Fluke 8600A - carbon resistors and tantalum caps, should they be changed? - Page 1
Forum section: electronics / repair

author: kblue
author_rank: Regular Contributor
timestamp: October 17, 2024, 12:13:38 am
content: Hello there!I've got another Fluke 8600A with a bit NiCd (Battery Pack Option) corrosion damage. Nothing to worry about (i hope). The pack is already removed and while i'm working on this unit, i wonder ... should i change the carbon resistors? Most of them read 15% high (with gold ring it should be 5%) or leave them be? Same question for the "ticks" (blue and orange tantalum caps). Some of them read "okay'ish" some have high'ish ESR compared to "modern new caps".The unit seems to "work" so far, but not quite right due to bad connections - already working on it.Best wishes and thanks in advance!/edit: Everything runs fine now ... tested some functions.5 VDC (Ref) = 5,0005 VAC (Ref) = 5,0001 mADC = 1,00001 mAAC = 1,0003100 Ohm precision resistor = 100,00 OhmOnly odd thing is, fresh from start every "dot" between numbers (except the one used for the actual range) is blinking. So its like ,5,0,0,0, Over time, while the unit is running, the dots not used for the selected range slooooooooowly fade away but never gone completely. Gives me the feeling of "bad cap" due to "things fading while something is running". Dunno.

author: blue_lateral
author_rank: Contributor
timestamp: October 17, 2024, 02:32:38 am
content: If it were me, since it is a piece of test gear, I would replace ANYTHING that is way out of spec like that. Are you sure? I don't think I have ever run into any single piece of equipment that had that many carbon resistors that far off. Since it's test gear, use something more stable.I would be more worried about the battery contamination on the board and especially in the switches. A little deoxit in the switches completely disabled one of my flukes years ago thanks to the high Z. It was conductive enough to be a big deal and it was hell getting it back out. It took lots of isopropyl.I suspect others will disagree, but it wouldn't take much to convince me to shotgun a bunch of tantalums in something that old. I suspect tantalums were poorly understood in years past. Engineers believed the datasheet and sometimes it did not go well. They fail short. If you aren't having that problem you can consider leaving them alone. If you replace, it is a good idea use higher working voltage.It's probably going to need calibration after a rebuild that invasive.

author: TERRA Operative
author_rank: Super Contributor
timestamp: October 17, 2024, 05:09:49 am
content: A 4.5 digit multimeter? I'd just replace the resistors with good quality metal films. It'll make the meter much more temperature stable for one.There is a chance that the temperature coefficient of some resistors was used to counteract the temperature coefficient of some other parts, but for 4.5 digits, it's not that critical and carbon composite resistors vary wildly in their tempco characteristics.As for the tantalums, if they aren't shorted, just leave them be, unless you really wanted to uprate their voltage ratings.I say go for it with replacing the resistors then recalibrate the instrument.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2024, 10:01:11 am
content: The carbon resistors should be in the non critical parts, like setting the display brightness. The actual result should not depend much on carbon resistors.So one could leave the resistors if they don't do harm if the drift further. At the supply regulation better replace them.For the tantalums, there is a difference between metal case ones and drop types. The wet types (metal case) are usually very reliable and no need to change them, unless there is corrosion damage.The plastic drop ones may fail short and if used with a powerfull (> 1-2A) supply they should be changed, as than a nasty failure mode with fire is possible.For the replacement "normal" low ESR capacitors can often be used, unless the meter also needs to operate below freeezing.

author: kblue
author_rank: Regular Contributor
timestamp: October 17, 2024, 10:09:55 am
content: The carbon resistors are somewhere in the power line, the battery power board and stuff, true. So far, they don't do harm - just reading 10-15% or more high (22 ohms are 30 ohms and so on). Thanks for the reply, tips and tricks!So far everything works fine, except for the decimal dots between numbers. With fresh start all of them are lit and while the unit is running, the "not used dots" fade sloooowly away but will never be off completely. The 8600 is just powered from my bench power supply and by accident, i noticed without the "battery power board" (missed to plug it back in) the decimal dots are as they should be while selecting ranges. But the instrument wont work correctly without it. Putting the board back in, the instrument works fine except for the decimal dots.Also this "battery power board" while powered from my bench psu via battery terminals is making somewhat of a high pitch noise (exactly 9000 Hz and 18000 Hz together).I could live with the "decimal dot problem" no worries, its just a bit odd. While fading, they do "pulse" with the actual measuring.Best wishes!/edit: The high pitch noise is comming from "T2" of the "battery power supply board". In the documents T2 is a "xfmr inverter"./edit2: In the original schematics, i see U202 and U203 voltage regulators. My battery power board has a single chip solution with a RC4195N. Heating RC4195N up with some hot air, makes the wrongly active decimal dots fade away faster and cooling it down, makes them brighter again. I wonder, if i can replace this part with "usual regulators" for + / - 15 V/edit3: Checked the +15 and -15 VDC from the RC4195N ... ripple @+15V is about 180 mVpp @-15V its about 500 mVpp. Goes down while heating up with a bit of hot air./edit4: Checked the RC4195N out of circuit and it seems fine (but this was just a simple test without load and stuff)

author: kblue
author_rank: Regular Contributor
timestamp: October 18, 2024, 04:21:51 pm
content: Ordered new +15 and -15 voltage regulators, the usual stuff after my observation (see all the edits in my posting before). All the additional pins of the +/-15V IC (2x comp, 1x balance) aren't used. The IC without any load is kinda noise-free, but as soon, as i put it back in (with socket) its lot of noise and it only goes away, if a bit of hot air hits the IC. Will report back, as soon as i have my parts.Best wishes!

author: kblue
author_rank: Regular Contributor
timestamp: October 19, 2024, 10:06:09 am
content: The board i have is a tad different from the official schematics with its IC-solution for +/- 15V. It seems, i need to make some physical changes. A quick question: In the schematics "Pin 2" of the +15 and -15 VDC regulators are the "output pins" but in the datasheet its for both always Pin 3 (7815 and 7915) and Pin 2 for the 7815 is GND and for the 7915 its In(put).Do i read something wrong in the schematics?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2024, 01:07:37 pm
content: Not all refgulators have the same pinout. E.g. the L4950 is one example with ground at pint 3 and output from pin 2.Excessive ripple could also come from to much ripple before the regulator due to a bad capacitor. It is rare that a regulator still gets the voltage abpout right but gives higher than normal ripple. The defects are more like no voltage at all or too much.

author: kblue
author_rank: Regular Contributor
timestamp: October 19, 2024, 02:14:01 pm
content: The caps before are already changed with quality panasonic caps. Still, everytime i give the regulator IC a bit of heat, everything is normal ... no additional dots and the readings are much more stable too. Ye, it's kinda odd, i agree with that. Thats why i want to try new dedicated regulators (no single chip solution), but from datasheet the ones i bough have different pinout compared to each other, which shouldn't be a problem. Changes in the schematic below (or how i think it should be hopefully).

author: Kleinstein
author_rank: Super Contributor
timestamp: October 19, 2024, 03:22:58 pm
content: Both regulator want a low ESR capacitor close to the input side. At least more / low ESR capacitance on the input side only helps.The 79xx regulators need a certain capacitance on the output side, a little like low drop regulator. They seem to want 1-10 µF of electrolytic or similar.

author: kblue
author_rank: Regular Contributor
timestamp: October 19, 2024, 03:45:14 pm
content: Will report back, once everything is installed. Thanks again for your help!/edit: As i dont have "Tantalum" caps on hand, some notes of these 78XX and 79XX say: "if tantalum isn't available, use electrolytic aluminum 10x higher than value given" so i used 22 uF for the 79XX and 3,3 uF for 78XX both with very low ESR. The "typical application" for 7815 had 0,22 uF in mind so 2,2 uF will be the 10x higher value (as i don't have 2,2 i used 3,3 instead) same goes for 79** "typical application" uses a 2,2 uF (times 10 its 22).As there are no usable traced nearby, guess i need to make my own for the output-capacitors.What am i thinking? There are already Caps on the output side (on the main PCB).

author: kblue
author_rank: Regular Contributor
timestamp: October 24, 2024, 02:57:50 am
content: The +15 / -15 V IC is changed / switched for dedicated +15 and -15 regulators. Everything works fine so far, except for something little. Down in the 200 (mV/uA/Ohms) Range and up to the 20-range (for every kind of input) i see numbers with shorted inputs. So i cleaned the switches with IPA and numbers went down but still not quite zero (e.g. 00,02 to 00,03 in VDC 200 mV range // 02,75 in VAC 200 mV range // 00,02 in Ohms 200 range etc pp). I also cleaned all PCBs with IPA. Mayhaps i try dedicated PCB cleaning materials.

author: kblue
author_rank: Regular Contributor
timestamp: October 26, 2024, 09:52:12 am
content: After letting the unit run for hours, DCV and Ohms are 00,00 with shorted plugs. If i turn the unit off for some more hours and back on again its 00,00 from start. So the "problem" whatever it was, solved itself. Except for VAC which still shows 02,60 - 02,75 in 200 mV range with shorted plugs but this could be some sort of noise i guess.