34401A repair - Page 1
Forum section: electronics / repair

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 10:50:15 am
content: Hi!I was waiting a good price for a failed 34401A for months, then found one on ebay sold as "used", powering on but not measuring anything in any mode/range...And got this:Tooooo bad!Burnt traces (the worst near the custom resistor network...), several component missing (realys, also the lm399 ref), missing shields, etc... i see also U105 burnt.Now i need to check if the custom resistor network is ko, and if it's worth repairing...

author: MadTux
author_rank: Frequent Contributor
timestamp: September 04, 2020, 11:07:34 am
content: Ship that bastard back ASAP!That one went like: Oops, I just toasted my DMM. Lets try repair it. Holy shit, relay, custom R-network and likely ASIC are toasted as well. Fuck it, lets find someone stupid on ebay who gives me money for it, but first steal LM399 and other good stuff.....

author: coromonadalix
author_rank: Super Contributor
timestamp: September 04, 2020, 11:21:36 am
content: yep the burn spot is under the special hp asic / resistor network if im not mistaken, that could sound like big problems for yousold as used is now a problems, people take parts in them and try to sell them, ask for a refund, if the description say parts missing ... thats another story, if not go for a refund battleEven the lm399 was socketed

author: Grandchuck
author_rank: Super Contributor
timestamp: September 04, 2020, 12:20:59 pm
content: One assumes a unit like that should be listed as "for parts only." 'Some parts missing' could also be added for full disclosure? Good luck; you deserve a refund.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 12:44:12 pm
content: If i'm not wrong this meter have the lm399 mounted on socket from the factory.If the resistor metwork is toasted there isn't much to do. Seem an impossible part to find.This evening i'll try it.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 04, 2020, 02:23:07 pm
content: the lm399 are directly soldered on the mainboard, i know, i have 34401a meters, and some google picture will show you thatcheck this threadhttps://www.eevblog.com/forum/reviews/hp34401-teardown/

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 02:49:10 pm
content: This is an old HP SN 3146AO2631, googling i've found some other with socket. And i not see traces of soldering around that area (while in other places is much evident).Also form this discussion:https://www.eevblog.com/forum/metrology/hp-34401a-hacks-and-upgrades/Seem that others here have found it socketed.The true problem is what is missing over the socketCan't wait this evening to check if the resistor network is gone. All other things seem that can be found.The seller asked me if i want a partial refound for the missing parts...Also if the asic resistor is ok, the other big problem is the lm399ah (1826-1249-5), seem that it's not a common part, it's selected by hp, and i think that is aged.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 04, 2020, 03:03:20 pm
content: Older HP units had the LM399 in sockets. But I have never seen an Agilent version with sockets.I would go for a full refund.Even if you would get it to work, you will have to spend a lot of time on this one.I have repaired one with broken traces before and it is not easy.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 03:17:17 pm
content: I've found the EMI shields, the COTO reed relay and some minor missing parts for 40gbp + shipping to italy. Need to add the vref source and some other relays.The item was in auction, ended at 159e (high feedback rate seller, was low for here in europe for a faulty unit... I've seen some FAULTY ending at 250+eur).Excluding the hours eventually spent, also if i'm luky and no other hard-to-find parts are missing, in any case the total can go quite high...Uh, found also CR306 3v3 zener gone (but the attached tantalum cap not seem shorted)

author: janoc
author_rank: Super Contributor
timestamp: September 04, 2020, 05:20:46 pm
content: Those are ridiculous prices for a broken, non-working unit. I have scored mine, fully working and still in cal on eBay for about 250 bucks delivered a few years ago. Working units commonly sell for 300-350 euro.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2020, 05:54:45 pm
content: From the picture this unit looks really bad. So bad that one could even complain if sold for parts only - there are obvious important parts missing. Chances are very slim this one could be repaired. With the burn marks under the HP specific asic / hybrid chances are high this one is bad. With some luck the display may be still working.The ADC itself should still work even with the input relays missing. So the initial test could be done without the missing parts from the front, a few bridges and a make shift reference (e.g. LM329).

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 07:02:04 pm
content: The asic resistor network is ok. Every test point match.I would say thanks to this guy:https://xdevs.com/fix/hp34401a/To have published all that work!There is only an error, the 11-13 is a series of 18k+2k so it must be 20k, while on the page is wrote 2k. 11-14 is 2k.The burnt trace is a gnd trace that run under the asic. I hate the idea of throw it away, i need to see how much he wants to refound if i choose to keep it.I know that it's a difficult thing to fix, but maybe worth a try...

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 07:03:52 pm
content: Quote from: janoc on September 04, 2020, 05:20:46 pmEvery single listing that i was following for a defective unit ended between 170-250e. Crazy. Never seen one working under 350eur.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 08:17:23 pm
content: This is a photo taken by the seller before shipping it.I not have the guts to power it on until i'm sure that there isn't any burnt component or any shortI'm really surprised to see something on the display in that conditions.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 04, 2020, 09:04:48 pm
content: These burn marks on tpop and bottom look as if very high voltage of several kV have been applied, I can't imagine another explanation.It also looks as if this HV has overridden, or arced over the protection circuit.Therefore, many of the semiconductors, especially the input multiplexer inside U101, are very probably toast.U102 , the resistor array is definitely defect, and as U105 inside the input amplifier is burnt, the whole front end is scrap.These two ASICs are unobtainium.I recommend to return the unit and get a full refund.. don't sink any further money on this lemon, please.Frank

author: janoc
author_rank: Super Contributor
timestamp: September 04, 2020, 09:59:17 pm
content: Quote from: masterx81 on September 04, 2020, 07:03:52 pmThey do show up here and there but usually sell quickly. And you may have to put up with things like missing handles/rubber bumpers and dim displays because they were pulled from measuring racks where they were on 24/7 for years. But that's not hard to fix.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 04, 2020, 10:50:54 pm
content: Quote from: Dr. Frank on September 04, 2020, 09:04:48 pmU102 is ok, all resistances are good. The problem is how to test u101...

author: coromonadalix
author_rank: Super Contributor
timestamp: September 05, 2020, 01:21:54 am
content: If the op wants to repair it, who are we to discourage him ? he'll learn the good way or the hard wayI bought 2x 34401a from israel, one was easy to repair, the other was not, paid 100$ usd + shipping, i had to put a new x-former in it, 100$ usd, it was a nightmare to have Keysight support if you're a single person, not an company, i had to buy bumpers 35$ usd, and had to put a new vfd in it, (luckily there are clones sold for a very cheap price, was bought 30$ usd) on Ali Express, a genuine vfd + board is 200$ usd at keysight, and they are going out of inventory ... and will be no longer available maybe this year or in 2021.The easy one costed me another 100$ usd to calibrate, plus 25$ usd to completly recap it (with Nichicon cap) before the calibration was done.Was it worth all the cash spent, hummm no, in the end it costed me as much as getting a clean and working one on Fleabay ... but it works very well and it's clean.You see you can easily spend many $$$ to get or repair some things / equipment used or for parts ....Since you have some special ic's in this meter, the xdev site link you have is very helpful, services manuals are available on the page. You'll have to folllow all the signal paths / input voltages etc .... the list goes on.As others members have written, this kind of damage can be very costly, and me too do think it was an high voltage spark, something like that to go all the way like this.I dont know your electronic skills and equipment you have at hand ... be prepared for a long runand maybe some costly repairs.Or you could be lucky to snatch another 34401a with a better survival chance and possibly merge some parts togetheror resell your meter, an good x-former could be sold for 50-60$ usd, some case parts ...

author: masterx81
author_rank: Frequent Contributor
timestamp: September 05, 2020, 07:38:30 am
content: Watching the schematics seem that u105 is after u101 (analog switch), so mean that also u101 must be blown. The part seem available in china.But maybe it's better to listen your suggestions and ask for a full refound. Around 30usd u101, few euros the (at least) 2 blown 34081, the missing relays, vref and shields, some otger minor parts. Easy to reach 100eur of parts. Plus calibration.

author: Gyro
author_rank: Super Contributor
timestamp: September 05, 2020, 09:58:46 am
content: It might be worth doing a cautions power-up to see if the display, in any way, matches what the seller posted in the listing. I'm curious to know whether such a 'normal' looking display would be possible without a voltage reference (maybe it is just 'stuck' at that reading).Be sure to include the internal photos of the damage and don't be afraid to involve ebay if he doesn't budge.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 05, 2020, 10:09:08 am
content: I've already sent him the photos to show all the parts missing.I not feel safe to power it on with at least 2 opamp badly burned, and at least 2 zeners dead short.I think the a/d converter is working (at least on the digital part), that explain the almost correct reading...

author: wd5jfr
author_rank: Regular Contributor
timestamp: September 14, 2020, 12:47:46 pm
content: Send it back before time runs out, eBay is pretty damn good on protecting the buyer.

author: masterx81
author_rank: Frequent Contributor
timestamp: September 14, 2020, 01:01:34 pm
content: Item already sent back and refound...Maybe almost for free was worth trying the repair. I like to fix "99.9% dead" things (after endless hours), but in this case too much moneys are needed...Waiting the next right offer on ebay and second hand markets...