Fluke 8502a repair assistance - Page 1
Forum section: electronics / repair

author: RichardM
author_rank: Frequent Contributor
timestamp: November 20, 2021, 01:35:04 pm
content: Hi All,Just a hobbyist looking for any suggestions.I picked up a cheap Fluke 8502a and after replacing most of the capacitors in the power supply & a voltage regulator I now have the specified voltages and the machine starts without error. The problem I have is that the DC-V LED is non-zero after booting (1167.0 V ?) in the 1000v range and has a different number/offset as I cycle through all the ranged from 100V to 100mv. The front panel responds to all the expected inputs, the sampling light is flashing etc but the values on the display remain unchanged no matter what I put into the input jacks ? The numbers for each range just remain static ? I suspect something on the A/D converter when I follow the troubleshooting outlined in the manual but I am hoping someone out there familiar with the Fluke range of meters may have some suggestions. I have read a couple of threads and it looks like the tantalum caps are a source of problems so I will work my way through them on e by one.Any help much appreciated.RichardP/S After struggling with the Fluke manual for a day I am really beginning to appreciate HP's service manuals !!

author: RichardM
author_rank: Frequent Contributor
timestamp: November 22, 2021, 01:47:43 am
content: Its almost as though (a) the Sram is not being cleared after recalibrating or (b) there is a huge offset that needs to be set using the filter module.This is going to be a huge challenge for a hobbyist. It could of course be related to so many components somewhere.

author: m k
author_rank: Super Contributor
timestamp: November 22, 2021, 09:24:56 pm
content: Make a list of exact values.Include ohms and currents if possible.Maybe there is a pattern.

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2021, 09:28:29 pm
content: Where are you located?

author: RichardM
author_rank: Frequent Contributor
timestamp: November 22, 2021, 09:39:46 pm
content: Newcastle AustraliaI am going to start checking logic chips on the A/D digital board. I thought I had found the smoking gun last night after noticing a missing solder pad on U25 but the board is multilayered and the connections are ok around this chip. Interesting that this is the only sign of a past repair so either I am on to something or someone else concluded a repair was not possible after replacing U25. The A/D board is made of 2 boards, a digital board and an analog board with a lot of rare looking opamp cans. Fingers crossed they are ok, all the input voltages are looking ok at least.

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2021, 09:58:13 pm
content: Quote from: RichardM on November 22, 2021, 09:39:46 pmOK, so I can't supply test parts very easily. But if you get stuck, let me know. I have one of these that I bought for parts for another unit, but it turned out to work just fine. Eventually I'll haul it in and go over it, but in the meantime if you need to compare something I can drag it in and do a measurement or two.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 22, 2021, 11:29:50 pm
content: That's very generous of you. It is a real pain to work on !! You may regret making that offerI would be very interested with measurements on all the pins on J1/J2 of the A/D board. I managed to probe J1/J2 from the underside and apparently the rise and fall pulses need <2us. My falling pulses are definitely 2us but he rise pulses are longer and curved, a bit like a charging capacitor is somewhere. I also do not see pulses on all pins but that may be normal.RegardsRichard

author: bdunham7
author_rank: Super Contributor
timestamp: November 22, 2021, 11:37:45 pm
content: It will be day or so (or possible even after the Thanksgiving weekend as I suspect my days will become busy) but I'll drag it in and have a look, unless someone beats me to it. These 850x meters have a small but dedicated following.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 23, 2021, 12:03:09 am
content: Thats good to hear. I was hoping there were some generous fluke enthusiasts out there. Out of interest, are there and known common failure points for these meters ?Richard

author: bdunham7
author_rank: Super Contributor
timestamp: November 23, 2021, 04:37:02 pm
content: Unfortunately it turns out my unit is an 8500A, not 8502A. Can you tell me the part number on the ADC module (outer shell)? I'll see if any of my units use that same p/n.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 23, 2021, 09:26:09 pm
content: Fast R^2 A/D Converter Module 383984I thought I found the smoking gun last night, U17 is a CA3183 transistor array that had 2 of 5 transistors blown. I tried to match them with discrete transistors with my curve tracer, cut the legs off the bad ones and used the matched discrete ones instead. No change in behaviour, boots without error but unresponsive to input jacks.

author: bdunham7
author_rank: Super Contributor
timestamp: November 23, 2021, 11:13:40 pm
content: Quote from: RichardM on November 23, 2021, 09:26:09 pmMy 8500A has the same p/n ADC.Have you checked the voltage at the input to the ADC? What happens if you select ACV?

author: RichardM
author_rank: Frequent Contributor
timestamp: November 24, 2021, 01:10:56 am
content: I have checked the voltages at the opamps (+-15V) and they look ok. Is there other rails worth checking ? Also the reference is -7V exactly.Regards,

author: Rax
author_rank: Super Contributor
timestamp: November 24, 2021, 01:33:45 am
content: Richard,Provided I get my own 8502A up and running, I will seek to measure voltages you may need (if you still do by then). I have the advantage of an identical unit.You did mention the negative voltage rail was "flaky" - is that the U7 7912?... That's where I saw some concerning measurements on the in-circuit caps.

author: bdunham7
author_rank: Super Contributor
timestamp: November 24, 2021, 02:01:13 am
content: Quote from: RichardM on November 24, 2021, 01:10:56 amNo, I mean tracking the input voltage from the input through the DC signal conditioner and analog filter to the ADC.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 24, 2021, 02:47:37 am
content: ok, I will investigate that path in the circuit(s). Thanks.

author: RichardM
author_rank: Frequent Contributor
timestamp: November 24, 2021, 02:50:28 am
content: Quote from: Rax on November 24, 2021, 01:33:45 amThanks RaxYes, I replaced the 7912 when I noticed lower than expected negative voltages. That seemed to do the trick. My -15V rail is a little low at about -14.87 but I am assuming that is within tolerance.

author: m k
author_rank: Super Contributor
timestamp: November 24, 2021, 10:09:27 am
content: Quote from: bdunham7 on November 23, 2021, 11:13:40 pmSeems to be a backbone.(0,2,5 and 6)What you have for active filter and DC signal conditioner?

author: bdunham7
author_rank: Super Contributor
timestamp: November 24, 2021, 10:42:19 pm
content: AF = 383976DCFilterSignal Conditioner = 383901.My 8505A indeed has the same 383984 ADC. I didn't realize that. Although I'm sure they have many revisions, that seems to imply that they're interchangeable. Hmmm.

author: m k
author_rank: Super Contributor
timestamp: November 25, 2021, 09:36:01 am
content: AF seems to be universal.DC SC in not, there seems to be old for 00 and 02 and new for 05 and 06.

author: asp
author_rank: Regular Contributor
timestamp: November 25, 2021, 06:07:50 pm
content: I did test a bad opto-isolator board in a 8505 and got the same 1167.0V so my 1 cent is on the opto-isolator board. I have no clue yet if it's the left part or the right part yet but if I manage to fix my broken opto-isolator board I will report back

author: RichardM
author_rank: Frequent Contributor
timestamp: November 25, 2021, 10:55:15 pm
content: Thanks. I am not sure if there is an opto isolator on any of my modules ?

author: bdunham7
author_rank: Super Contributor
timestamp: November 25, 2021, 10:58:40 pm
content: They actually aren't "opto", they're inductively coupled isolator boards. It is a module towards the back marked "Isolator/Trigger".

author: asp
author_rank: Regular Contributor
timestamp: November 26, 2021, 06:52:34 am
content: Sorry you are right it's not an opto just an isolator board. This is the boardhttps://i.ebayimg.com/images/g/SPMAAOSwIqlfo2Rp/s-l1600.jpgI have 2 versions/revisions of the same board and the right part is the same and the left part is completely different I will try to take some pictures later on today and post them here with the 2 boards

author: m k
author_rank: Super Contributor
timestamp: November 26, 2021, 08:50:13 am
content: It's clearly a magneto-isolator.

author: asp
author_rank: Regular Contributor
timestamp: November 26, 2021, 06:01:28 pm
content: Here are both boards as promised. Which version do you have?

author: RichardM
author_rank: Frequent Contributor
timestamp: November 30, 2021, 11:44:13 am
content: Does anyone have an idea what would control the rise pulse time for the pulses on J1/J2 of the analog A/D board ? I have gone full circle and I am back looking at the A/D board. The pulses are showing on J1/J2 but the rise time is much greater than the 2us specified. The fall times are 2us. See attached.RegardsRichard

author: m k
author_rank: Super Contributor
timestamp: December 01, 2021, 07:11:54 am
content: What side it the triggering side, both?If all signals are equally slow then it is more likely from the digital side.Analog side has discrete FETs and resistor networks.Digital side has trasistor networks and U17 is special.Thoug, analog side has Q2 and Q3 driven by I-signal.Can you add a small circuit between the cable and its connector?Then you could isolate the load.@aspUpper board has a printed number, low right corner, what is it?First number is clearly 3.

author: asp
author_rank: Regular Contributor
timestamp: December 01, 2021, 03:30:42 pm
content: Top one is 334099 (if I remember it correctly) (also 455832) and the bottom one is 486415

author: RichardM
author_rank: Frequent Contributor
timestamp: December 02, 2021, 12:28:03 pm
content: Could my problem be a RAM issue ? i.e. large zero offset stuck in faulty ram ?I replaced every single logic chip (brute force option) on the A/D digital board without resolving the issue. Still stuck with non-zero values after boot and they return after ever re-start.

author: m k
author_rank: Super Contributor
timestamp: December 02, 2021, 08:12:40 pm
content: Assuming the machine is booting without errors.Every displayed string is first constructed to RAM.So if something is displayed all RAM used controller signals are operational.I have only checked 506_607.bin and it has(clearing ram)Code:0x009C 11 00 40 LXI DE, 0x40000x009F 26 42 MVI H, 0x420x00A1 AF XRA A0x00A2 12 STAX (DE)0x00A3 1C INR E0x00A4 C2 A2 00 JNZ 0x00A20x00A7 14 INR D0x00A8 7A MOV A, D0x00A9 BC CMP H0x00AA C2 A1 00 JNZ 0x00A1If RAM is cleared at the beginning you can generate an error by removing any of them.If RAM is not cleared at the beginning you can possibly take two chips out.Then the machine can still contruct screen strings but everything else is unknown.Then you can also check if less than 3 ships are faulty by swapping them.Manual says that 2111-1 has 0,5us access time.You can easily replace those 4 with a single chip but since you need an adapter board anyway maybe two boards with 8 data lines are easier.

author: asp
author_rank: Regular Contributor
timestamp: December 09, 2021, 10:02:45 am
content: You could also test the ram chips using an arduino mega board or an uno. you can find projects like that on github. I have a project like that if you want I could try to upload it to github as well. it tests both logic chips as well as 2111 chips but you would need a 2.4 lcd with sdcard for it to work(without changing the project). also you can swap the 2111 sram chips between them and see if it makes any difference

author: RichardM
author_rank: Frequent Contributor
timestamp: December 13, 2021, 11:21:53 am
content: OKI am back up and running again after replacing U5 of the active filter with an OPA445.I started checking the DC voltage readings and it appears the readings are off on the 100V and 1000V range (i.e. when input voltage exceeds 20V). Both the 100V and 1000V read the same value and are initially >10V off before drifting down to around about 2 to 5 volts above what they should be reading. Its a little random but that best describes it.I have checked transistors Q3 & Q4 and they are fine. I have no idea how to check K2 (relay) but it appears to click in and out of the ranges as expected.I have an MC1536 coming to try in place of the OPA445 but I would welcome any suggestions on what could be checked. Not sure if there is a specific range adjustment to be made, the manual is a little hard to follow.Regards,Richard

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2021, 11:45:05 am
content: One could check the input current in the 10/20 V range. Too high a current there (e.g. from a blown JFET switch or flux residue or bad JFETs) could also effect the read out of the divider and thus the higher voltage ranges.For the test one can use something like a 10 M resistor or alternative some 10 nF capacitance at the input.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 13, 2021, 01:56:36 pm
content: Thanks for the reply KleinsteinSo do you just mean the current showing on the bench top power supply when it’s connected to the inputs and supplying more than 20V ?Richard

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2021, 06:49:28 pm
content: It is about the current showing up one pos side input, when measureing voltage in high Z mode. This should be less than some 20 pA or so. 1 pA would be 10 µV on a 10 M resistor or a rate of voltage change of 0.1 mV per second. The measurement is not about precision, but the order of magnitude: if something is broken this could be 10s of nA and this would confuser the reading from the divider that is at some 150 K Ohms.The test with the resistor would measure at 0 V. With the capacitor one can check other voltages too - wait for the drift of charge for suitable start.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 13, 2021, 09:40:45 pm
content: Quote from: Kleinstein on December 13, 2021, 06:49:28 pmOk thanks. Could you provide a sketch of the test setup you are declining ?The divider appears to be working fine. I.e the voltage on the bus when in 100V or 1000V range is /8 or /64 respectively. It’s just that the voltage measured is incorrect to begin with and drifting down etc.Thanks againRichard

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2021, 11:43:23 pm
content: I don't hink it needs a schematics for the test: Just have the resistor or capacitor across the input for voltage (in the high Z mode, not the 100 V range) measurement. For the capacitor maybe charge to a voltage you want to start with and than disconnect. Keep in mind that PCV isolation is far from perfect and it may show more leakage that the meter input current.

author: m k
author_rank: Super Contributor
timestamp: December 14, 2021, 05:57:29 pm
content: Quote from: RichardM on December 13, 2021, 09:40:45 pmBus where, RT1?When you select different signal paths you can compare how K1 and K2 are driven, circuits are equal.You can also check what happens over R7 and R8 compared to R31.But since it's only >20V maybe you should concentrate to those dividers.Do you have an Ohms Converter, how is it doing, can you fake it?It uses those same components through RT1.If it operates correctly then you know that the problem is out of its path.Since it's fading maybe you're lucky and the connection is so weak you can disturb it with a scope.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 14, 2021, 09:23:00 pm
content: Quote from: m k on December 14, 2021, 05:57:29 pmMeasurement of multiplier as per manual is between RT2 and RT6 and RT5.I have noticed the reading eventually settles on the correct value but it can take a minute or two regardless of whether its warmed up or not. Flipping between modes (ie. AC volts etc) on the front panel sonetines settles it quicker.Yes I have an Oms module and it is working fine.Richard :

author: m k
author_rank: Super Contributor
timestamp: December 15, 2021, 08:17:44 am
content: o| Ohms ConvertervRT1|K2|--K1--ov| DC Signal ConditionervRT6v| Active FiltervRT5|> A/D Analog

author: RichardM
author_rank: Frequent Contributor
timestamp: December 19, 2021, 11:39:22 pm
content: I think my 100v/1000v DC range issue may be due to relay K2. I took off the two side stickers and gave it a squirt of contact cleaner then dried with a hair dryer and it worked for 2 minutes perfectly. I was about to report my success before turning it off and on again and the issue returned, a bit worse actually. The dc reading is very slow to drift downwards and almost never settles on the correct input value. Any idea on how to best tackle a relay repair or replacement ? Does it sound like the relay?Richard

author: m k
author_rank: Super Contributor
timestamp: December 20, 2021, 09:34:56 am
content: Temperature oriented problems can be incredible nasty.Intermittent ones can go hiding if you stare them too much.So if the cleaner went somewhere else also the problem can be there.Contact cleaners in general are not what they used to be.It feels that you should flush those surfaces, not very practical.Was the thing warm when it was dried?It pretty much seems to be a relay with dirty contacts.Do it again and let it dry in abnormal orientation with vertical contact surfaces.There are nothing special in relays, they just make a good contact or not.Maybe you should try a solid one, at least you can see the difference then.You can also short the old one away and see what happens then.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 21, 2021, 08:32:53 am
content: Dare I post an update in case the unit starts playing up againAnyway, I suspected the K2 relay so I finally got the courage to de-solder it. I then soaked it in a bath of cleaning vinegar before flushing it with contact cleaner, blowing with a hair dryer, and baking it in the hot Aussie sun for 3 hrs. I am glad to report the unit now reports the correct voltages on the 100V and 1000V ranges. It was floating a bit initially but I cycled between the ranges to activate K1 then K2 etc That seemed to have flushed the gremlins from the K2 contacts for now, fingers crossed. Now for a longer burn in test to see if any errors are thrown etcRegardsRichard

author: m k
author_rank: Super Contributor
timestamp: December 21, 2021, 11:32:40 am
content: Hip hip hooray but dang, it's 15 below here.

author: RichardM
author_rank: Frequent Contributor
timestamp: December 21, 2021, 07:41:10 pm
content: Quote from: m k on December 21, 2021, 11:32:40 amBoy that is coldAs expected, after about 40mins the display goes haywire and starts flickering and then throws an error 8 and becoming unresponsive. If you turn it on and off again you get about 2 mins of normal behaviour before it does the same thing. Sounds like a temperature related issue perhaps? The heatsink reaches about 50deg Celsius or so which appears hot but it is a linear power supply so I thought that was within specs? The card that gets the hottest overall doe appear to be the controller card but that has the CPU etcAnyone else have comments on possible temperature issues or thing to check.?This is like a 10 part mini series.Richard

author: SilverSolder
author_rank: Super Contributor
timestamp: December 22, 2021, 02:02:40 am
content: Quote from: RichardM on December 21, 2021, 07:41:10 pmSounds like the controller board is crashing. Reseat the EPROMs as a first stab in the dark?

author: m k
author_rank: Super Contributor
timestamp: December 22, 2021, 10:40:51 am
content: I think 50C is not much.Over 100C and transformer is still happy many times.(maybe not here)How many different problems you can count?How is the machine doing with minimal configuration?DC SC in first slotAFA/DBus IC/IsolatorController in last slot(s)You can also use controller boards without covers and also so that between them are empty slots.But anyway, good power first.Are all leds flickering equally, power led also, is Vcc (TP1) healthy?Leftmost digit and sign can be a bit different, same with decimal points.If storage capacitor can't handle it anymore you should see the change during that 2 min.U18 pins 9 and 10 can be one place to check also.Controller TP2 and TP3 are clocks, maybe you can see some slowing down there.Do not treat all Vcc equal, it's a complex chain, GND also.Maybe you should check the situation more closely when that F-display is present.Check points that are as far as possible from the power supply.Controller board and its upper edge TTL-chip is one good place for 5V, compare it and GND near by, then some GND far away.

author: RichardM
author_rank: Frequent Contributor
timestamp: January 02, 2022, 10:14:09 pm
content: I am confident my repair is now complete. The issue with the overheating and crashing was a shorted tantalum C29 on the controller board. I noticed how hor the -12V regulator was getting and followed it from there. It appears this was also the issue with the floating readings on the 100V and 1000V DC range also and not the K2 relay ?Thanks All.Richard