Keithley 619 Repair- CPU bad? - Page 1
Forum section: electronics / repair

author: joel16
author_rank: Contributor
timestamp: April 02, 2025, 06:01:01 pm
content: Hello all,I have been working on repairing a Keithley 619 electrometer over the past few days and have run to a standstill. I have replaced a handful of shorted tantalum capacitors, the 9.5V supply rectifier (also shorted), and two electrolytics on the electrometer boards that were bad. At some point in the process, I believe after replacing the electrolytics, the display stopped coming on. Measuring on the CPU board there is no clock on the CPU (pins 38 and 39), but the board is receiving power. Powering the board out of the assembly did not work. I removed the CPU, a Motorola 6808, and wired it into the test circuit shown below to run NOPs. It drew a couple of milliamps, but there was no action on the address bus. Testing with the crystal oscillator as on the Keithley schematic did not work as well. I do not understand what would have caused the CPU to go bad, is the test circuit incorrect? I have trouble believing that the CPU went bad without cause, if it is dead than what might the reason be?EDIT- the supply voltages were all good after the display stopped workingTLDR;1. Am I correct in how I am testing the CPU2. What could cause it to die3. Are there any stupid mistakes in my methodologyAny help would be appreciated, thanks.Datasheet for CPU-https://www.jameco.com/Jameco/Products/ProdDS/43502.pdfManual for Keithley-https://chrisgrossman.com/manuals/keithley/Keithley%20619.pdf

author: srb1954
author_rank: Super Contributor
timestamp: April 02, 2025, 11:56:54 pm
content: The 6800 outputs have very limited drive capability in the high state so you are unlikely to get enough current or voltage to drive you indicator LED the way it is connected in your test circuit. Either interpose a buffer e.g. 74LS04 or reconnect the LED/resistor to Vcc. The sink current of the 6802 outputs is 1.6mA which would be enough to drive a sensitive LED provided that the series resistor limits the max sink current to 1.6mA.Do you get any clock output on the E pin? This should have a 1MHz square wave.Trying putting a power-on reset circuit on the RESET pin, The 6802 may need a valid reset pulse before it can start up correctly.

author: joel16
author_rank: Contributor
timestamp: April 03, 2025, 10:44:57 am
content: Repeated with reset circuit, it still did not work. There is no clock on E.

author: srb1954
author_rank: Super Contributor
timestamp: April 03, 2025, 08:41:19 pm
content: The chip is dead.It may have been damaged by an excessively high supply voltage or a negative supply voltage when your rectifier failed short.

author: joel16
author_rank: Contributor
timestamp: April 03, 2025, 11:10:00 pm
content: It was working for a while after the rectifier was replaced, do you think that the rectifier failing could have caused damage that was not immediate?The regulator appears to be working, but perhaps it let a spike through (7805, not sure how fast those are) or is intermittent? I will replace it either way.I will get a replacement and update. Thanks for the help.

author: joel16
author_rank: Contributor
timestamp: April 21, 2025, 11:45:21 am
content: Update: received new CPUs from AliExpress, surprisingly they are working.There are some other issues with the device still, I will update this thread when I get those sorted out.