[Repair log] Another faulty Keithley 2000

[Jens01]

Hi guys,

Given the enormous source of information about the K2000 already available on this forum and on TiN's website, i decided to take a gamble and bought a broken unit which was available for a reasonable price on the local ebay. The seller noticed that the unit boots fine but showed 'Ovrflw' on all ranges. Because the unit is optical in a good condition i can resell it for parts when the unit is unrepairable, so there is nothing to lose.

The display is very nice given it's age ('97!), the whole front panel is in an good shape except the yellow tint on the front panel. The unit runs Firmware A13 and boots without start-up errors. There is a sticker on the front indicating a calibration in february 2001; i really hoped this was not the last calibration done on this unit, but the calibration menu noted 12-01-2001 as date of last calibration. Bummer



The unit switches ranges but displays overflow on every range. Starting the self-test resulted in every possible error so there is clearly something wrong with this unit. The display and front buttons self-test completes without problems. Next step: opening the unit. 



After opening it up it's clearly visible why it isn't working 

U165, The ADC-ASIC is missing. As there are pen markings applied on the board near the optocouplers (including the '?' ) i suspect the previous owner already did an repair attempt but sold the unit before finishing the job. The rest of the board is very clean; there are no traces of electrolyte or dust on the board. Further optical inspection did not showed any sign of broken components, burned traces, damaged packages or any other sort of known problems. I will replace the big cap's when the board is repaired, but leave it as-is for now.

The obvious next step would be contacting Keithley/Tektronix and buy a replacement for U165, but first i decided to check as much as possible other things, like the power supplies:

+5VD	5.0115V
+37V	36.462V
+5V	5.0079V
+15V	+15.887V
-15V	-15.646V

The +-15 are a both bit high but as the schematic shows these voltages are lifted a bit by design so nothing to worry about. On the digital side i verified if the RS232 port was working and measured the logic signals between the main-cpu and the optocouplers, which were also fine. If there is something more broken, i would not suspect it to be in the digital part of the unit.
The analog part is a bit more tricky, because the missing ASIC controls a big part of the circuit. I decided to do one last step before buying the replacement part: check the +-14V and +7V supplies.


(source: http://dev.xdevs.com/attachments/download/672/K2000.pdf)

On startup the Vref rail is not active and meauring it gives less then 1V, so it seems like there is some action needed to activate the reference voltage. The Vref (+6.95V) zener reference is generated by clamping a resistor connected to the +14V rail, and the +14V is generated by multiplying the zener reference..  My guess would be that the +ICB signal is used to start the reference into a stable state, but i am not really sure. For now i bridged R315 with a 2K resistor and the reference and +14V/-14V circuit seemed to work OK:

+Vref	6.9447V
+14V	13.8434V
-14V	-13.8027V
+7V	6.9446V
+13.3V	13.1536V

although the +-14V rails are not completely accurate i think the circuitry is working fine.
There are other supplies around the board:

+V U123

17.32VV

Lowering the unregulated +20V is needed for not damaging the op-amp U123, rated at max. +-18V. The measured value seems low enough.

There is another local supply for U113, which is clamped to max. 6V by two diodes. This circuit is also working fine;

+-V U113

5.128V

Next step: order a replacement U165. To be continued!

[klaus11]

Testing Caddok resistor, possible short between shallow surface rails.