Repairing Tektronix DM501 and How to Clean a Board - Page 1
Forum section: electronics / general

author: David Hess
author_rank: Super Contributor
timestamp: February 27, 2025, 07:08:35 pm
content: I am finally getting down to the nitty-gritty of repairing a pair of Tektronix DM501 multimeters, which appear to have high leakage around the high impedance attenuator. I have established that the leakage is not from the input buffer, and is not from the input protection diodes, which I think leaves surface leakage on the printed circuit boards from surface contamination.Now it may come down to scrubbing the boards with soapy water, rinsing, and then drying, but some of the components may not survive immersion. What about steam cleaning with something like this?https://www.amazon.com/dp/B0DCHBC8JJSo far it seems to work well, but I will not have results until later today. I need a better oven for baking out boards.

author: mawyatt
author_rank: Super Contributor
timestamp: February 27, 2025, 08:40:03 pm
content: Some of the Enclosed 3D printers like the Bambu XC1 have a Filament drying mode. This is for drying out Printer Filaments which are plastic, so might work well for a populated PCB since the temperature is programmable to ~100C.Best

author: EC8010
author_rank: Frequent Contributor
timestamp: February 27, 2025, 10:41:47 pm
content: Seriously, why would the board be dirty and have surface leakage? A freshly hand-soldered board will have contamination from flux loaded with solder pellets, but a board that was clean once?Cleaning a PCB is not trivial. I have made electrometers that are good down to <1fA and they require clean boards after hand-soldering. I use defluxer to dissolve the flux and scrub immediately with a toothbrush. I hold the board vertical, with the most sensitive bits at the top so that dirty liquid runs away from them. Then rinse with copious isopropyl alcohol to wash/dilute the muck away and immediately dry with pre-warmed hair dryer to prevent water (and further muck) condensing from the atmosphere. The other side now needs to be done because all that fluid undoubtedly got onto the reverse side. And in doing the reverse side, its muck got back onto the first side, so both sides need to be redone. If that sounds overly complex, I'd point out that all twenty of my electrometer boards worked first time.Beware that many older components do not survive flux cleaner. Many boards do not survive water, either. Years ago, we had an ultrasonic cleaner and it was known for finishing off boards, yet it was only water plus detergent.Think very carefully before letting liquids loose on boards...

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 28, 2025, 12:17:19 am
content: I just use isopropyl alcohol on these boards, washed standing on end as EC8010 says.They are FR4 boards and pretty robust. There's nothing too sensitive there.To clean the gold contacts on the rotary switch, wet some strips of standard white printer paper with isopropyl alcohol, place it in between the contact leaf and the PCB, gently press the leaf to the closed position with your finger and carefully drag the paper out.This is just enough abrasion to clean the contacts perfectly with no damage.Also while you are at it, replace all those nasty Texas Instruments IC sockets, they are averycommon failure point due to their crappy design.

author: Victorman222
author_rank: Regular Contributor
timestamp: February 28, 2025, 01:16:12 am
content: For me using lint free wipes with a solvent stronger than IPA - for example ethyl acetate mixed with IPA 50/50 works well for cleaning without immersion.I have a test board that has flux on it giving leakage settling from 4 pA at 1 minute to 0.6 pA after 5 min with 9V applied, i just cleaned it with two wipes and it measures in the noise floor after 1 minute.Keep in mind i have a very simple picoammeter doohickey that is a build of the "Gyro picoammeter" with a chinese 1gig "1%" thick film resistor. 100fA is about the noise floor of the thing.And of course, there still may be contamination left, just that the solvent made the board dry, thus increased the resistance..I want to emphasize it is important to avoid spreading the contamination around, for example we do not want the solvent to dissolve the contamination, dry up and leave it right back on the board.I like EC8010s thorough approach.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2025, 02:19:34 am
content: The steam cleaning definitely made an improvement, so I am going to scrub the boards in soapy water, rinse, and then blow clean using the steam cleaner. I am going to pick up a food dehydrator for baking out the boards.If that is not enough, then I will try scrubbing with a solvent, likely starting with ethyl alcohol because I have that available. I might also rinse with acetone on the solder side.Quote from: mawyatt on February 27, 2025, 08:40:03 pmI am now thinking of picking up a food dehydrator for baking out boards.Quote from: EC8010 on February 27, 2025, 10:41:47 pmThese multimeters have an uncertain history before I acquired them, and it is not unusual for old test instruments like these to be stored poorly.One of these multimeters arrived with welded switch contact for the amps mode but I had no trouble fixing it. I have done some other maintenance which includes soldering, but nothing which would have affected the high impedance paths.One thing both boards have in common is that I had to clean and lubricate the switch which selects between the internal and external inputs. The cleaning fluid or lubricant could have contaminated the board over time, and at this point that is what I suspect happened.Quote from: TERRA Operative on February 28, 2025, 12:17:19 amI am familiar with how to clean the Tektronix switches. Non-clay bearing paper like vellum or card stock is suppose to be used. Unfortunately my problem is not dirty contacts.QuoteThe Texas Instruments edge wipe sockets are terribly unreliable, and some of the maintenance I have done on these has involved replacing them with machined collet sockets, but I have not replaced them all yet. The leakage does not involve any circuits which use ICs or IC sockets.Quote from: Victorman222 on February 28, 2025, 01:16:12 amI usually scrub and then rinse to a downhill corner.

author: laichh
author_rank: Regular Contributor
timestamp: February 28, 2025, 05:58:47 am
content: Check out the TI OPA928EVM user's guide, page 13 for detail cleaning procedure.https://www.ti.com/lit/ug/sbou282a/sbou282a.pdf

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2025, 03:40:37 pm
content: I reassembled the meters for testing; one is much worse, and one is much better, but not good enough.I think I am going to have to do a thorough cleaning which will require removing sensitive parts, and an extended bake out. This is now a long term repair. Using the steam cleaner definitely had a positive result, but was not enough by itself.I am also going to complete my hack picoammeter, which should have a usable resolution of 10 femtoamps, to test the dual JFET transistors used for buffering, and other sections of the input circuits.Quote from: laichh on February 28, 2025, 05:58:47 amThe procedure described in the manual for the OPA928 Evaluation Module is what I usually do to good effect, but in this case the boards have parts which an ultrasonic cleaner will likely destroy, and parts which may be damaged by complete immersion or a temperature of 100C.

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2025, 10:58:56 pm
content: Do you think a vacuum chamber could be used to dry and remove the water from a cleaned board in place of baking the board out in a low temperature oven?The board which got worse has slowly returned to its previous leakage of 2 nanoamps. I am going to test the input JFET since I did not swap the ADC on both bad meters.The board which got better after cleaning is slowly getting better. it is down to 0.5 nanoamps where before it was 2 nanoamps. I suspect I did not bake it out well enough.Update: Just to be clear, I think the two DM501 multimeters have different problems; one has leakage in the main board, and the other has leakage from a bad JFET input buffer. While the first dries out, I am going to directly measure the gate leakage of the JFET and qualify some replacement input protection diodes for low leakage.It just occurred to me that if the JFET is bad, I could add a modern operational amplifier configured as a follower to buffer the JFET removing the effects of its gate leakage. Replacementdual JFETs exist, but cost like $6 each and I would have to pay Digi-key shipping.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2025, 08:08:34 am
content: Electrolytic capacitors may not like vaccum. I would consider a metal container and dry silica gel or similar to keep the board in a dry environment for quite some time.If one suspects leakage through a semiconductor, one can do a quick test with local heating (no much needed). The leakage current of semiconductors goes up with temperature quite a bit (e.g. double with 10 K more) and thus also applies to most bad parts. So leaky fets would likely also be temperature sensitive.1 nA of leakage is a lot for a dirty PCB. This would be very dirty of maybe a chrack or overheated part of the PCB.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 01, 2025, 12:49:18 pm
content: There are some good advice here, but keep in mind that the DM501 is a mere 3.5 digit DMM and not a pico-ammeter. I received one which obviously spent decades in not-so-favorable conditions (stored in humid garage, I'd suspect). I had to repair the PSU and there's some rust on the case, but the DMM still works fine.

author: David Hess
author_rank: Super Contributor
timestamp: March 01, 2025, 01:46:38 pm
content: Quote from: guenthert on March 01, 2025, 12:49:18 pmThe DM501 is 4.5 digits, and 2 nanoamps is 20 counts for it.

author: factory
author_rank: Super Contributor
timestamp: March 01, 2025, 02:26:06 pm
content: This might not be the cause of the leakage, but there is foam in the 307-1015-00 resistor network block, that can go conductive, see this repair for more info;https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg5681927/?topicseen#msg5681927David

author: EC8010
author_rank: Frequent Contributor
timestamp: March 01, 2025, 04:25:20 pm
content: Quote from: David Hess on February 28, 2025, 10:58:56 pmInteresting. I've generally found a hair dryer good enough to dry a board. Something I found useful when designing fA-level stuff was to rout a large hole under the (SOIC8) electrometer IC. The trouble with cleaning boards is that it's easy enough to dissolve the contamination, but actually getting it all off the board is another matter. It easily gets trapped under components. I rout slots under gigohm resistors for the same reason.

author: maat
author_rank: Regular Contributor
timestamp: March 01, 2025, 08:28:06 pm
content: From my experience, a board that was cleaned in water is technically not soaking wet as it takes a good deal of time for the water to get into the board.Here is how I dry my boards after washing: I dump them in 99.9 % Isopropyl. This stuff is very hygroscopic and will pull out any moisture that is inside the PCB and on the PCB. You can get it on the cheap via Ebay.Do watch out for polystyrene or other stuff (e.g. rotary encoders) that does not play well with IPA, though. If there is contamination on the board that is not dissolved by IPA, you can try 60/40 IPA/distilled water. This normally does the trick.If you leave the board in IPA for an hour or so (close the lid), it will be super dry and you will see that leakage will slightly drift upwards for the next day or so until the board is in equilibrium with ambient humidity again.

author: David Hess
author_rank: Super Contributor
timestamp: March 01, 2025, 09:44:28 pm
content: Quote from: Kleinstein on March 01, 2025, 08:08:34 amThat is a good point. The DM501 design is old enough to only use solid tantalum capacitors. I will have to remember that for the future because I am definitely going to try it at some point, if not for this project if I can find a large enough vacuum chamber for the whole board.QuoteThat is why I was so alarmed, and knew there was a problem. The working DM501 has less than 100 picoamps of leakage even after warming up to a 40C operating temperature.Quote from: EC8010 on March 01, 2025, 04:25:20 pmWhen I designed fA circuits, I air wired critical nodes, but back then, everything was still available in DIP packages making this easy. I can imagine cutting a slot in a board to air wire a lead off of an SO-8 package.My normal board cleaning procedure is to use an ultrasonic cleaner first with distilled water with a little bit of dish-soap, and then an ultrasonic cleaner in distilled water, followed by baking out. At the moment I lack a suitable ultrasonic cleaner and bake-out oven at home, and there is some question about how delicate the board is without removing too many parts.Cleaning in production worked, but there was a perpetual problem with the distilled water not being changed often enough.Quote from: maat on March 01, 2025, 08:28:06 pmI have been pulling the rotary cam switch assembly off of the board before cleaning. I am mostly worried about many of the trimmers. Polystyrene capacitors are also damaged by temperatures above something like 90C.QuoteThat seems like a good alternative. The only alcohol I have at the moment that I trust is 96% grain alcohol. I was thinking of rinsing in acetone to do the same thing but the acetone may be too harsh.Quote from: factory on March 01, 2025, 02:26:06 pmI have dreaded having to replace the custom resistor network. So far all of mine are good, and this would not affect leakage, but I will take a look.One of my networks has the original maker's name on it. I think Tektronix initially had this part contracted, and then may have brought production in-house.Tektronix has a history of rotting potting material, but it was an intermediate time before these multimeters.

author: David Hess
author_rank: Super Contributor
timestamp: March 02, 2025, 03:20:24 am
content: One of the bad DM501s is fixed. There is now no detectable drift or input current.DM501 #1 showed about 0.5 nanoamps of leakage after cleaning. DM501 #2 showed the original 2 nanoamps of leakage after cleaning. I was swapping the ADC boards around between the three DM501s to make sure it was the main boards and not the ADC boards, and #2 outright failed to operate. It would start, but as soon as I changed the range switch it would stop, and then it failed to work at all.So I checked the voltages and the main -12 volt regulated supply was missing. I checked the schematic, shown below, and the first thing I tested was the Vbe current limit shunt, and it was open. I replaced it and the meter works again, and the drift and leakage are gone. The pulled resistor now tests good, except maybe if heated.So what must have been happening is the Vbe current limit resistor was warming up and gradually going open, which dropped the -12 volt supply which is also used to generate the isolated floating supplies for the ADC. The lowered supply voltages to the ADC caused a zero shift as the unit warmed up. The resistor was located close to the edge connector where mechanical strain would be greater than normal. When I tested the supply voltages, the meter was always cold so nothing was revealed.Now I am back to the remaining "leaky" DM501.

author: lowimpedance
author_rank: Super Contributor
timestamp: March 02, 2025, 08:51:52 am
content: Well that was a sneaky b@st@rd ! Looks like one of those horrid carbon composition types that love to drift high with age.Good to see the 501 back in action

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2025, 06:44:07 pm
content: I removed and bypassed the input selection switch for both DM501s because it is a difficult to clean part and I have never used this function. Both DM501s still showed leakage; #1 is up to 3 nanoamps, #2 showed 1 nanoamp.I disconnected the input to the ADC boards from the main boards and added a 10 megohm resistor across each ADC input. I did this so that the inputs are disconnected, but the ADC boards are still powered from the main board as usual. I also replaced the 15 microfarad solid tantalum capacitors on the floating power supply for #2 with 100 microfarad low impedance electrolytics and 1 microfarad X7R ceramic capacitors. One of these tantalum capacitors had already shorted and been replaced a while ago, and when removed the second tantalum capacitor tested weird. #1 now shows no leakage, and #2 shows 0.6 nanoamps. So I finally have some results that I can use. Removing the input protection diodes from #2 got rid of the leakage, so now DM501 #2 works. The floating power supply for #2 might have been noisy.I calibrated DM501 #2 and #5 for DC volts, and ohms. DC current and AC calibrations will have to wait. Both meters have less than 1 count of noise and drift.So I am left with DM501 #1 with 3 nanoamps of leakage somewhere on its main board. I am going to wire the ADC directly to the input terminals for tests. I suspect that I will need to remove sensitive parts and blast it with an ultrasonic cleaner.The photo below is from before I removed the protection diodes from DM501 #2. Shown also is my 9.94 megohm 0.25% calibration resistor. After DC calibration, both meters were within tolerance of the test resistor.I may rebuild the high frequency inverter based floating power supply on all three units using polymer aluminum electrolytic capacitors. This seems like a good place for them.

author: David Hess
author_rank: Super Contributor
timestamp: March 09, 2025, 07:35:40 pm
content: I hacked together a picoammeter with 100 femtoamp resolution (1) as shown below. It only needs more shielding to get a useful 10 or even 1 femtoamps.The first protection diode from a DM501 that I measured was 4.5 picoamps, so not the problem, but now I am thinking that with a suitable probe, I could localize where the leakage is on the main board of the leaky DM501.Update:The good protection diodes that I pulled from the DM501s leak about 5 picoamps. One was bad at 30+ picoamps which could account for all of the remaining leakage on DM501 #2 after its main board was cleaned.My collection of "low leakage" 1N458A and FDH300 diodes are about 30 picoamps. A random new 2N3906 was 1.0 picoamps so I will be testing and using small signal transistors as low leakage diodes, as usual for me.Update Update:Found a Fairchild BC550 with leakage below 0.05 picoamps. New Diotec BC546C was 0.2 picoamps.(1) Current resolution extends to 1 femtoamp, however practical considerations of shielding prevent useful results below 100 femotoamps, which is good enough for what I am doing. Calculated resolution from noise and drift is significantly below 1 femotamp, revealing how real picoammeters can achieve this. To do so here would require better construction and a shielded enclosure. Ground plane construction was my compromise.

author: EC8010
author_rank: Frequent Contributor
timestamp: March 10, 2025, 04:58:19 pm
content: Quote from: David Hess on March 09, 2025, 07:35:40 pmWas that collector connected to base, using the base emitter junction as the diode? And at what reverse voltage?

author: David Hess
author_rank: Super Contributor
timestamp: March 10, 2025, 08:19:43 pm
content: Quote from: EC8010 on March 10, 2025, 04:58:19 pmThat was 3 volts across the base-collector junction, and I did not bother connecting the base to the emitter, and if I do there is practically no difference. 3 volts is about the level where the DM501 protection circuit operates and it allows measurement of the base-emitter junction. The ohms converter protection diodes operate at 12 volts, however that input is connected to the negative terminal so cannot affect the ADC reading.I measured a BC337 (800 mA) at about 0.02 picoamps, so 20 femtoamps. It is difficult to get a reliable reading at this level without some improvements. I would not expect such a large small signal transistor to have such a low leakage but it came from Mouser and appears to be correct.On some parts the base-collector and base-emitter junctions have comparable leakage, and on some the base-emitter junction has more leakage.

author: David Hess
author_rank: Super Contributor
timestamp: March 25, 2025, 05:13:57 am
content: I made some more tests and my collection of Diotec BC337 transistors (NPN 800mA) all have a leakage of about 50 femtoamps at 20 volts. My smaller Diotec BC546C transistors (NPN 100mA) are about 300 femtoamps at 20 volts.Having gotten good results, I guess I should assemble a better version with a fully shielded enclosure to extend resolution to 1 femtoamp.

author: EC8010
author_rank: Frequent Contributor
timestamp: March 25, 2025, 07:50:09 am
content: Interesting stuff. A tobacco tin should do nicely as screening. Beware that they are often coated with epoxy, so the lid is insulated unless you explicitly solder a wire between tin and lid.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 29, 2025, 09:57:14 am
content: Quote from: David Hess on March 01, 2025, 01:46:38 pmI finally got around to have a look at my instrument and it turns out to be a DM502. Sorry for the confusion.

author: David Hess
author_rank: Super Contributor
timestamp: March 29, 2025, 04:20:35 pm
content: Quote from: guenthert on March 29, 2025, 09:57:14 amI have two DM502s and about 4 years ago replaced the aluminum electrolytic capacitors which were worn out and bridge rectifiers because one of them failed. I also added TSV diodes to the supply rails to try and prevent damaging anything if the regulators failed, and replaced some Texas Instruments edge wipe sockets with collet sockets, and replaced some solid tantalum capacitors.My DM502s only require calibration, which I have done, but the trimmer resistors need to be replaced.At one point I had plans to modify one of my DM502s to extend its AC low frequency response down to 2 Hz for audio work, however that proved not to be necessary.