Keithley 486 - Repair (Need help) - Page 1
Forum section: electronics / repair

author: Sbampato12
author_rank: Regular Contributor
timestamp: August 04, 2017, 06:31:16 pm
content: Hello,I'm trying to repair a Keithley Model 486 Picoammeter.Until now I've found the manual, opened it and start probing.I've isolated a problem (at least one, but it seems to be the 'only' one) to the digital board.Probing the voltages, I've saw a problem in a specific rail, and managed to find a very hot +5V regulator. I've take out the regulator and put a DC Power Supply (with current limit, at the maximum of the VReg), and the Power Supply showed that it is in CC mode, telling me that something is drawing to much power.Then with a termal camera I've found 1 very hot IC (with 10 seconds it get over 100ºC). The problem is: It is a PAL (IC U507 wich is a PALCE16V8H). In the manual, there is a Keithley part number, but I've got in touch with them and they told me that there is no chance to get this part, since it has been discontinued since 90' and since 2008 they do not do service in this model.I've never worked with any PAL circuit, but someone have any clue if it could be possible to get this program in any way?Thank in advance!

author: alm
author_rank: Super Contributor
timestamp: August 05, 2017, 11:55:31 am
content: Cloning might be possible if you had access to a parts / known good unit and the PAL does not have its security bit set (probably unlikely). Alternatives are a procedure likethis(also requires access to a working PAL, possibly from a pal), or trying to reverse engineer its programming from the schematics. Maybe you could compare the schematics to earlier units like the 480 or 485 in the hope that they might have a similar circuit in discrete logic? Is there a theory of operations describing what the PAL does? Is it function obvious from the schematics or circuit around it?Pre-programmed parts are unfortunately parts that are often very hard to source. At least EEPROMs can be easily read and copied.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 05, 2017, 12:01:31 pm
content: I have 487, the difference is that mine has a voltage source... is there a difference in the software, I don't know, I've never even seen 486

author: Sbampato12
author_rank: Regular Contributor
timestamp: August 06, 2017, 02:29:54 pm
content: Quote from: alm on August 05, 2017, 11:55:31 amThank you for the answer,interesting link about PAL, I've checked it, it has an interesting way to solve, but as you said I would need a pal with a PALI've been thinkin about reverse engineering the circuit, but it seems to be a little complex. It has 8 outputs signals (Status, IEEE 488, GCLK, Display, VIA, EEROM, RAM and ROM), I don't think I'm able to replicate these signals without more info.I will try to find 480 or 485 info, to see if this could clarify a bit.

author: Sbampato12
author_rank: Regular Contributor
timestamp: August 06, 2017, 02:33:05 pm
content: Quote from: Ordinaryman1971 on August 05, 2017, 12:01:31 pmOrdinaryman1971, the 487 has the same 'logic board' as the 486. So the PAL should has the same program.Could you do some tests, or try to help with info (I know it is time consuming, so I really understand if you couldn't)?Thank you.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 08, 2017, 01:49:13 am
content: Let me take some pictures of the inside so you can stare and compare. Will post some picts later on today or tomorrow.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: November 30, 2019, 03:27:30 am
content: Well, two years later I’m getting back to the subject of 486. I’ve got so busy with other projects I haven’t even open the meter.And I’ve realized actually I have 486 that has been waiting for its turn on the bench.The problem with mine is lack of reading. The meter zeroes out but there is no meaningful read outs on any of the ranges. Most of the ranges show overload.I was wondering if somebody experienced this problem.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: November 30, 2019, 03:54:42 am
content: I guess this picoammeter I have was overloaded. I’ve bought it long time ago but I’ve never had time to look at it and here we are...I’m suspecting one of the opamps. I see two of them right by the input. One I can’t see clearly, have to take the board out to be able to read it but the other is AD711knAny chance on schematics of the input stage..... is it designed similarly to the electrometer input?

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: November 30, 2019, 03:59:57 am
content: Here are some pictures of board

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: November 30, 2019, 04:00:38 am
content: And closeups.

author: picburner
author_rank: Frequent Contributor
timestamp: November 30, 2019, 03:36:39 pm
content: Wishing you to find the fault soon, could you post the eprom dump?The K486 firmware is one of the few that is not on the net yet.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: December 03, 2019, 04:53:18 am
content: Waiting for some parts, as soon as I have board out.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: December 18, 2019, 08:22:58 pm
content: I've checked and the Eprom says 487 800...

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:20:22 pm
content: Here is a schematic of the analog board for both meters. Still getting basically erroneous readings out of mine... have changed two AD711KN opamps...

author: Sbampato12
author_rank: Regular Contributor
timestamp: March 31, 2020, 07:52:28 pm
content: Thank you Ordinaryman1971 for the photos. Right now this would be a little complicated for me.As I think many of us have a time now, with this corona virus...I entered in contact with Keithley in Brazil, but they said that as this equipment is no longer manufactured since 1997 (or something like that), they no more do any kind of repair, (imagine selling parts...), except for calibration.Then, since august last year I moved from Brazil to Italy, and didn't have time (neither money) to bring my lab yet. Just some small things.But I will search for some photos I've taken that time.But could you dump the memory of that PAL (I mean, have access to a programmer, and the availability for do that)?

author: Sbampato12
author_rank: Regular Contributor
timestamp: March 31, 2020, 07:58:42 pm
content: I would say to check carefully the 'mechanical' parts too. Because any little grease, moisture and things like that can also inflect errors.But as you said, the problem looks like a more big problem.Tomorrow I will download the schematics you've posted and try to check possibles causes.There is some months I didn't opened the forum, unfortunately.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 01, 2020, 03:47:20 pm
content: I've done some testing two days ago... it's been a month since I've looked at it... and I've noticed that the overload is present only when certain relays are engaged. All the switching is done with U145 which is DG411DJ DIP-16 Precision Monolithic Quad SPST CMOS Analog Switch.That would be my next suspect. Unfortunately I don't have any of those... maybe somebody out there has some in stock.I will make a copy of the eprom tonight and post it.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 01, 2020, 05:15:59 pm
content: For quick check one could use other switch chips instead of the DG411. They have a little higher resistance but the main function should be OK with DG211, DG441 or DG444. DG201 could also work.Instead of random switching parts one could do a few more measurements to narrow down the fault.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 02, 2020, 07:05:08 am
content: So, here is firmware, I have C1 on mine.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 02, 2020, 07:12:33 am
content: and here is first meaningful measurement. Well, I know it's unconventional but.... there were two DG411DJs. I've just desoldered them both and swapped them.There is still few ranges that are showing overload but I've proved the point....

author: Sbampato12
author_rank: Regular Contributor
timestamp: April 04, 2020, 01:10:24 pm
content: Nice to hear that!thank you soo much for the dump.

author: Sbampato12
author_rank: Regular Contributor
timestamp: April 07, 2020, 09:56:51 am
content: Ordinaryman1971, can I disturb you a little bit more?Could you dump the PAL16 IC from the digital board?I don't know if you are available to do that (I could argue saying that this could serve as backup even for you, rsrs).If you are, I don't know what is your programmer.There is an adapter that could make this PAL readable with a bunch of programmers in this link (only need some resistors, and a socket for commodity) as a 27C020:http://dreamjam.co.uk/emuviews/readpal.phpI attached the adapter schematic anyways.I've checked and this IC is the same for the models 486 and 487.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 10, 2020, 06:51:58 pm
content: Desoldered the PAL16 and I’m trying to find it in the ic choices on my programmer I have mini pro tl866cs... have a newer one also but didn’t check with the other one.Can’t find it in cs, am I missing something?

author: miken
author_rank: Regular Contributor
timestamp: April 11, 2020, 05:43:00 am
content: I don't know if any modern programmers support PALs "natively". The page Sbampato12 linked to shows a method that requires making an adapter to read the PAL as a 27C020, and uses software to reinterpret the readout into logic.On the schematic it looks like it says 16L8, so hopefully the implementation in the 16V8 is all combinational logic. Another way to read it out is to use a microcontroller to drive the inputs, while reading the outputs. Then the logic can be derived from inspection of the results. I think I may even have some Arduino code somewhere for doing this.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 12, 2020, 03:25:29 am
content: I wonder if maybe somebody made adapters for it.... want to buy one.

author: Sbampato12
author_rank: Regular Contributor
timestamp: April 12, 2020, 11:37:27 am
content: Quote from: miken on April 11, 2020, 05:43:00 amYep, thats right. But even the 16L8 is ok. The problem is with the 'registered' logic (which has flip-flops on the outputs) 16R devices.Being a 16V8 is even better maybe because is easier to program, if i'm not wrong.There is another website, that I would need to search, from an italian group of rom dump, that has an arduino script to do that.

author: Sbampato12
author_rank: Regular Contributor
timestamp: April 12, 2020, 11:42:56 am
content: Quote from: Ordinaryman1971 on April 12, 2020, 03:25:29 amIn fact, is just a couple of resistors, and 2 sockets for the IC. It uses a 'normal' programmer to set all the possible inputs, and read all the outputs.Then build a truth table of the IC.After, the guy did a software that generates the equations for the PAL compilation from that table.The good news is that the TL866CS should work nice for this. From that website there is a table that show this programmer works in this case.

author: ni.st
author_rank: Newbie
timestamp: April 25, 2023, 05:25:29 pm
content: I know this topic is quite old, but were you guys sucessfull in reading out the PAL or reverse engineering the logic?I'm asking because I also have a 487 with defective U507

author: Sbampato12
author_rank: Regular Contributor
timestamp: May 23, 2023, 10:23:23 pm
content: Lets hope some news appears. I kept the 486

author: Kleinstein
author_rank: Super Contributor
timestamp: May 24, 2023, 07:16:26 am
content: The PAL is mainly doing the address decoding. With the informations from the manual it should be possible to conclude what programming is needed. Most of it is standard interfacing of the CPU to RAM / ROM and IO chips. So with a little help of the 68xx CPU manuals one should get most of it relatively easy. Only the display part with the shift done it a bit odd and may need a little guess-work.