Recently, I acquired a broken Keithley 236 SMU, which is normally able to source voltages from -110V to +110V, and currents from -100 to +100 mA with ranges down to 1 nA.
The seller stated, that it is not possible to turn it on. He was right on this. The fuse carrier was missing, as well as the fan and all the screws for the top and bottom cover. The good thing is that there were no signs of previous repairs or any kind of abuse, excluding the factory bodges to fix the errors in Rev. D.
After carefully inspecting the power supply section, I spotted an unhealthy looking power resistor.
Fortunately the full service manual with schematics is available on ko4bb.com. This power resistor R73 is used to derive the -15 V rail from the -150 V rail together with the 15V zener diode VR7, which leads to a power dissipation of up to \(P_{max,R73}={(150 V -15 V)^2 \over 15 k\Omega}= 1.215\) W. Due to overheating the resistance of R73 dropped to 67 \(\Omega\), which overloaded VR7, failing also short.
After replacing both parts and the Capacitor C58, which sits right next to R73 on the pcb and was obviously exposed to some thermal stress, the unit turned on without any problems.
The lithium cell which is used to store the user settings during power down, seemed to be fine with 3.1 V but under a 1 k\(\Omega\) load dropped to 2.5 V. The missing screws are unfortunately 4-40 and 6-32 UNC and not metric screws, so I had to order them separately.
The last step was to upgrade the firmware from A6 to A10, thankfully provided on xdevs.com. As I wanted to keep the original Roms, strangely consisting of a UV-EPROM and a OTP-EPROM, I saved a backup and copied the new firmware on a W27C256 and a W27C512 EEPROM from Winbond. Maybe I will also desolder the 2816A calibration EEPROM, to have a backup in case of a failure. Even though the manufacurers claim a data retention time from over 10 years up to 200 years!
Now comes the difficult part, finding some reasonably priced 7078-TRX high voltage triaxial cables and building a shielded measurement fixture.
Repair Costs
| Part | Ref. Des. | Price |
|---|---|---|
| OA80AP-11-1TB | Fan | 15 € |
| 1N5929BRLG | VR7 | 0,40 € |
| LGR2D561MELZ45 | C95,C96 | 10 € |
| RS00515K00FE73 | R73 | 2,50 € |
| INL 33378S | Fan Filter | 3 € |
| MOLEX 9503031 | Fan Connector | 0,10 € |
| MOLEX 8500108 | Crimp Contacts | 0,25 € |
| SCH 311661 | Fuse Carrier | 1 € |
| Total | 32,25 € |