seem to fail very often and reverse engineering is easily done as the components are mostly on the top side. The list of designators in K2400 service manual did not match the numbers on the PCB, but it did match with the designator list in the K2410 service manual. Obviously the high-voltage part will differ, but the < 30V part seemed to match well.
Eventually i found out that my unit had a fully working output stage. if i knew about that EAMP testpad before i started..
The setpoint dacs seemed to work so the feedback path was the next step to debug. The current feedback path is located near the blue resistor on the front side of the PCB and the voltage feedback path is located next to the black relay on the right side.
This component, Q233 on TiNs photo, became very hot when driving the output to a high voltage. As the voltage reading was always wrong, i eventually found out that op-amp U219 (OPA124) had a large voltage difference between the inverting and non-inverting input. As this op-amp is a simple high-impedance voltage follower, i tested it out of circuit to make sure the output was not externally clamped. With success, the output was stuck at -15V
Replaced the opamp and now she's fine again.