HP34401A Ohms function problem - Page 1
Forum section: electronics / repair

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 04, 2021, 12:48:29 pm
content: I recently acquired a HP34401A with a failed Ohms function. The DC and AC voltage and current functions work fine. The ohms function shows approximately 3 ohms when the probes are shorted, but resistance readings are all over the place. I researched the problem online and on this site and found that the usual culprits are either the COTO9947 relay contacts or the AD706 op amp in the Ohms current source circuit. The relay is good and I have just carefully replaced the AD706, but the problem persists.The usual reports of an AD706 failure indicate that there is no current present on the terminals when Ohms mode is selected. In this case, there is approximately 10ma present rather than the expected 1ma, both before and after the replacement of the AD706 so clearly something is still wrong with the current source.Voltages around the AD706 with reference to AGND (pin 4) terminals open / shorted are:1. 4.09 / 4.072. 6.92 / 6.923. 6.92 / 6.924. 0 / 05. 8.28 / 8.766. 7.65 / 8.787. 13.25 / 12.448. 18.88 / 18.88The 18V line looks a bit high but other than that I am not sure where to look next and any insights would be appreciated. There is some suggestion on one thread that the problem might be U101 (custom HP chip 15K6-0001), but how do I determine this?

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 04, 2021, 01:21:11 pm
content: Hello,you certainly refer to e.g. this thread here:https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg489687/#msg489687I you don't mind, please get a copy of the schematic, measure all these voltages indicated in the diagram, write them down and copy this here.Your description of the test current is not clear, i.e. is it faulty, or not? Do you use 2W or 4W Ohm, and what's the difference in behavior?Please chose 4W Ohm mode and measure all different test currents in each range with another DMM, from 100 Ohm up to 10MOhm. The nominal values are listed in the service manual or in the specification.In first place, I would not suggest that U101 is faulty, there are some other components around which might cause trouble, more probably.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: May 04, 2021, 01:34:57 pm
content: It looks like there is more current flown than supposed too. So the first suspect would be Q202, as this is the transistor (P-JFET) to control the current. With some 12-13 V from the AD706 the JFET should be essentially tured off all the way. So the voltages around Q202 would be interesting.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 05, 2021, 08:19:12 am
content: Quote from: Dr. Frank on May 04, 2021, 01:21:11 pmThank you for that link. I hadn't come across that particular thread yet.Quote from: Dr. Frank on May 04, 2021, 01:21:11 pmOk, I will work on that and post later.Quote from: Dr. Frank on May 04, 2021, 01:21:11 pmI have measured on both 2W and 4W settings and the results are approximately the same.Auto 7.861 mA100Ω 7.924 mA1kΩ 8.516 mA10kΩ 8.541 mA100kΩ 8.494 mA1MΩ 8.490 mA10MΩ 8.509 mA100MΩ 8.507 mAThe current readings are approximate as the current starts high and gradually sinks by a few 10ths of uA.The following errors are reported:+612,"Ohms 500 nA source failed"+613,"Ohms 5 uA source failed"+615,"Ohms 10 uA source failed"'+617,"Ohms 100 uA source failed"+618,"DC high voltage attenuator failed"+619,"Ohms 1 mA source failed"Quote from: Dr. Frank on May 04, 2021, 01:21:11 pmI am certainly hoping that the problem is elsewhere and not U101....Quote from: Kleinstein on May 04, 2021, 01:34:57 pmYes, the current is much higher than it should be on all ranges. The voltages around Q202 measured with reference to AGND are:Q202 (01H - MMBF5460 - P-Channel general purpose amplifier)G 12.80D 8.15S 4.38So yes, there is some 12-13V from the AD706 but evidently the FET is not turned off. The 18V feed is present at R290. There is a 0.78V drop across the diode and the R290 tests good. Its a P-Channel FET so as you point out, a positive voltage at the gate should turn it off. Looks like Q202 ought to be the next candidate for replacement.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 05, 2021, 08:33:19 am
content: The gate voltage at Q202 still looks quite low. It looks like the current from R290 frows towards the gate. The zener diode should not conduct much with a low voltage, at least not to lower the voltage.Before odering new part, I would also check Q203-Q210 in circuit. There is a chance they can fail from too high external voltage. This should not cause the extra current from the ohms source, but it could be a path on how to damage Q202.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 05, 2021, 09:07:47 am
content: Hello,these 8mA can only be delivered by R290 and U201-B and only if Q202 is damaged, i.e. the Gate is conducting towards its Drain.Maximum current delivered by U101-E, from the 5k resistor would be 3.6mA.Therefore Q202 is damaged probably by overvoltage, so you should check the HV protection transistor array Q203 .. Q210, Q211 and CR202 as well.Frank

author: Keith956
author_rank: Regular Contributor
timestamp: May 05, 2021, 01:09:51 pm
content: I would check the +7 REF is OK, if that's out then the whole ohms current source will be wrong.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 09, 2021, 05:08:05 pm
content: The reference voltage is 6.927V so pretty close to 7V. Not sure how far out it would need to be before obvious problem show. Voltage and current readings are spot on.I checked the HV protection transistor array. Q203, Q204 and Q210 have at least on junction with resistance across it in revered bias mode. The remaining ones test good. Resistors R203 to R206 appear to be fine. Diode CR202 tests good. I am uncertain about the Zenner DR201A. Q211 is impossible to test in-circuit. I have been trying to work out the purpose of that arrangement?A short, high voltage event would seem to be a reasonable explanation.I will be ordering some replacement parts in due course.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 09, 2021, 09:29:10 pm
content: CR202 acts as a protection against a high positive voltage when in Ohms mode. That is the easy part.Q203-Q210 arct a bit like a high voltage darlington. R203-R206 divide the votlage approximately equal over the transistors (actually not very accurate as the base current is neglected). Under normal conditions (no negative external votage) Q211 does no conduct and the votlage drop over the transistor string is relatively small (~1-2 V).With a larger neg votlage at the terminal Q211 will start conducting and will limit the votlage for the emitter of Q203 to some +0.5 V or so. So the actual current source aroudn Q202 will not see the high votlage.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 10, 2021, 03:59:49 pm
content: Q202 (MMBF5460) is proving rather difficult to source and showing as no longer manufactured at main suppliers such as Farnell. The only UK source, Littlediode, charges an extortionate £5 + postage for a single unit - and I rather suspect they are just re-packaging from China (generic graphic, no brand etc)! I am not sure whether to trust the cheap ones from China on eBay. I saw some branded NOS from Israel which would cost £13 to obtain with postage and + unknown tax, so still not cheap, but at least it is 10x rather than just one piece.Has anyone successfully tried any alternatives? The IDSS current of 1mA min and 5mA max is proving impossible to match. How important is this? I have seen situations e.g. in oscilloscope inputs where the IDSS can be quite critical.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 10, 2021, 07:24:51 pm
content: Sorry, no, haven't tried an alternative.. maybe I need to do that in a few days.The 34401A and the identical 34970A schematics both indicate a 2N5461, sel., i.e. IDSS= -2 .. -9mA.I assume, that only the minimum current parameter is crucial, as at least 1mA is required for the test current. I would estimate, that hp selected them for 3mA at least.Maybe it can be replaced by a p-channel MOSFET, as this is drawn alternatively in the 34970A schematic.FrankPS: Maybe the J175 / J176 could be a direct replacement, as these are in series production and cheap.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 10, 2021, 08:08:03 pm
content: The OP can provide quite a large voltage range. So this should also work with a MOSFETs.A much larger JFET may compromise the protection when the supply is off. However normally then the relay should also be diconnected.I also don't see an obvious reason why a different p-JFET like J176 should not work. It is larger current, and thus a little more leakage, but the 34401 is not using such a small Ohms test current.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 11, 2021, 05:24:20 pm
content: I am going to try a J177 that I have been able to source (pmbfj177, 1.5mA to 20mA). The MOSFET is an interesting observation as well which I will keep in mind should the jfet not work out.Does it make sense to replace all eight transistors in the Q203-Q210 array, or just the faulty ones? My thinking is that although the remaining ones have not failed, they will have been stressed. I have had to order 10-off anyway, so it makes little difference how many I use.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 11, 2021, 05:52:56 pm
content: Chances are the others are still OK. With these transistors it is not about leakage or noise. Failure was likely from exceeding the SOA from high votlage and than more thermal.So chances are the other are OK and tested.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 12, 2021, 09:48:59 pm
content: Hello,today I got my 'new' 34970A on the desk, similar Ohm mode fault.The AD706 very probably is broken, as the voltages on the 2nd OpAmp pin 5, 6, 7 are similar to the defective 34401A here:https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg489687/#msg489687.The transistors of the +/- inputs are obviously blown. Characteristically, the +input is on a lower potential than the -input, but the output on pin 7 is positive, instead of being negative.As this is the first instrument ('401A or '970A) which I have to repair myself, I found out that the defective inputs obviously drive or draw excessive currents, because I measured an excessive current out of pins 33, 35, 36, 39 of U101-E, which could only be induced via pin 40.This aspect was new to me.Your replaced AD706 seems to be fine, as the voltages on pins 5, 6 are nearly identical, and the output seems also to be reasonable. Therefore, the j-FET seems most probable to be the root cause, as a few of the Q203 .. Q210 were also damaged.But anyhow, according to Murphys law, I propose that you should de-solder the j-Fet quite carefully so to be able to reuse it, in case your already replaced AD706 is also defective, and might show a different defect characteristics at the moment, compared to the usual ones, as described above.Frank

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 13, 2021, 10:52:34 am
content: The FET has been replaced, as have the three faulty transistors in the array.There is now no current whatsoever between LO and the output from the array.The array tests good except that Q209 seems to be showing about 2.7 volts across B-C in the reverse direction on diode test when all other transistors show infinity on both junctions. Transistors Q203, Q204 and Q209 were replaced as they showed anomalous readings. Q210 has also now been replaced for good measure so I am puzzled by this reading.Voltage readings are as follows:AD706:-1. 4.082. 6.923. 6.924. 05. 8.7746. 8.8007. 2.8628. 18.88Q202 (01H - MMBF5460 - P-Channel general purpose amplifier)G 7.568D8.284 0S08.284Voltage across the Zenner CR201-A is exactly 4.70V.Full voltage reading details are contained in the attached image.Since there is now no output from the FET it would whereas there was an excessive output previously would seem to indicate that the original FET was faulty, but wouldn't there have to be a negative output from U101 to turn it on and produce current.The ohms reading now fluctuates randomly unless I short the leads when it shows something like 0.2ohm.This now appears to be the classic AD706 failure mode, although that IC has already been replaced. It is likely to have failed again?

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2021, 12:01:42 pm
content: Hi,it may be really helpful, if you would measure the voltages and write them into the schematic , like in the other thread. It's quite confusing to analyze this way.Also, you obviously swap the measurements of Drain and Source voltages of this FET, as Source goes to the ASIC, at about 8..9V and Drain is the output and should have a low voltage.pin5 and 6 should be identical, if the circuit works correctly, but pin7 seems to be about the correct output voltage to drive the FET open, as its Gate is at about 8V.Maybe the transistor array is still not working correctly.You can check the current source by an external DMM, current range, if you connect minus to the low input of your 34401A (4W Ohm mode), and the current plus in mA range directly to the Drain of the FET, i.e. the output of the FET, pin2 and Q210, pin2.Frank

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 13, 2021, 12:03:08 pm
content: Thanks. I was working on it after the last post. I have attached the result.I checked the output current of the FET by directly measuring it from the drain as suggested but am still seeing no output.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2021, 12:32:32 pm
content: Have you tried another range?the new FET seems not to work properly, as its Gate voltage is too low, in conducting mode.have you selected the FET somehow, for minimum 2..3mA for example?the current seems to be sunk by the opamp, instead of output to the jacks

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2021, 01:08:15 pm
content: The JFETs seems to be not working correnct - maybe broken or wrong pin connection (as D and S are essentially symmetric it would be that gate that would be wrong).The voltages for the OP make sense, so likely no porblem there.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 13, 2021, 03:03:07 pm
content: Thanks.Quote from: Dr. Frank on May 13, 2021, 12:32:32 pmIf you mean the different resistance ranges on the meter, then yes, the measurements are with the DMM set to Auto, but I did try all of the manual ranges with the same result.The J177 is in the same line as J175 and J176, just has a minimum IDSS down to -1.5mA whereas the minimum for J176 is -2mA and J175 is -7mA. I have made no selection other than to choose the J177 because it goes to -1.5mA. I had to purchase a minimum quantity of 10 so I have spares available and could try another one from the pack?BTW, Dr. Frank, does the 34970A manual list the alternative p-channel MOSFET part number please?Quote from: Kleinstein on May 13, 2021, 01:08:15 pmI did check to make sure that the pinout of the J177 is the same as that if the original part.Quote from: Kleinstein on May 13, 2021, 01:08:15 pmThat is good to note.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 13, 2021, 03:31:27 pm
content: unfortunately, no alternative part mentioned.this CC circuit might have a parasitic modus operandi, which will depend on the jFET characteristics. maybe you select your FETs for different pinch off and zero GS currentFrank

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2021, 04:32:23 pm
content: The source voltage should be at some 9 or 12 V depending on the range for the current source.So a depeletion JFET would need a gate votlage from some 15 to about 9 V.An emhancement MOSFET should need some 6-11 V.With the current zener diode the OP should be able to work from about 7 V to 17 V. With MOSFET one may have to change the zener to a slightly lower voltage one.There are not that many small THT p-MOSFETs to choose from. I don't think the parameters should be that critical. Maybe avoid a modern type with low limit on the gate voltage and possibly a little more gate leakage. A rather high high threshold could be a problem to.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 13, 2021, 06:12:06 pm
content: I think I rather overlooked Q211. I figured since source and drain where shorted it was impossible to test in circuit and didn't check between gate and source/drain. This measures 80Ω in both directions on resistance and 0.08V on diode test which presumably cannot be correct. A leak/short would pull Q204 base to ground which would presumably be enough to shut down the array? Initially I will just remove Q211 to see whether that makes a difference but I don't think this would explain the absence of current from Q202? Still, its another item to sort out. I do have some J390's to hand but the operating voltage is 25V rather than 40V for the MMBF4117.I will have to give the Q202 FET issue some thought and maybe order those NOS MMBT5460s from Israel. At least then we know its the correct part.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2021, 06:44:22 pm
content: Q211 is uses as a low leakage diode. It does not absultely have to be a JFET. A low leakage diode would work as well. They just selected the 2N4117 because it is sopposed to be tested for low leakage. A different FET may not be tested as well. For a substitute one could consider the collector - base diode of the jmall BJT like 2N3904 or BC548.For a curde test to see if the rest is OK one could also consider a PNP transistor instead of Q202: it should also operate the current soruce, just not very accurate.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 14, 2021, 08:51:31 am
content: Quote from: WaveyDipole on May 13, 2021, 06:12:06 pmIt's strange that the voltage at the Drain of Q202 (pin 2) is exactly 0V, maybe something is still shorting it to AGND. Q203, Q204 and Q211 might be short, so if you remove Q211, it should work.In the BOM of the 34401A and the 34970A, there's definitely a 2N5461 required, as the 2N5460 has 1mA IDSSonly.The cutoff voltage should be maximum 4.5V, better 2V only, to be selected from the batch, otherwise the FET will not be able to operate properly in the 10MOhm and 100 MOhm range.Frank

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 14, 2021, 10:21:36 am
content: Quote from: Kleinstein on May 13, 2021, 06:44:22 pmFrom the datasheet the Idss of the 2N4117 is only -30-90μA and Igss -10pA compared to the J309's -2-25mA and 1nA. so I take your point about the part in this position having to be a low leakage part! Thank you for pointing it out. Makes sense given the tiny currents involved, especially on the high resistance ranges.I do have a question though. You mention the base-collector junction specifically. The circuit board shorts the S and D pads as per circuit diagram with G opposite:G-----S--DIf a BJT were substituted as you suggest, it would have to have B on one side and C + E on the other.B-----C--E (or E--C)If soldered in directly, both junctions would be in play so in this case, the E pin would presumably need bent up out of the way or cut off but either way disconnected leaving only the B-C junction in circuit? Also, am I right in thinking that B would connect to the G pad? My thinking is that for an NPN transistor the base connects to the P material, as does the gate for an N-channel FET?So far, I have checked the datasheet for the 2N3904 and the SOT23 pinout has C at the top so might not be a suitable physical fit. I can't seem to find the BC548 in a SOT23 package but so will continue to search for something that might fit. You mention that another FET may not have been tested as well, but, in principle, is another N-Channel FET with similar Idss/Igss characteristics likely to be suitable?Quote from: Dr. Frank on May 14, 2021, 08:51:31 amQ211 has been removed and confirmed leaky between G and S/D. However, unfortunately there is still no output from Q202.Quote from: Dr. Frank on May 14, 2021, 08:51:31 amThank you once again. Embarrassingly, I had noted down and subsequently incorrectly referred to Q202 throughout as an MMBF5460. Having just re-checked the BOM list after your last post, I now realise that it should, in fact, be an MMBF5461. The only explanation I can think of is that I had mis-read the 0 from the line below, besides my mind is very much distracted at the moment with caring for an elderly parent who has been in and out of hospital several times over the last few days. Whatever the case, this has led me down the the path of a part with a slightly different specification.Unfortunately, I have the same problem with MMBF5461 as I had with MMBF5460 in that both show as no longer manufactured and I can't find the SMD package anywhere. It is available as a NOS TO-90 package, but again, the J176 is readily available.So, the bottom line is that I now need to do my homework and find and order something suitable and low leakage in place of the 2N4117 and get a suitable substitute for the 2N5461.Quote from: Dr. Frank on May 13, 2021, 12:32:32 pmQuote from: Dr. Frank on May 13, 2021, 03:31:27 pmAre we talking about a paper based excecise, i.e. looking at datasheets, or some kind of testing?If datasheets, then what parameters am I looking for?The all seem to have a range for both Idss and Igss, e.g. the one I chose has Idss between -1.5mA to -20mA.I anticipated that 2..3 mA would be covered within that 1.5-20mA range?Does this involve some kind of testing to select a particular FET from the 10 that I received in the batch?Probing loose SMD parts is particularly problematic as they tend to ping off in all directions never to be seen again....

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 16, 2021, 09:48:05 am
content: Farnell still have a stock of 2SK545 which has a higher min Idss (50uA vs 30uA) but has a similar max Idss (95uA vs 90uA). The Igss is much higher though at 500pA vs 10pa so I am wondering whether it is a suitable replacement for the MMBF4117? It seems to be the only other FET I have found so far with Idss measured in uA rather than mA.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2021, 10:03:38 am
content: Using a BJT as a substiture low leakage diode, the gate of the 4117 would correspond to the base of a NPN or collector of a PNP to get the same direction.The BC548 is the To92 case version - the SOT23 equavalent is BC848. Other small type should be similar.From the usual pinout a PNP would likely be easier.An alternative would be a low leakage diode BAV199 - not tested by the manufacturer, but typical very low leakage.MMBF4117 (or 4118, 4119 - just with higher threshold) are usually still available relatively cheap. However I am not sure if all manufacturers actually test them for very low gate leakage.

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 16, 2021, 12:28:10 pm
content: Quote from: Kleinstein on May 16, 2021, 10:03:38 amThank you for the confirmation and further detail concerning the PNP device.Quote from: Kleinstein on May 16, 2021, 10:03:38 amAh, different number. Thanks. I see that these, along with the previously mentioned MMBT3904s are readily available.Quote from: Kleinstein on May 16, 2021, 10:03:38 amAgain, these seem to be readily available.Quote from: Kleinstein on May 16, 2021, 10:03:38 amHave been really struggling to find MMBF4117 anywhere. They come up as "No longer manufactured". I did find a NOS supply in the USA on eBay, but the cost was for 50-off and with considerable postage and import duty costs. The total cost would come to somewhere near 50GBP. If you or anyone is aware of any other source I would be interested to know.The diode seems the most favourable and convenient in the circumstance. It fits the profile and has two diodes in series with the common at pin 3 which corresponds to the G position. It would be just a matter of soldering pins 2 (cathode) and 3 (anode) and cutting or lifting pin 1 (anode) to leave it disconnected.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2021, 04:09:21 pm
content: Mouser still has some mmbf4117 in stock and is supposed to get new ones in august, with no sign of an EOL. Similar the european site TME.eu still has some in stock at some 25 cents, but maybe 10 GBP for postage.For the low leakage diodes one has a typical / expected leakage level and a test limit that can be quite a bit higher, as testing to very low current is slow and thus cost money.The original 2N4117 are supposed to be tested to a 10 pA (or similar) limit. For the mmbf4117 they still give the same limit, though I am no so sure they actually test them all at the low price. The SK545 may be a very similar chip, just not tested to the same limit but a much higher 500 pA. Still chances are low to find one with a leakage current higher than 50 pA.For the expected / typical leakage current I would expect the BAV199 to be lowest - after all it is made just for low leakage. The mmbf4117 is probably similar.The BJTs leakage is likely slightly higher, though very likely still good enough ( e.g. < 10 pA). For a quick test one can measure the CE leakage, which is higher by a considerable factor.There are also special low leakage diode tested to low pA levels (e.g PAD5) - but these are expensive an internally they may be 2N4117.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 17, 2021, 05:34:45 pm
content: I've successfully repaired my DAQ34970A:https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg3571485/#msg3571485AD706 was broken, as usual.You will definitely need a junction FET for Q211, as the BC diode of a bipolar transistor usually breaks down (zener) at about 5V.. but the current source may deliver up to +13V to the transistor array.I used a certain FET to improve the Ohm mode protection circuit of the 121GW multimeter, which could withstand 25V. Please search for that thread..I assume, that you could also use a p junction FETs, when reversing the polarity inside the circuit.Frank

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 18, 2021, 01:51:17 pm
content: Yey! I have it working!Additional FETs arrived today so the J177 previously soldered into position Q202 was replaced with a J176 (MMBFJ176) as was originally suggested back in your posts on the 10th May. Since I am still waiting on the arrival of MMBF4117s, Q211 is currently not present in circuit.On power up, the meter performed its boot-up routine as usual and the resistance function was now working and the continuity/diode test function was now returning sensible results as well. I measured a handful of resistors from ohms to meg-ohms all of which returned appropriate results. The test current levels for each range were checked and seem to be in the right ballpark.There was still one error present:620 - AC rms zero failedChecking the code in the service manual, it states "The internal residual noise of the ac section is measured and checked against a limit of –10 mV to 70 mV at the output of the rms-to-dc converter."The meter was on the bench, opened up and shields had not been replaced. Also the LED bench light, which is known to generate some noise, was turned on. It occurred to me that perhaps the meter was picking up EMF from the environment and sure enough, when the TEST ALL sequence was performed again with the lamp turned off and the case covering the chassis, the test sequence returned a PASS! Success!As it turns out then, this repair required a little more than just replacing the AD706. In addition, at least 3 transistors in the array Q203-Q210 had failed along with the FETs Q202 and Q211. Fortunately it looks like U101 was still fine.On the Fluke 187, the 100Ω 1% tolerance resistor being measured in the photo returned a reading of 99.91Ω. The Brymen 869s returned a reading of 99.79. The reading on the 34401 falls between the two which looks sensible. At this point I have only carried out 2W measurements but once the MMBF4117s arrive, the repair can be completed and the meter tested thoroughly on all ranges in both 2W and 4W modes.Thank you Dr Frank and Kleinstein for helping me get to this point. Initially ordering an incorrect FET notwithstanding, I am nevertheless thrilled to have the meter fully operational again.

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 18, 2021, 02:24:24 pm
content: Hello WaveyDipole,it's always a pleasureto do such remote failure analysis, which by itself is not easy, due to limitations in communication and lack of direct measurement investigation..This time it was even more thrilling, as more components were affected by a real HV overload, and not only this boring AD706 deterioration.So, congratulations and have fun with your fine piece of equipment!Frank

author: WaveyDipole
author_rank: Frequent Contributor
timestamp: May 21, 2021, 01:48:25 pm
content: Just as a final postscript, the MMBF4117 arrived today and has been installed. The Ohms function was re-tested and continued to work as expected. The ohms source current was measured again to make sure installation of the new FET had not affected anything and the measurements were found to be correct on all settings. The meter was then re-assembled and a final self-test performed which returned a PASS. Lastly, all functions were briefly checked and found to be working correctly. The repair is now complete.