Keithley 195 stopped working after a lightning strike near my house. - Page 1
Forum section: electronics / repair

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 04:00:42 pm
content: So about 3 months ago I bought a used Keithley 195 for $60, which was a pretty good deal. I was pretty happy with it, but then last week there was a storm and a lightning hit a building nearby, and after that it stopped working. When I turn it on it doesn't show anything on the display, but if I turn it off and then on again in a short amount of time it will light up all the display elements at once.I measured the 5V digital rail and got 4,8V, so power supply should be ok. Now, I noticed that the F6808P processor gets warm when I turn it on, so I thought maybe it was damaged somehow. I bought a replacement on ebay, but when I replaced it, I just got the same behavior.Is it possible that the lightning strike somehow cleared the EPROM, so now the CPU doesn't do anything, because there is nothing to do? If so, is there any way I could get a replacement / the binaries for the EPROM?Any help is appreciated!!!My source for schematics:https://download.tek.com/manual/195_901_01C.pdfSite 113 to 121

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: April 18, 2024, 04:15:31 pm
content: - Have you measured ALL voltages? Page 116.- 4,8V may indicate there is something wrong, it could be a large current dragging the 5V rail down. Disconnect the load if possible and measure again.

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 04:20:53 pm
content: Now, I measured every voltage after I replaced the Processor:5V digital: 5.02V5V analog: 5.00V-15V analog: -14.98V15v analog: 15.01V

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: April 18, 2024, 04:25:51 pm
content: Do a reverse check with the old processor, see if 5V drops again.If it does: That's bad news. In that case, the 5V rail was affected by lightning strike and anything powered by that rail may be damaged.

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 04:36:44 pm
content: It drops to 4.96V, better than 4.8V (Which I am no longer sure about, I did measure it with a cheap DMM because I didn't have access to anything better at the time, now using Fluke 83), but not the same as the 5.02V with the new processor. I think that 6mV difference may be because the F6808P that I ordered is not the exact same as the original one (made in Singapore vs original one made in the Philippines and date code is 8319 vs 8035)

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 04:38:38 pm
content: I can try and read out the EPROM, but I would rather eliminate every other possibility first, since I don't have an EPROM programmer / reader and would have to set it up using an arduino or sth.

author: wasedadoc
author_rank: Super Contributor
timestamp: April 18, 2024, 04:42:04 pm
content: Quote from: jezel123 on April 18, 2024, 04:36:44 pmLooks like your calculator is faulty too.

author: BreakingOhmsLaw
author_rank: Frequent Contributor
timestamp: April 18, 2024, 04:45:04 pm
content: Leave the old one in, I don't think it's dead, and the new one may cause more trouble than good.I do recommend getting a proper programmer like Xgecu t48 (around 50 euro on AliX). It allows you to health check logic chips as well.Do you have access to an oscilloscope?

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 04:45:15 pm
content: 60mV not 6mV, my bad, forgot a 0

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2024, 04:52:18 pm
content: A lower power consumption is possible for the newer chips. 60 mV difference may also be thing from a different temperature.I would not expect EPROMs to be especially sensitive to loosing it content from a glitch. AFAIK they are not especially sensitive.If one has a scope, one could look at the data / adress bus to see if the CPU is running or somewhat stuck. If stuck the clock and reset signal are further points to check.The signal quality at the bus could also show if there is a bus conflict (usually causes intermediate signal levels).

author: jezel123
author_rank: Contributor
timestamp: April 18, 2024, 05:35:22 pm
content: I don't have access to a scope at the moment, but I can borrow a picoscope from school tomorrow. I did however find my LA 1010 logic analyzer and hooked it up to the A0-A7, and it looks like the lines are dead. There is a little something when I turn the Keithley 195 on (330ms mark in the data), but nothing more than that. In the zip you will find .csv and .kvdat with the data I recorded on the logic analyzer if you want to take a look. Sample rate is 32MHz.EDIT: Added png's of the signals

author: Kleinstein
author_rank: Super Contributor
timestamp: April 18, 2024, 07:16:58 pm
content: The long pauses in the signal look odd. This may indicate a problem with the reset or clock.

author: jezel123
author_rank: Contributor
timestamp: April 19, 2024, 05:04:12 pm
content: I got the picoscope and measured the reset and one of the clock lines. Red one is the clock, 4MHz as expected, blue is the reset line, and there is a sawtooth signal. According to the schematic, the IC controlling the reset line should be a ICL8111 (Site 108, U144 in the manual), but the part number on the IC that is actually soldered onto the PCB is ITS6728. I measured the resistors R162 - R164, and got R162 = 15.02k, R163 = 1.000 M, R164 = 15.06k, which is way off, but I am not sure if I can trust the measured values without having to desolder the resistors. Also, R163 looks suspicious, and maybe the last owner replaced it or something (see photo). I also did a diode test on the CR111 and CR112, both have about 0.59V in one direction and 2.85V in the other. R107E reads 2.75k, but, again, not sure if something else in the circuit is affecting the measurement.Any ideas?

author: wasedadoc
author_rank: Super Contributor
timestamp: April 19, 2024, 08:05:42 pm
content: Scope the +5 power, threshold and hysteresis pins of that reset chip.Could be a fault with the power supply unable to supply sufficient current when the processor takes more after coming out of reset. Reset chip sees the voltage fall and resets the processor. Rinse and repeat.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2024, 08:10:22 pm
content: The clock looks OK, but the reset signal looks suspect.Another point to check may be supply if there is significan ripple - too much ripple could cause the reset problem. The ripple before the regulator can give a hint on how good the filter caps are still.

author: jezel123
author_rank: Contributor
timestamp: April 22, 2024, 03:09:02 pm
content: I scoped the 5v power line and the voltage looks clean except for some sub 30mV peak to peak 160kHz ripple. On the screenshot, you can see the threshold (red) and hysteresis (blue) signals.

author: lowimpedance
author_rank: Super Contributor
timestamp: April 23, 2024, 04:00:38 am
content: Contained in the zip is my K195a rom dump , .bin .If you or anyone else needs it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 09, 2025, 07:05:57 am
content: Hey @Jezel123, did you ever find the issue?I have a similar fault, the relay clicks and that's all, voltages are good so I suspect a faulty EEPROM, but I couldn't find any dumps in the net.@lowimpedance I don't think the 195A images are compatible, they're different DMMs.

author: picburner
author_rank: Frequent Contributor
timestamp: April 09, 2025, 08:26:33 am
content: The eproms for the K195 (without A) can be foundhere.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 09, 2025, 02:30:31 pm
content: I had already checked that site, it only lists the EEPROM IC types, but no files?

author: picburner
author_rank: Frequent Contributor
timestamp: April 09, 2025, 03:54:31 pm
content: That's right, my bad, I saw there was a link but I didn't check if it actually led to the eprom dumps.In the "ROM Images" section of that site there are some Keithley but not the K195 without A.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 09, 2025, 09:03:44 pm
content: I checked the schematics and they seem to be identical, except the 195A using a 2764 eeprom vs 2764+2732 in the 195.Is the 195A a cost-optimized version? Maybe the simply optimized the program to fit in a single eeprom?195https://neurophysics.ucsd.edu/Manuals/Keithley/195a%20multimeter%20instructions.pdf195Ahttps://w140.com/tekwiki/images/8/88/Keithley_195A-901-01F.pdfThe strange part:The 195A schematics describe the NVRAM board (Page 131), but it's missing in the available pictures, the NVRAM is placed in the board directly, and this manual is missing the board view...And my 195 has the NVRAM board, the schematics don't mention it, but the board drawin does...I'll try the 195A software... I'm pretty sure there's a problem in the eeproms because I see a very short pattern in the address/data lines, like it's trying to boot, but resetting in a loop.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 24, 2025, 09:07:31 pm
content: Well, it seems the 195A and 195T are identical to the 195, but the A and T use a 16K*8 ROM instead two (8+4).I flashed a new EEPROM with the available 195A dump, nothing changed.Replaced most of the 74xx TTLs, tried a new sram, no change.I find very strange that data/address lines behave differently everytime I power it up... Like it's jumping to random program areas.Some data lines act strange, varying between 5V and 2-3V.I removed every IC except the CPU, installed new rom and ram, same behavior.I thought, maybe it was the bus going Hi-Z, added a 1K pulldown resistor, still sitting at 2-3V.I don't think the CPU went bad, but go figure!

author: aeg
author_rank: Regular Contributor
timestamp: April 24, 2025, 10:44:21 pm
content: Just a thought. I don't know anything about the Keithley 195. Do you have a way to monitor ALL the chip select / output enable lines and correlate with those low-amplitude data line pulses?

author: srb1954
author_rank: Super Contributor
timestamp: April 24, 2025, 11:10:05 pm
content: Quote from: aeg on April 24, 2025, 10:44:21 pmOr just check the chip selects one-by-one until you find the one corresponding to the bad logic levels.Judging by the frequency of accesses I would expect that it is a memory chip access that is causing the problem rather than an I/O chip so I would start with the chip selects to the memory chips.

author: aeg
author_rank: Regular Contributor
timestamp: April 25, 2025, 10:43:37 pm
content: Quote from: srb1954 on April 24, 2025, 11:10:05 pmIt could be one chip with a weak output, or it could be contention. That's why I suggested watching all of them.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 26, 2025, 11:39:02 am
content: As I said, I removed everything conected to the cpu, placed new sram and eeprom, and replaced the TTL ics generating the chip selects.Exactly the same!The chip select signals looked fine, but I'll check again next time I have some spare time.Those repair sessions only happen when I have nothing else to do at work

author: srb1954
author_rank: Super Contributor
timestamp: April 28, 2025, 01:22:35 pm
content: Quote from: DavidAlfa on April 26, 2025, 11:39:02 amDid you replace the two 4503 chips? These also drive the data bus and, if faulty, could load the bus signals down.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 29, 2025, 05:02:41 am
content: I didn't have replacements, so I simply removed them for testing.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 30, 2025, 05:24:05 pm
content: Update: The weird levels were caused by a bent CE pin on the TMS9914, completely invisible as it went under it.But still not working... The data/address lines still hace different patterns everytime device is turned on.Again, replaced the sram, EEPROM, tried removing all peripherals ics, also only one at a time, no wayI don't think this old device deserves this much work...