Keithley 237 repair - Page 1
Forum section: electronics / repair

author: rolkinas
author_rank: Contributor
timestamp: November 22, 2019, 08:47:15 pm
content: Hello,I bought Keithley 237 from ebay "used and 100% working" condition.It appeared to be not outputing any voltage/current due to some failures such as all caps in resonant converter are dry and converter not working, 4 caps 100uF for +-30V are dry and there are no 30V present, triac in 50V supply is leaky and one 1N4577 in output stage was open.bit too much for used and perfectly working condition (as stated in listing) from Italy (:fit temporary parts to see functionalityNow I have a question. if I input by keypad 1 volt - unit outputs 10V. Moving decimal point to right I manage to get 1 volt but in other range. the same is with all other voltages from 1 to 11 volts. 12V and up - are fine (12V means 12V 50V means 50V etc. everything below 11V are multiplied by ten except when decimal point is before 2 digits right to left. The same is for negative and positive values.current programming is OKJust for clarification I added pictures.Will be looking for issues

author: rolkinas
author_rank: Contributor
timestamp: November 29, 2019, 01:09:17 am
content: Moving towards last steps in repair.Replaced all electrolytic caps in resonant converter. In power supply section replaced C52, C52, C53, C54, C56, C57, Q7Replaced shorted Q33 and defective U31Last issue almost solved. It is burnt 2.77K PTF56 series resistor in feedback divider network.Have anyone idea where to get stable 2.77K ?Also, what is original fan specifications in regards of required air volume for K237 ?

author: coromonadalix
author_rank: Super Contributor
timestamp: November 29, 2019, 02:09:14 am
content: For the 2.77 k resistor you have a fewVishay Dale RNC50J2771BSB1431PHILIPS B0207C2K770B1A metal film resistor. 2.77 kOhm1/2W 0.100% toleranceIt depends on the wattage and availbility in your countryDigikey, Avnet, Arrows, Future Electronics

author: rolkinas
author_rank: Contributor
timestamp: November 29, 2019, 09:21:09 pm
content: I looked in Farnell, Mouser, Avnet, Digikey, RS, TME, Distrilec, but unfortunately they have no stock of 2.77K . Only lead time of approx 20 weeks+I am open to offers for 2.77K 0.125W - 0.25W

author: MiDi
author_rank: Frequent Contributor
timestamp: January 04, 2020, 11:53:38 pm
content: Hope it is ok to hijack this thread for this interesting unit:If it would be only that, I would be happy...Clear indication to turn it apart before turning it on - as I would not do that anywayThe sticker that states "does not turn on" probably has falling off somewhere...After pulling out the transformer - which was an easy part as preowner did preliminary work already - the first check was resistance of all windings followed by checking voltages @242V~:RD - BK6121V~RD/WH - BK/WH5.5121V~GY - GY0.3511V~YE - YE/WH - YE25each157V~ eachOG - OG/WH - OG2.7each21.8V~ eachBN - BN0.710.7V~BU - BU/WH2.242V~VTHigh-Z-Seems to be reasonable, but transformer drawing 580mA and 9.5W (real power) without any load might be a bit high.After initial checks on all other boards and cleanup of the inside of the whole unit, the tranformer went back into it.First only digital board was connected to see if unit powers up.It did without any problem, everything nice and cool despite the LM323 Heatsink which gets rather hot - this seems to be ok.The other boards were connected and unit came up without any issues - sorry, no magic smoke escaped for youPower consumption is 58W and ~600mA @~230V when outputting 10V.The fan consumes a whopping 18W @ 121V~, so rest of unit should be ~40W.It came with a 1A fuse installed, but for 230V a 0.5A fuse should be used, which would not last long in this particular unit.So current drawing from line might be a bit to high -would be nice if someone could measure current @230V~/50Hz to compare.First test of source part succeeded and gave reasonable values - just some digits of.Have to test the measurement part and will give an update...PS: Have taken pics from all boards top/bottom and details - so if there is interest, I can post them, but there are already a couple e.g.Keitley 236 Source Meter - Teardown and Review.

author: rolkinas
author_rank: Contributor
timestamp: January 05, 2020, 12:35:45 am
content: Hi, feel free to hijack this threadMy 237 consumes about 330mA either in idle and voltage source mode (without any load)I can't remember exact power of fan. I replaced it while it was not original and worn out.I suggest to check temperature of MAC223 triac. It may be semi open and cause high intermittent current draw as in my unit. Therefore I had to replace it.

author: rolkinas
author_rank: Contributor
timestamp: January 05, 2020, 12:48:23 am
content: Small follow up to my repair.2.8k instead of 2.77k resistor in input divider worked well. Calibration-adjustment of lower ranges were performed without any problems.In my case upper voltage range initially was quite off. Looks like already failing unit was calibrated by previous owner. It might be that Keithley has calibration boundaries in terms of call- to call value difference and in terms of absolute value. Had to perform several adjustment sessions to be able to adjust it to real value. Adjusting firstly negative and then positive 1000V (as opposed to manual) helped a lot.

author: BU508A
author_rank: Super Contributor
timestamp: January 05, 2020, 02:05:05 am
content: Quote from: rolkinas on November 29, 2019, 09:21:09 pmHow about combining a 70.6R and a 2700R resistor?Okay. the 70.6R has only 100mW but I think in series with 2700R resistor this shouldn't be an issue.All come with 0.1%.70.6R:https://www.mouser.de/ProductDetail/Vishay-Dale/RN55C70R6BB14?qs=sGAEpiMZZMsPqMdJzcrNwsAoK60ltMOoV2kySbHX6ZI%3D2700R:https://www.mouser.de/ProductDetail/Vishay-Dale/RN60C2701BB14?qs=sGAEpiMZZMsPqMdJzcrNwsDBSEuVlBlrkPFjbJ8wHr8%3D

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: January 05, 2020, 06:23:03 am
content: Quote from: BU508A on January 05, 2020, 02:05:05 am2700R + 69.8R, but only in 1% 50ppm/°K 0.125W

author: rolkinas
author_rank: Contributor
timestamp: January 05, 2020, 04:08:41 pm
content: 50ppm/C might be too much for such use.I put this one:https://www.mouser.de/ProductDetail/Vishay-Dale/PTF562K8000BZEK?qs=41svs21nsHB8T7Nf7ZeGtg%3D%3DPTF56 series as original but 2,8k instead of 2,77Currently I am going to leave that as is. At least since 2,77 with better than 5ppm/C becomes available somewhereAlmost forgot to mention that before performing calibration unit being in voltage source mode started to be noisy after several hours being ON. That was caused by faulty 150pF NPO !!! capacitor.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 05, 2020, 07:29:48 pm
content: I have found it to be easier sometimes to buy a second unit in working condition and then compare signals.My problem usually is, that I keep the second instrument as well.

author: rolkinas
author_rank: Contributor
timestamp: January 05, 2020, 11:48:13 pm
content: Quote from: HighVoltage on January 05, 2020, 07:29:48 pmI have same sicknessMy inventory tends to be growing in pairsunfortunately (or maybe fortunately) there is only one K237 in my lab this timeK237 is quite straight forward unit. But as always we need some time to familiarize with design

author: MiDi
author_rank: Frequent Contributor
timestamp: January 06, 2020, 12:23:53 am
content: Quote from: MiDi on January 04, 2020, 11:53:38 pmWould appreciate if someone could do measurements on unit, especially transformer to compare:-Current/Real power consumption from line and line voltage-Voltages on all tranformer taps (see above table)-Resistance of primary windings (unplugged!)-Bonus: Temperature difference on top of LM323 to environment (warmed up unit)

author: rolkinas
author_rank: Contributor
timestamp: January 08, 2020, 03:37:55 am
content: Quote from: MiDi on January 06, 2020, 12:23:53 amMeasurements taken on my unit:current draw at 240Vac0.33A with fan,0.25A without fanMy fan is 230Vac, It's connected directly to mains not through transformer as it should be when using 120V fanRD - BK 8.7 ohm 120V~RD/WH - BK/WH 8.2 ohm 120V~GY - GY 0.3 ohm 9.1V~YE - YE/WH - YE 25 ohm each 134V~ eachOG - OG/WH - OG 2.7 ohm each 18V~ eachBN - BN 0.8 ohm 8.9V~BU - BU/WH 2.0 ohm 40V~LM393 temperature approx 29 degree C above ambient (top cover removed, fan running).Ambient 20 degree C, actual temperature on LM393 - 49 degree CTransformer is barely 5 degree C above ambientTemperatures taken on unit which was powered for about 40 min.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 11, 2020, 09:11:57 am
content: Thanks rolkinas!The transformer seems fine, despite it is a Japan Version (TR-262 100/200VAC) instead of usual (TR-261 115/230VAC)This explains the ~20% higher voltages on secondary, the elevated current & real power draw (saturation of core) and the higher temperature on LM323 - a new TR-261 is on its way.

author: rolkinas
author_rank: Contributor
timestamp: January 11, 2020, 11:14:29 pm
content: Wow, what a finding...What about fan? isn't it 100Vac ?

author: MiDi
author_rank: Frequent Contributor
timestamp: January 12, 2020, 08:14:22 am
content: Guess the fan is same on all units: Papst 8500 C 115VAC 16WLine filter: Delta 115/230V 6A 2x0.55mH 15nF-X2 2x3.3nF-Y

author: rolkinas
author_rank: Contributor
timestamp: January 12, 2020, 10:21:11 pm
content: Looks great, especially that gorgious PAPST fan

author: balage
author_rank: Regular Contributor
timestamp: January 19, 2020, 09:38:32 pm
content: Quote from: MiDi on January 06, 2020, 12:23:53 amHi Guys,My 237 was repaired before I've bought it. The fan is connected to the PCB as it is a 125V one.-Current consumption on a 230V line is 320mA in idle. When I turn the output on (without load) it draws around 330mA.-The LM323 is around 40°C hot, ambient is 25°C.-Resistances and voltages are the followings:RD - BK ?118V~RD/WH - BK/WH ?119V~GY - GY ?9,4 V~YE - YE/WH - YE ?each 132V~ eachOG - OG/WH - OG ?each 19V~ eachBN - BN ?8,7V~BU - BU/WH ?41V~I haven't got time to disassemble the instrument that is why the resistances are missing.

author: MiDi
author_rank: Frequent Contributor
timestamp: January 19, 2020, 10:40:38 pm
content: Thanks balage,error was a japanese Transformer:Quote from: MiDi on January 11, 2020, 09:11:57 amGot a new TR-261 and now power supply is fine (and RoHS compliant):Instrument consumes ~350mA and ~50W, without fan ~260mA and 34W @235V.LM323: 40°C, dT=20.5KMAC223 triac: 31.5°C, dT=12KTransformer is drawing 230mA and 2.4W (real power) @241V without any load.RD - BK121V~RD/WH - BK/WH121V~GY - GY9.6V~YE - YE/WH - YE136V~ eachOG - OG/WH - OG19V~ eachBN - BN9.3V~BU - BU/WH41V~Next is performance verificationPS: If someone needs TR-262 - PM me

author: aleruggeri87
author_rank: Newbie
timestamp: March 12, 2023, 07:17:19 pm
content: Quote from: MiDi on January 11, 2020, 09:11:57 amHi, sorry to reopen this but, where did you find it?I fear, I burned mine yesterday...Thanks,Alessandro.

author: merox
author_rank: Regular Contributor
timestamp: March 12, 2023, 07:56:37 pm
content: Quote from: aleruggeri87 on March 12, 2023, 07:17:19 pmI think I should have a spare one somewhere in my lab, I'll have a look for it...

author: DavidKo
author_rank: Frequent Contributor
timestamp: March 13, 2023, 07:32:04 am
content: Quote from: aleruggeri87 on March 12, 2023, 07:17:19 pmI think I have found the source (on ebay) - they do not have another one.

author: aleruggeri87
author_rank: Newbie
timestamp: April 23, 2023, 05:10:33 pm
content: To follow up on my previous post:Quote from: aleruggeri87 on March 12, 2023, 07:17:19 pmAt the end of the day the transformer overheated a lot, but it is still fine.The 10 mF capacitor on the unregulated digital rail (C4) released some electrolitic, which went under the diode bridge CR1 and shorted it out.The bridge exploded and the PCB was also damaged (see picture below).I replaced these components and the two 100 nF capacitors near the bridge. It took about 1 month for the burning smell to pass, but everything work like beforeI also replaced the two 560 uF on the ±150V rails (C58-C59), as they are sitting really close to the 15k drop resistors for the ±15V rails, which get very hot during normal operation (~120 °C).My unit (actually of the workplace) is a kei236, and power consumption from the 230VAC is 280 mA when ouput is on without any load current.