HP 3478A Multimeter - Parts for overvoltage protection circuit - Page 1
Forum section: electronics / repair

author: 11mrjones
author_rank: Contributor
timestamp: August 17, 2024, 10:23:16 pm
content: Hello all- First repair of any type of instrument and I need some advice on replacement parts. Meter had a problem with 3K ohm range not reading accurately and unable to successfully cal it. Found the problem was the ohm current source being pulled down by the over-voltage protection circuit. The schematic is attached. So may need to replace any or all transitors Q201-Q205. From the parts list in the service manual. I can see Q201-Q204 are 2N6520, so no problem there (see attached parts list).However, I am having trouble identifying Q205, it is supposed to be a JFET n-channel, depletion-mode device. The parts list gives just the H/P P/N as 1855-0298. Looking at a HP P/N cross-reference (https://xdevs.com/doc/HP_Agilent_Keysight/HP%20Master%20Xref.pdf) it looks like it would be either: 2SJ2472 (a jfet p-chan), MBR2545CT (a schottky barrier rectifier), MJ12005 (a npn xtor, hi curr, hi. voltage). So none of these are a JFET n-channel, depletion-mode device.On the Q205 physical transistor itself, markings are just last digits of the HP P/N, ie, National Semi Logo and 204 5 5 0298.Does anyone know the generic part number for Q205 , or what I can use as an available replacement for it? I can't find the datasheet for it since I don't know the P/N, so I can't get the specs to try and find an equivalent replacment either. In the end, testing it in circuit looks like it is ok, but I would like to order a Q205 part (and replace it with new) as I need to order Q201-Q204 (p/n 2N6520) anyway.Thanks for any advice/help.Victor

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 17, 2024, 11:41:10 pm
content: This fellow has some HP and Tek Parts:https://www.eevblog.com/forum/profile/?u=141173;area=showposts;start=75But I do not think he has your JFET. I would try a common JFET in this position. If you get too much "leakage" then we can look for another low leakage Jfet. The transistor is used as a diode and is probably a low leakage part since they did not use a common diode.

author: pqass
author_rank: Super Contributor
timestamp: August 18, 2024, 12:22:15 am
content: I couldn't find a datasheet for J2472 nor a substitute. I just found a product obsolescence notificationhere.You may be in luck as there still could be new-old-stockhere. The owner passed away and the site is winding down.If that fails, the same protection circuit is in the 34401A and a lookup of theschematic(9-10 or pdf page 153) and parts list (page 129) shows: "Q211 ... Transistor J-FET N-Chan D-Mode ... MMBF4117A". Its still available from Digikeyhere.IMSAI Guyvideo #1818 (sort of) explains how the circuit works.

author: squadchannel
author_rank: Frequent Contributor
timestamp: August 18, 2024, 12:50:58 am
content: There are still NOS products on fleabay.https://www.ebay.co.uk/itm/233385727562

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2024, 05:34:51 am
content: The JFET is used as a low leakage diode. So the exact type does not matter. The MMBF4117/8/9 should be OK as small and low leakage types.There is no need for a specific, NOS type.If THT is needed one could also consider a dedicated low leakage diode (may need a cover when in glass case).For a quick test one could also use the base collector junction of a transistor (e.g. 2N3904, BC547,...) as a relatively low leakage diode.

author: floobydust
author_rank: Super Contributor
timestamp: August 18, 2024, 05:35:44 am
content: All I could find is: HP 1855-0298 Siliconix/TEMIC J2472 J-FET N-ch VGS(th)=3.0V, IDSS500uA... which looks to be in the PN4119A league.Same part in others like 34401a is 2N (PN)4117A.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 18, 2024, 12:09:45 pm
content: As I alluded to before, I had a similar problem. Kleinstein did help. Here is a link to the thread:https://www.eevblog.com/forum/repair/transistor-jfet-hp-equivalent/msg5521660/#msg5521660floobydust has it right about the FET.....I usedThompson 2N4117 FETSas clamping diodes. I was able to get them on ebay.They have four leads, one for the case, which I left flying.

author: factory
author_rank: Super Contributor
timestamp: August 18, 2024, 12:39:53 pm
content: From the HP microfiche that CuriousMarc scanned.David

author: squadchannel
author_rank: Frequent Contributor
timestamp: August 18, 2024, 01:15:09 pm
content: I too looked up curiousmarc's fiche. but i could not find it.It needs to be in PDF and OCR...

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2024, 03:43:33 pm
content: From the data it looks the HP fet is kind of a JFET from the 2N4117-4119 family, not selected for the threshold, but for low leakage. So kind of a selection for diode use.

author: floobydust
author_rank: Super Contributor
timestamp: August 18, 2024, 05:46:38 pm
content: Isn't the IDSSimportant? I see it as lost current as the output stage is after the CCS, so it would be corrected for?Weakness in these circuits is they wrongly assume there will never be a -ve overvoltage seen, only +ve which CR201 blocks fine.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 18, 2024, 07:25:49 pm
content: Idss is not important for a JFETs used as diode. The current that is lost is the gate leakage current. Even here it is only the change in the current (e.g. with temperature) that really matters. Any constant part would be included in the calibration.R_on may be a littler as a small part of this will be in series to the diodes. Still the JFET as diode will only see a low current (e.g. 1 mA range) from the 240 K resistors.Of cause there is also protecton against a negative voltage. In this case Q205 will start conducting and Q201-Q204 will more or less turn off, so that most of the voltage will drop at the BJTs and not get to the MOSFET for the regulation.What may be a problem are fast transients from positive to high negative, as the transistors may not be that fast to turn off. Also CR201 may not be that fast blocking.

author: 11mrjones
author_rank: Contributor
timestamp: August 18, 2024, 10:17:20 pm
content: Thanks for all the great advice everyone. So I'm going to:1. inquire about orig 1855-0298 (Sphere Research, if available). per pqass2. order a couple 2N4117 and 2N4119 (Digikey). per Flooby and Wallace3. Try BC jcn of BC547 (I actually have some of these) and look for some low-leakage diodes on DIgik to try for kicks per Kleinstein.Background fyi:The 3478A was bought on the bay and seemed to be working well. A while late I noticed for the 3K ohm range, at approx 2.5-3.0K ohm load the meter read low by about 10% and was not able to successfully finish cal on the 3K ohm range.On the 3.0K ohm range for load resistor = 0, 1, 2, 2.2, 2.5, 2.8, 3.0 K, I measured 1, 1, 1, 0.99, 0.95, 0.93, 0.85 mA, respectively, for the ohm current source (the meter read the resistor value low approx by the same %). All the other ohm ranges worked perfectly, with the current source constant over 0 to fullscale.A shorting clip applied between Q201 emitter and CR201 anode restored the ohm current source to 1mA across the whole 3K ohm range and proper working order and successful cal. I'm guessing one of the transistors is not completely bad but is borderline, but I'm no expert.In the meantime I replaced the 4 Rifa caps, the 4 big electrolytic caps, and battery (temporary battery in place during R/R so as not to loose cal data). So I should be good to go shortly.It will be a while till I get some parts but I'll let all know how it all ends up. Again big thanks for everyone's help, such a great community here.Victor

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 18, 2024, 11:29:22 pm
content: I had a very similar experience with a 3456. The high ohm readings were bad. There was a big low leakage diode in the 3456 over voltage circuit and that diode was bad. The Service Manual was very helpful.I think someone had connected this unit to lots of volts while in resistance mode.There were other FETS that I replaced and HP used FETS at several places as clamping diodes. I also replaced the Pre Charge FET., Used 2N4117A. Now I have two 3456 that agree with one another and have been calibrated to a volt standard that I have had professionally calibrated. (I bought it from Sphere)

author: pqass
author_rank: Super Contributor
timestamp: August 19, 2024, 01:28:52 am
content: @11mrjonesFYI:You can extract the calibration data from the 3478A via a GPIB adapter; either commercial or even an Arduino UNO-based (AR488 project).Seehereand follow the links.

author: 11mrjones
author_rank: Contributor
timestamp: August 19, 2024, 03:40:05 pm
content: Thanks pqass, I'll check it out

author: Kleinstein
author_rank: Super Contributor
timestamp: August 19, 2024, 03:55:34 pm
content: If the problem is with the 3 K range and not the higher ranges like 300 K, the diode used FET is likely not the problem. Its more a thing of the BJTs Q201-204, like one of them shortet / lost collector connection or too low gain.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: August 19, 2024, 10:31:59 pm
content: In my 3456, the ranges affected were the ones with the highest volts used for the resistance measurements and the problem was definitely the diode in the over volt circuit.

author: 11mrjones
author_rank: Contributor
timestamp: August 20, 2024, 01:02:44 am
content: Well an interesting development. I removed Q201-Q205 and replicated the protection circuit on an external breadboard. Was planning to swap out individual xtors to see what happened.With a simple multimeter diode mode test all xtor junctions appeared to be normal in circuit when tested the previous day. When a removed them all and tested device junctions out of circuit today, again nothing abnormal. I checked both forward and reverse bias for all junctions and all were ok.So I then built the protection circuit on an external breadboard (see pic) with all the original xtors and after meter power on everything is working normally, ie, no shorting clip to bypass the protection circuit required to get proper readings.Perhaps as Kleinstein suggested a collector lost connection, cold solder joint?, i definitely dont know.Should I just resolder all the xtors back in place and call it a day. Or should I run some tests on the various xtors while I have them out before putting them back. What is advisable?See attached pic.ThanksVictor

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2024, 09:22:09 am
content: It could also be one of the 240 K resistors that went too high. With the transistors removes one should be able to check the resistor in circuit now.While still on the breadboard one could check the voltage drop at the protection with the 1 mA and maybe some 1.5 mA test current.A check of the transistor gain could also be a good idea. The transistors work as pairs, and swapping them could have avoided using 2 weak ones together.I would consider butting the transistors back, and maybe get new ones only when ordering other parts anyway.

author: 11mrjones
author_rank: Contributor
timestamp: August 20, 2024, 11:12:09 am
content: On initial checks when first looking at this the resistors measured per the schematic in circuit and also the same now with transistors removed.Regarding swapping transistors and possibly avoiding using two weak ones as a pair, I rebuilt the external circuit exactly as it was in the meter in terms of transistor placement. That is, Q201-in-circuit => Q201-external-board., Q202-in-circuit => Q202-external-board, etc. (The resistors were not removed, so the external board has new resistors from inventory parts.)I will see about measuring voltage drop and transistor gain .Thanks,V.

author: 11mrjones
author_rank: Contributor
timestamp: August 21, 2024, 04:39:32 am
content: Measured Hfe was 81, 79, 94, 90 for Q201, Q202, Q203, Q204, respectively (at Ic=10mA, Vce=10v). Spec is 30-300.Also measured voltage levels of protection circuit with a 3k resistor load. See attached pic. Two voltage levels are given corresponding to 1.0 and 1.5 mA test currents. For example at test current 1.0 mA Ve(Q201)=5.3v and at 1.5 mA test current Ve(Q201)=7.0v.ThanksVictor

author: Kleinstein
author_rank: Super Contributor
timestamp: August 21, 2024, 04:57:08 am
content: The measurements look good and even with the higher 1.5 mA current there is not much voltage lost at the protection. Chances are the defect before was something like a missing link or similar that no longer there. Even if the error would happen again, it would not be a damaging defect, but only wrong readings. Chances are one would want to have a reasonable good 3 K resistor at hand for an occasional check anyway - with the old parts as well as with new ones.

author: 11mrjones
author_rank: Contributor
timestamp: August 21, 2024, 05:40:16 am
content: Well thanks for all your advice, looks like I'll put things back in place and include some replacement parts in any next digikey order for backup.Thanks everyone else also.V.

author: 11mrjones
author_rank: Contributor
timestamp: August 21, 2024, 09:00:46 pm
content: Resoldered all transistors back into meter and all works proper.Bad joint somewhere initially? I don't know.Is the lesson here to reflow all the joints as first step even if the look good?Anyway , success, job doneThanks allV.