(Fixed) Keithly 192 Glitchy Front Panel and Reset Cycle - Page 1
Forum section: electronics / repair

author: NathanFulton
author_rank: Contributor
timestamp: April 06, 2022, 02:20:54 am
content: I scored a potentially very nice 6.5 digit meter that has issues. I've already taken it apart and started to do some quick checks with a multimeter but haven't 'scoped it yet.At power-on, the display is a little glitched-out but I can fairly clearly make out the "-1.8.8.8.888" and "F60-C4" self-test sequence mentioned on page 8-5 ofhttps://xdevs.com/doc/Keithley/192/KEITHLEY192.pdfso I'm feeling optimistic. This makes me think that the CPU and ROM are probably fine.It starts to display some other code that I can't make out, maybe complaining that the analog board isn't plugged in (because it isn't). When it was all put together it seemed to be trying to display measurements but they were garbled too.After a few seconds, the display becomes more and more glitched and multiple selector LEDs start to rapidly flicker-on, and then eventually it starts what I think is a self-reset cycle and gets no further.Before the reset-cycle begins, I can press the buttons and the LEDs light up appropriately.At first I thought it might have low/dwindling DC power over time, but I checked the digital +5v rail and it's reading a fairly steady 4.97v - 5.01v. The other analog voltages seem okay as well.The PCB around the LM7805 is darkened from heat so I might just replace it since I have a pile of new ones. The area around the two big rectifier diodes is a little dark too.I measured the axial capacitors in-circuit with my cheapo ESR tester and they seem fine. I might try replacing them with ones I have on hand. I don't have a spare capacitor that matches the huge 11,600uF one across the digital 5v rail but might try a 4700uF just to see. I'm guessing it's such a high uF as to give the digital part some time to shut-down after the power goes out..?I'll check the power rails with a 'scope to see if they're really noisy or something. There's also a table 8-14 in the manual about checking things with the scope so I'll do that too once I'm set up. If anybody has any other ideas please let me know!-----I also went ahead dumped the two ROM chips labeled "PRO-104-C4" (HN462716) and "PRO-105-C4" (HN462732) just in case I screw them up. I'll post them here if anybody needs them in the future... I haven't been able to find them anywhere online to compare what I have. Is there a popular central place to upload ROM dumps for obscure test equipment? Archive.org maybe?Thanks!

author: NathanFulton
author_rank: Contributor
timestamp: April 06, 2022, 03:47:01 am
content: Well I guess this will be a quick thread...I started probing everything listed in table 8-14 and it was all looking good until I got to U131 pin 40 which is the reset line. It starts cycling at about 4hz, so it was a reset cycle as suspected. But why?The next step is to probe the pins for the front panel and... hey... wait a minute. Someone has been here before!What a mess! I wonder if the flux is being a little bit conductive... Cleaned it up and:Well, that appears to have solved that whole problem completely! How freaking lucky is that!?Guess I'll put it back together and start running some tests on it!

author: lowimpedance
author_rank: Super Contributor
timestamp: April 06, 2022, 04:20:21 am
content: So those pins that were re-flowed go to the IDC cable connecting the front display/button PCB and main board ?.I would be more suspicious of the IDC cable and connectors than the flux, (although it was a mess indeed!).Is the DIP header plugged into a socket or soldered direct ?, if in a socket that would add to the possibility of an intermittent connection.So this 'fix' maybe only temporary until enough oxidation of the contacts, either in the IDC part where the cable is crimped in or the socket its plugged into, builds up again.If the issue does return replace any dip socket if used and redo the IDC cable.

author: NathanFulton
author_rank: Contributor
timestamp: April 06, 2022, 04:52:41 am
content: Yeah it looks like they installed individual round female pins where a DIP socket would have gone. It'll definitely be the first place I look if the problem crops up again. So far the rest of the meter is looking really good. Been letting it run for 20 minutes or so and hasn't done anything funny. It's even in really close calibration with my 189. Can't complain!

author: lowimpedance
author_rank: Super Contributor
timestamp: April 06, 2022, 05:34:25 am
content: Nice !, while these vintage Keithley's wont win any beauty contests they are great performers for the price, and no VFD to shag out !.I have a 195,196 and 199 so there must be something that appeals.

author: NathanFulton
author_rank: Contributor
timestamp: April 06, 2022, 05:57:57 pm
content: Quote from: lowimpedance on April 06, 2022, 05:34:25 amAhh very nice collection! I think the red LED Keithley's look pretty great, but I love the looks of retro tech/computers/musical equipment in general. It's my first benchtop meter and I'm really impressed with its performance despite its age.So your prediction about this being a temporary fix was correct. Very slowly, the garbled display started to come back, where certain segments of certain numbers would be wrong and it made 0 = 8, 3 = 9, etc.So I took the front panel back apart and went ahead and removed those individual pins and replaced them with a proper DIP socket. I noticed some of the solder joints on the board were looking a little rough so I went ahead and reflowed all of them while I was at it. I also noticed the one bright red tantalum cap near the connector and pulled it out to test it. It measured 3 ohms ESR so I replaced it with an equivalent electrolytic cap.When I first cleaned up the flux mess, I hit it with hot air to melt it and make it easier to clean. I wonder if heating up the cap right next to it caused it to temporarily become a little better too?Anyway after doing all that it has been running great for several hours!

author: free_electron
author_rank: Super Contributor
timestamp: April 06, 2022, 07:16:48 pm
content: you bet flux is conductive ! especially when it is carbonised like that ...Same for water-soluble flux. That stuff is hygroscopic and saponifies...