Keithley 240A Repair assistance (fixed) - Page 1
Forum section: electronics / repair

author: smgvbest
author_rank: Unknown
timestamp: June 26, 2016, 06:07:55 am
content: HelloI recently got a Keithley 240A off eBay. Supposedly working. well you know the rest of that part.So I've spent time trying to repair it thought I had it figured out but still no go.Overall symptom is that from 0V to 1200v on the dial gets me from ~400mv to 5V out and that is after replacing some partsas below.going through the service manual in 4.7 The Voltage Control Amp will not stabilizeand shows a constant overcurrent.I tracked down the 8086/0A2/0B2 Tubes (got spares just in case).The 0A2 Tube appeared bad and replaced it. but no go.So followed the service guide and as stated above the Voltage Control Amp will not stabilizeI can not adjust pin 5 of the 8086 at all. I have a constant -40vDC on Pin 5.Continuing on D107 has -0.8V on the Anode and -0.2V on the cathode with respect to ground. So it is reverse biasedper the manual. The Diode also tests good out of Circuit on meter and peak tester.On V101 the 8068 Series Regulator, I have2-7 Heater 6.3vAC3 -0.45mV this should be -0.1vDC5 -40vDC this should be -20vDC8 -112vDCTrying to attach a copy or the schematic I've been editing with notes and voltage readingsThe part I'm not getting is why after R103 (which can not be found on the board) the voltage goes from 112Vto .7v but should be 20V. if I'm reading this correctly that is on the main supply rail and is probably the sourceof the problem. but I can not figure out why I can't get the 20V on it.If any one has any experience with one of these I could use some advise.I know I'm missing something basic here.I do have a HQ scan of the original manual for one of these but it's HQ and large, like 160MB in PDF form. If anyone wants it I can FTP it to my site and share the link.Edit:The Power section seems fineI've got1750VDC for the 8068150VDC112VDC6.3VACall out and in spec of +/- 15%

author: TiN
author_rank: Super Contributor
timestamp: June 26, 2016, 08:15:22 am
content: Sure we want it.Perhaps something taking too much current after R103? Is D101 ok?

author: bitseeker
author_rank: Super Contributor
timestamp: June 26, 2016, 08:20:46 am
content: smgvbest: Might want to correct the thread subject (204A -> 240A) to help future searches match better.

author: smgvbest
author_rank: Unknown
timestamp: June 26, 2016, 09:40:37 pm
content: Quote from: bitseeker on June 26, 2016, 08:20:46 amThank you and fixed

author: smgvbest
author_rank: Unknown
timestamp: June 26, 2016, 11:45:19 pm
content: Quote from: TiN on June 26, 2016, 08:15:22 amUploading to your site now. in /Keithley/240A/Keithley240A.pdfD101 was pulled and checked as ok but went ahead and replaced with new diode and same result.I checked R122, R143, R132 and D112 as they all appeared hot running. Value wise they are fine. all within spec. D112 is one of the rectifier diodes. it checked fine. I pulled D105 the main reference Zener and it test fine on the Peak tester, but on simple Multimeter test it shows open on 3 different meters. Replacing it changed nothing.

author: TiN
author_rank: Super Contributor
timestamp: June 27, 2016, 04:58:08 am
content: File available here :PDF, 180MB

author: Kleinstein
author_rank: Super Contributor
timestamp: June 27, 2016, 04:10:46 pm
content: If D112 is really OK (and the right way around), my guess is there might be a problem with the GND wiring. Otherwise there should not be a negative voltage over D101.

author: smgvbest
author_rank: Unknown
timestamp: July 02, 2016, 03:17:13 am
content: I have the 240A setup as the service manual indicates which is output set to Negative, Fine Tuning to Cal, Voltage Selectors to 0 0 0 which would output negative voltages.If I change the output to positive D112 reads 191v on the anode and -0.2 on cathode if I remember which side was which correctly.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 02, 2016, 09:25:38 am
content: 191 V at the cathode side and close to 0 at the anode side of D112 is more reasonable. The anode side is essentially AC from the transformer.Still strange how to get -112 V at pin 8 of the tube - should be more like +112 V.The low voltage across D101 indicates a problem, likely something shortening out the supply that should be +20 V, or R103 really missing / fallen off. For a short, I would suspect Q107.

author: oldway
author_rank: Super Contributor
timestamp: July 02, 2016, 01:14:57 pm
content: In my opinion, there is an insulation failure of the screen in the power transformer.Disconnect the screen from D111 and check the insulation resistance between the screen and windings and between the screen and the core of the transformer with a 1000V or more Megger.NB: be very careful, there may be unespected high voltages with such fault...Voltages in this power supply are lethal

author: smgvbest
author_rank: Unknown
timestamp: July 02, 2016, 04:39:28 pm
content: Quote from: Kleinstein on July 02, 2016, 09:25:38 amI'll have to recheck all the voltages with it in the positive output. they could have all changed.R103 is on the schematic but with two markers for notes, but I can not find those notes anywhere in the manual.when you look at the parts reference pages. R103 is blank for which of the PCB photo's it is onlooking at all the PCB photo's (and the PCB itself has no part designators on it, you only have the reference photo's in the manual)so they show R103 on the schematic but no where else is it seen.I can check Q107, its a A1380 the only replacement I've found so far is a SA1380 and is a different package.editwell the SA1380 is PNP and the A1380 is NPN so that won't workfound the BC108 is a replacement but it's also hard to find but did find some on ebay (of course) so just in case I got some.

author: smgvbest
author_rank: Unknown
timestamp: July 02, 2016, 05:12:36 pm
content: Quote from: oldway on July 02, 2016, 01:14:57 pmI don't currently have a insulation resistance tester.Since this would be very limited usage for me. I'm thinking of one of the single purpose units like the Vc60bcan get on amazon pretty inexpensively.

author: oldway
author_rank: Super Contributor
timestamp: July 02, 2016, 07:36:31 pm
content: First, check the voltages you measured.The diagnosis I have done is based on the values you provided.If these values are wrong, the diagnosis may be wrong.Have you really a negative voltage at pin 8 of the tube ?NB: on tubes, voltages must always be measured with reference to cathode (pin 3)Negative voltage at pin 8 of the tube, that's not possible if you measure with reference of cathode.Did you find R103 ? It's a power resistance (probably 5W)The value is well 3K ? Did you check this ?You said you replaced D101...It should be a zener diode 20V 1,3W ...Is this right?

author: smgvbest
author_rank: Unknown
timestamp: July 03, 2016, 04:25:36 am
content: ok,R103 is not marked on the board at all. it's not in the Doc except for on the schematic and in the parts list. but is not in any of the photo's of the board in the manual. why? because it sits right against the center bulkhead underneath V101 so it can not be seen in any photo's.R103 is reading 5Mohm instead of 3K. it is a 5W power resistor.Looks like Digikey may have a suitable replacement so I will order it and wait until it arrives to do any more troubleshootingif I'm lucky this is all that is wrong.I had to trace out the circuit board to find it.checking from R101 to Pin 8 of V101 was ok.then checked from R101 to cathode of D101 and continuity showed a open.flipped over board and visually traced it and that is when I found the pins for R103. flipping it back over showed it under the V101 tube. I would have thought Keithley would have called that out in the manual.Will update after I've got R103 back in place.Oh and yes, my info was bad, it is 112 V on pin 8 not -112

author: smgvbest
author_rank: Unknown
timestamp: July 07, 2016, 11:23:25 pm
content: I am happy to say that R103 fixed it. the part I suspected from the beginning but I could not locate.I let it warm up then went through the cal procedure. Everything well within the 0.5% stated in the manual.Screen shots are at a few test points plus the 12.5mA cal reading.Overload is working as it should. So very happy now.totally unrelated.The old 34401A vs the 34465A.I could go over range on the 34401A to 1199vDC but on the 34465A only to like 1020vDC. that extra 200V is nice