Fluke 8600A repair - Page 1
Forum section: electronics / repair

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: July 31, 2025, 02:29:45 pm
content: Fluke 8600A meter has an intermittent varying DC voltage at TP11(R52) in any mode, shows best in 200mV or 2V DC range.Note: cleaned contacts, checked solder, input is shorted.I suspect Q13 J2318 N-CH JFET as its resistance between drain and source is very low but so many FETs are connected to the same point its difficult to determine which could be intermittent.I could use a replacement or substitute part for the FETs other than NSN types, these are J2318, J2068, J2069.Fluke could have included a lot more needed information in their service manuals, I spend more time adding info to speed troubleshooting than doing troubleshooting.Jeff

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 03, 2025, 01:12:01 am
content: I am going to try removing the FETs one at a time and monitor TP11 for DC each time and see what happens.Hopping the offending part will show up this way.Any other ideas?Jeff

author: elecdonia
author_rank: Frequent Contributor
timestamp: August 06, 2025, 01:16:33 pm
content: JFETS are difficult to test for source-drain leakageThis is because JFETS are "normally on" when nothing is connected to the gateA substantial negative gate voltage is necessary in order to switch them off. JFETS may require -5 to -10V on the gate to completely switch off.Technically JFETS operate in "depletion mode." They require a negative gate voltage to turn offIn contrast most MOSFET devices are "enhancement mode." They require a positive gate voltage to turn on.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 06, 2025, 10:28:39 pm
content: I am also working on a 8800A with a similar problem however it is not intermittent and should be easier to find the problem.Its a shame these parts are difficult to test and locate, I have always had a FET to replace a suspect one, now I will be lucky if I can find a replacement after finding the defect.Jeff

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2025, 12:54:12 am
content: It's normal for a JFET to be low resistance D-S out-of-circuit. They are normally on devices.You can use a scope or audio signal tracer and listen to the noise voltage at the JFET. Heat can also flush it out, you can put your finger on it. A soldering iron can be a bit too much but a few taps will warm it up.What do you have for data on the dual JFET? I would say it's not critical other than the IDSSspec. The op-amp does a lot of the work.Those p/n's I can find nothing for. The part is quite old, circa 1976.Fluke 379321 Dual N-ch JFET - E7024, SU2443, FD1757, ITS30778, DN1811I did see 25V 10mA, 6-lead mentioned.Linear SystemsorInterfet(config "dual") will have something suitable for a replacement.You could email them a request a substitute.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 07, 2025, 07:13:07 am
content: Most of the JFETs are used for signal switching. For these Idss is not critical. Instead enough maximum voltage (e.g. >= 35 V gate to source) could be more relevant. The availability of TO92 case JFETs is quite limited. In most places something like PN4393 (very similar to J113 or BSR58) should work. Where leakage is critical may be a J201.

author: floobydust
author_rank: Super Contributor
timestamp: August 07, 2025, 04:29:47 pm
content: It looks like a high speed FET input op-amp is what Fluke built there, by adding the JFETs to the LM318 input. The JFETS are source-followers feeding an LM318. There is a fair DC offset for the stage as well.Section schematic in other threadhttps://www.eevblog.com/forum/repair/fluke-8600a-with-a-single-problem-in-amps-ac/msg5975503/#msg5975503What do you think - swap in a high speed FET input op-amp, jumper out the old JFET's and wrestle with covering the stage's DC offset, at some NULL pins?This could be much lower cost than looking for exotic dual JFETs, they are quite expensive nowadays. Last time I orderedLSK389in TO-71 can, I cried a liittle bit at how expensive they are after the exchange rate and shipping. I see Digi-Key has them $22.U2 is a LM318H (can). You can buy TO-99 to DIP-8 or SOIC-8 adapters boards.ACV is rated to +/-1% at 100kHz and seems to rolloff around 150kHz with the 15pF cap C5.If you think about it, one of the diff. amp JFETs the input is grounded, so it seems to be just for PSRR.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 08, 2025, 11:28:27 pm
content: Thanks for the replies, I have to get to the point I know what to look for first.Removing parts was not to helpful although Q13 missing did help, the next one sent it into OL.Then I found a part number that was listed incomplete so search was negative on 1102.Tuesday is my next time to look but it would be nice to have datasheets on all of these parts first.It seems Fluke used different parts in the same board layouts so G,D,S, could be in any order in the partand then positioned to the PCB. I find it easier and quicker to ID the order from the part.Then I found an early type FET with the top epoxy separating on the AC converter board.Bad part but not the problem I am looking for? is this a common failure in these parts?I do not see others doing this but? may not be visual at first.Jeff

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: August 08, 2025, 11:37:38 pm
content: I saw both J113 and J201 while searching they did look interesting for subs.Thanks for the ideas.Jeff

author: elecdonia
author_rank: Frequent Contributor
timestamp: September 29, 2025, 09:47:33 pm
content: Quote from: Jeff eelcr on August 08, 2025, 11:28:27 pmTo the best of my knowledge the 8600A will operate in DC volts mode (only) with the AC converter and ohms converter boards removed. Therefore, by testing the DC volts mode with the AC converter and/ohms converter boards removed this could help identify where the fault is located. My collection of 8600A units is large enough that I have extra ohms converter and AC converter boards on hand which are known to be in working condition.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: September 30, 2025, 11:50:04 pm
content: Yes, they can be removed to check DC operation only.Jeff