Keithley 616 repair  - Page 1
Forum section: electronics / repair

author: Ahax
author_rank: Newbie
timestamp: September 07, 2025, 11:20:08 am
content: HelloI recently got a broken keithley 616. No mater what I do it always shows 001,7 on the display. Except when swiching range to ohms it sometimes shows something else. Zero check also shows the same, and I cant correct it with the potentiometer. I'm guessing the display drivers are fine. when I cleaned the swich it began measuring on the lowest ohms range but it was way off. Nothing else works. Voltages seem fine except the Q105/106 output when I put it in low ohms range and swich to normal mode. Then the output is 280V (supposed to be +-200V). But on no other ranges do they deliver such high voltage, so I'm guessing they could be fine. The input amlefier is not shorted but I don't know how to test it another way. The instrument was working before it went into storage about 10 years ago. Any Ideas?Ahax

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2025, 11:23:48 am
content: voltage output above the max can mean there is some shorted pass element. Are the rails 280?I got severe stress just thinking about working on that instrument, I dismantled mine and I am still not yet regretting it.seriously i feel like stress in my forearms thinking back to that thingTypically for a voltage circuit, you have a rail voltage, some dropout, then your span. It could either be stuck at max, or just shorted.

author: coppercone2
author_rank: Super Contributor
timestamp: September 07, 2025, 11:29:23 am
content: Also check that voltage for ripple. It says the unregulated voltage is 275V with 2Vpp ripple. I don't know how much dropout the circuit has, but 280 sounds too high to be functioning properly.?So compare the rail voltage to the amplifier voltage when its stuck to see if it makes sense that its even going through something that causes dropout . I just mean to subtract the output voltage from the rail voltage and see what the difference is, to check for a dead shortand check for AC ripple on the voltage rails. The 275v rail is 2Vpp

author: Ahax
author_rank: Newbie
timestamp: September 07, 2025, 12:40:45 pm
content: Okay, so upon further inspection I have -+295V on the 275V rails. Riple is 2Vpp. Big problem is I get 295V at the amplefier board! yikes!!! yet its interesting that no transistors/diodes or capacitors are shorted. I continuity checked them all. The amplefier also seems to be putting out different values yet the display shows the same thing. As for the transistor output. It does seem to change, regarding what mode and range I set the instrument to.Any further sugestions?

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2025, 01:11:39 am
content: Well its unregulated, so you can have higher voltage with higher line I guess @ 295V.IDK what to say, because I renember all the parts being some keithley PNI think it might be possible to bypass that little amplifier board near all that high Z shit on the big switch, and feed probe signals into the machine for troubleshooting. Determining if the problem is on some crazy dead-bug shit or the PCB would be helpful. I think it was some sort of high impedance amplifier at the input on its own little card connected to the big resistor network on the switch. It connects to the PCB. If you can figure out what the signals that are expected from that module are, then you can simulate them and see if the machine responds correctly.

author: MathWizard
author_rank: Super Contributor
timestamp: September 09, 2025, 02:20:45 am
content: You can try modelling some of in a simulator like LTSpice.Does it have an ADC, is it working ? Hopefully it's not the ADC or MCU. Have you checked the data getting sent to the LCD ? Maybe it's frozen, but yeah you get some changes so that's probably ok.https://download.tek.com/manual/28039I(Model616).pdf