Hints for fixing a 34465A? (blown input?) - Page 1
Forum section: electronics / repair

author: K
author_rank: Contributor
timestamp: February 24, 2025, 10:38:56 pm
content: Hello!Recently I came into a posession of one "not working, for parts" 34465A, with the obvious problem of a smashed-in screen. The unit appeared to turn on and click the relays, as they do, and I took that as a sign it is not completely dead. So I went on and opened it. Found that whatever impact the screen got smashed by also broke the ferite loop on the display FPC cable, but that likely is not super critical right now. I replaced the broken screen with a generic 4.3" one I had. After partial re-assembly and power-on the new screen appears to be working as expected.Alas, the problems are not over. The instrument fails the "full" self-check (see photo of the fault list) - there is a slew of voltage and resistance autocal errors e.g. "autocal 1V range full scale", "autocal 10kOhm range reference".Now it does not look quite so good anymore, eh.Starting from the top, I checked all the voltages from "Keysight Truevolt Series Operating and Service Guide" - those appear to be in spec. Nothing gets hot to touch either.With a sigh I powered off for a careful second look.One thing that I immediately noticed is that main board has 34460-... etched on it, but also has a paper label with 34465-..., in the corner. The display PCB has a label with 34465-... on it (photos). Not sure what, if anything, it means. Do I have an early revision/unit? Maybe all 3446*s have the same main PCB? Or is this instrument just two not-working pieces put together? FWIW the brains in there do think itself a proper 34465A, on the "about" screen (see photo).Then, somewhat even more disheartened I took out the magnifier and went over the main PCB. Sure enough - found a blown via and some destroyed components near it on the main PCB, both sides (see the photos). Also noticed there actually is a blowout mark on the bottom of the cover as well.To my understanding the area where the damage is related to input switching - as are relays close. The destroyed components are 3 inductors and 3 resistors. By sleuthing the PCB images on the internets I am quite convinced I have deduced the nominal values for the completely destroyed resistors and that one blown inductor.Catalogued carnage:Destroyed inductors:- top side - L201: 10uH (blown case, 470 ohms?, nominal value from internets)- top side - L202: 1uH (broken case, open, marked 102J)- top side - L203: 1uH (broken case, open, marked 102J)Destroyed resistors:- top side - R201: 316ohm (appears to be intact, but is parallel with L201, and measures 470ohms, marked 3160)- bottom side - R207: 3.16kOhm (blown/charred, open, marking unreadable, value from internets)- bottom side - R208: 3.16kOhm (blown/charred, open, marking unreadable, value from internets)Although there is no schematic for 34456A on the internets, I figured that Keysight likely did not invent the same horse twice and went on to look at schematic for a 34410A. In that schematic, on page "FUNCTION SWITHCING" there are 4 relays (albeit one is a different model), and some paralell/series inductors/resistors, with same values as I have (see attached schematic fragment). So it kinda makes sense. Also somewhat good news is that the blown via does not (should not) have any hidden traces or ground/reference planes.Also tested the relays, as they are in the supposed line-of-fire. Relays are of "double-winding latching type", with two sets of contacts. All but one tested ok - 0.2ohm one side, open on the other. One side of the supposedly not ok one (K204) had 104ohm on one side and 0.2ohm on other (see attached picture for placement on main PCB).I could somewhat easily replace the destroyed components and fix the via - although there is an uncertainty regarding other parmeters for the replacement inductors. Seems to be worth a try at least.But then there are questions:- The main one - what caused this? Did someone do a silly and put high voltage into current/sense ports? Or did something blow on it's own?- Is there more damage deeper inside? Transistors/ICs?- Is that one relay half-dead?- And what about 34460-... business on the PCB?Found somewhat useful photos of main PCB here -https://www.jabse.com/L0FnaWxlbnQvcGhvdG8vMzQ0NjVBAnswers, hints, theories, comments are welcome.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2025, 11:17:36 pm
content: AFAIK the 34460 / 34461 / 34465 and 34470 shared the same PCB layout. There is a difference with some parts not populated and possibly also different types / grades for some parts. AFAIR the different model marked was also noted in an early teardown.The inductors have a lable with the value. They are used in the protection and are not super critical in the value. The maximum voltage rating may be more relevant that the inductance.A damages relay contact is possible, e.g. from a persistant discharge from a high DC voltage. To higher voltage to the ohm sense inputs kann absolutely make sense.There could well be more damages. It would defintely check the transistors in the ohms current source protection.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 25, 2025, 12:23:24 pm
content: You can find hi-res photos of all versions, comparison of the different models in this long thread from 10 years ago:https://www.eevblog.com/forum/testgear/keysight_s-new-34465a-(6-5-digit)-and-34470a-(7-5-digit)-bench-multimeters/E.g. page 5 and 7 . You should be able to find the correct component values there.Dave still provides these on his server.Yes, the schematic of the 3446x/470 probably is very similar to their predecessors 410/411, as they at least share the same MultiSlope A/D version.All your errors are very helpful to finally understand the ACAL method in the 465/470FrankPS: you have FW 2.14, which contains a lot of bugs.Please upgrade to 3.1, or the latest version, which will then give additional memory at least, maybe also the digitizing function.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2025, 01:15:18 pm
content: The ADC is very similar with the 34410, but the signal switching is different: the 34410 still uses custom switching chips, while the 34465 and similar are using more standard CMOS switch chips.There are a lot of ACAL error messages - possibly most steps failing, which would give a list of the ACAL steps.The ACAL part is quite a bit further down the path. So the damage would likely extend to beyond the relays.It could make sense to identify the main amplifier output / ADC input as test point.

author: K
author_rank: Contributor
timestamp: February 25, 2025, 02:12:04 pm
content: I think I will start by removing the damaged components, and tack on some leaded low value resistors, fix the via, and run the tests again. If that (most likely) does not help - identify and check the transistors and switch ICs, as Kleinstein suggested. Will try to make some sort of schematic as I go on.Also will order proper replacement SMD parts and also couple of relays, just in case.If there is some kind person with a 3446*A who could check the in-circuit resistance on the destroyed inductors (L201, L202/L203), I would be really thankful as that would make choosing inductors less of a guesswork.FWIW - yesterday late I removed the shields from main PCB and measured all the marked voltage test points (low side to shield contact), and all of referency-ones - +7V, +/- 10V, +0.7V - appear to be spot on (+/- hundereds of uV) on my semi-trustworthy Solartron 7150. +/- 16V test points also measured sensible values.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2025, 05:06:26 pm
content: The inductors are in the protection. The series resistance and exact inductance should not be important. For L201 one may want an inductor that can withstand high peak voltage. For the start a slightly higher inductance should be OK. I think it is still early to order parts.Though not effecting the self test errors, the MOSFETs next to the blown inductors and U201 are worth testing. They are likely used as part of the protection. If they short out, L201 would be next to blow.Chances are that U401 is used as the main input amplifier / first buffer. For some odd reason they seem to have the amplifier near the reference.With so many self test errors is there any measurement getting any remotely right result ?

author: K
author_rank: Contributor
timestamp: February 25, 2025, 08:55:52 pm
content: Ran some tests in DCV mode with different voltages, using a lab PSU and Solartron as a control.The good news is that HighZ appears to work just fine - up to 10V, higher than that the instrument switches to 10Meg. I see the HighZ working a sign that major internals are intact. Values displayed are stable, occasionally bumping up or down the last digit.On the other hand - in 10Meg values are ~2.3 times lower, and last 3 digits are not stable at all.When shorted, in both modes, value goes down realy close to to 0V: 30uV for 10Meg, 50uV for HighZ.Tried 2-wire ohms mode - it does not work at all - jumpy negative values with no discernable pattern.I have attached a photo of my hand-noted measurements for DCV mode.Now will check the BJTs, those four MOSFETs and nearby ICs.

author: K
author_rank: Contributor
timestamp: February 27, 2025, 12:22:14 am
content: Desoldered all the damaged components and also those 4 MOSFETs and rectifier module.According to my transistor checker and DMM - all MOSFETs are ok.Rectifier module also is OK.Attempted in-circuit testing "output" side of MOSFET driver IC U201, marked "A V621" - did not find much the exact markings, but I think this is a reasonably similar spec sheet -https://docs.broadcom.com/doc/AV02-0259EN. Anyway, with MOSFEts removed, in diode mode, one way it is 678mV, the other way 773mV, both "channels". However, likely the actual innards are not diodes, and this test is not very conclusive, if any at all.Started to draw up schematic for this corner of the PCB. Discovered that the blown up via did have a track attached to it, in the inner layer. I think I found the correct relay pin it goes to (via some picture of PCB from a 34461A). To repair it a bodge wire will be needed. I will have remove the some of the relays anyway, just to check.I have attached a result of my work in this evening. There is progress, but it is slow.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2025, 08:09:52 am
content: The FETs could have been checked in circuit - they usually fail short. The circuit around the FETs looks sensible.Similar the BJTs in the Ohms protection can be checked in circuit.The photovoltaic opto-coupler should produce something like 5-10 µA and some 7-9 V at the output when activated. Some models have extra circuitry to turn off faster and this may cause the 700 mV range reading in diodes test mode. I would not expect this part to get damaged. The extra zener would protect it. One could check the function by looking at the output voltage when activated (e.g. in circuit when the path is used).The part with the 1 µH coils likely has a mistake and has inductor and resistor in parallel. The resistors are there to dampen a possible resonance. At least this is how this part is made in the 34401 and similar.For the readings with the 10 M active there should be a difference between the 10 V range, where the 10 M are are just parallel to the input and the 100 V range where the signal is taken from the divider. If already the 10 V range in 10 M mode is wrong, this would point to something like an nearly open part in the protection (e.g. bad relay contact).

author: K
author_rank: Contributor
timestamp: February 28, 2025, 12:09:18 am
content: Desoldered relays K201 and K203. Tested actuation with lab PSU (5V, 0.025mA) - both appear to be ok, albeit closed contact resistances seem a bit high - ~0.6ohms. Used "grabber" probes, maybe that adds some.Also tested, in-circuit, protection transistors in "OHMS CURRENT" section. Two of those, closer to the diode CR404 - Q410 and Q416 - measured ok in forward mode, but also had strange values in reverse. Not sure if half-dead or just shunted in some wierd ways. Will add to the order list a bunch, just in case.The diode CR404 is ok, both ways.Worked on schematic, mostly on "OHMS CURRENT" section, but am not sure if I got it right, esp. the part after protection transistors. Having a light under PCB and constantly turning it around makes it hard to follow vias.Next (tomorrow) I will desolder two remaining relays to see what happens to the trace from the other end of MOSFETs after it goes into a relay. If these relays are ok too, will put all the desoldered parts back, add bodge for the blown track, and put some THT resistors for blown resistors/inductors, and and see how it goes from there.Attached schematic so far.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2025, 10:58:12 am
content: The BJTs with the ohms part are for the protection. One could normally test them in circuit.The reversed plan is probably wrong with R421 - chances are the resistor goes to the other side of the zener. At least there should be something going off from there, to soemthing like a comparator of BJT to turn off the current source when external voltage is detected.Chances are the source of Q406 is connected to the base of Q405.I don't think the current source would go towards the LO terminal. This should be the high side terminal.

author: K
author_rank: Contributor
timestamp: March 01, 2025, 03:42:14 am
content: Removing of all relays helped to see which traces go where. Updated and final (for now) schematic is attached below.Re-did reay test setup, with properly nulled/filtered Solartron - all of the relay contacts appear to be in spec. - less than 50mOhm. As no other components were found to be dead, I put back all the removed stuff, and (temporarily!) added 1ohm resistors in place of inductors. Also added a bodge for the blown via/trace.Put the instrument back together, powered up, ran full self-test - great success! - the test passed! DCV 10M mode works as well. Aint that a glorious friday evening!Now what is left is to order proper replacement SMD parts. And also fan. It is quite loud.The blown via is "fixed" with a 0.2mm strand of wire. It is quite thin and flimsy. Not sure if that is proper enough. Nothing more thick will not go through the via hole. I guess I could carefully drill out the via, to 0.3mm, and put a AWG 22 wire in.Big thanks to Kleinstein and Dr. Frank for advice and hints.P.S. Would it be of any use to add the schematic file itself here? It is somewhat messy.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 01, 2025, 07:55:35 am
content: Just to be sure it works, it may be worth testing the protection a little. This would especially be the active turn off part that seems to be used for the Ohms current source and likely also with the 4 FETs in series.For the ohms source a test could be done in the lowest Ohms range in 2 wire mode. Measure the current with an external negative voltage (e.g. 0-12 V). I would expect the current to go down, once the external voltage goes beyond some -5 V.For the normal voltage (e.g. 10 V manual range) input one could check with a 0-30 V external voltage (maybe some 10 K in series for protection) and measure the current. The current should not go up very much (e.g. less than 100 µA at 30 V)p.s. A possibel scenario to cause the damage could be that the instrument saw some high voltage while powers off, after getting transported or subject to shock. This could bring the latching relays to an unsafe state and this way defeat the protection.

author: K
author_rank: Contributor
timestamp: March 09, 2025, 01:37:44 am
content: Got around to do the protection tests:1) In lowest range of 2-wire ohms mode (100 ohms) current, initally 1mA, went down to some 10-20uA when external voltage went below -8.4V.2) In manual 10V mode, the current went up to ~200uA till aprox. 25V, after that it went down.And, finally, the proper replacement parts arrived. Replaced the temporary THT resistors with SMD inductors and resistors, replaced the fan (the old one was quite loud) and added the ferrite onto display FPC cable.Also upgraded firmware to v3.03. Everyting appears to be running as expected.I am considering sending the instrument to Wellectron for calibration, but that can wait.Again - big thanks to Kleinstein for holding my hand, so to speak.