HP3458A Repair.  - Page 1
Forum section: electronics / repair

author: sprit
author_rank: Regular Contributor
timestamp: January 05, 2025, 05:05:48 pm
content: Hello everyone, have a great day.After a visit to my uncle's house, I found an old HP 3458A sitting in the corner of the storage room. It looked lonely so I asked if I could take it home. After a quick check the voltage regulator board for the AC board was broken, the rectifier diode was burnt. I replaced them and tried to turn it on again and it started normally except for error 202: AC Board fail. I checked the voltage part -17V Fuse F702 was burnt, I kept replacing it but the problem persisted, then I checked Q702 and it was also broken After replacing it I tried turning it back on, but the fuse kept blowing, I tried shorting the fuse and this time the Q702 exploded. After some fiddling with the -17VA line, I found that it connects to U401( MC34072) and U301(LT318 but It is labeled as LT1826-1265, I think it's a custom part or something like that. ) After continuing to replace them, the fuses no longer blew,The -17V voltage has stabilized but it still keeps giving error 202: Slave Test: AC Board Fail. The next day I tried measuring again.I measured pin 1 of U405(MC34072) and found out it has a very high voltage, around 6-7V DC. While normally I only see it around 5-60mV. Too high. I searched and continued to replace it (U405) and U401 and it returned to normal, around 60mV on the AC function scale. U404(EL2039) was also replaced but it still could not pass the test.Can anyone give me some suggestions? Which area should I dig into now? Thanks for your help. Sincerely, Feng.

author: MathWizard
author_rank: Super Contributor
timestamp: January 05, 2025, 06:21:41 pm
content: I have to get something from that era that's broken, it looks so fun to work on, and a real easy size for cutting traces, and narrowing down stuff or adding on a bodge.

author: cruff
author_rank: Regular Contributor
timestamp: January 05, 2025, 07:35:41 pm
content: Have you obtained a copy of the service manual? Should be downloadable PDFs available with a search, physical copies are probably also findable.

author: Swake
author_rank: Frequent Contributor
timestamp: January 05, 2025, 08:23:00 pm
content: Quote from: sprit on January 05, 2025, 05:05:48 pmCareful, this is a fantastic and expensive device but requires special care to stay fantastic.Before taking any step, even just dismantling something, inform yourself, take your time and ask the specialists here what they think about that next step and how to best approach it.

author: Swake
author_rank: Frequent Contributor
timestamp: January 05, 2025, 08:29:07 pm
content: Quote from: sprit on January 05, 2025, 05:05:48 pm1826-1265 is an HP code for a component.In this case it would be a LT318AN8. They added the word 'selected', don't know on what criteria this is 'selected'.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 05, 2025, 11:14:23 pm
content: could be bandwidth response, slew rate, noise etc .... they had their own criteria(s) at the time to use it in this metersur you could do a test with a brand new one to see if it solve some problems ... best would be finding a working board ..... but priceyyou could ask keysight if they are available but ....and if there is no a3 board drift, you could/may have find a dormant pearl

author: IanJ
author_rank: Unknown
timestamp: January 06, 2025, 05:16:31 am
content: Before you go any further, download the cal ram data via GPIB from the 3458A.Or, if it’s in a socket then back the cal ram IC using a suitable eprom programmer.Ian

author: sprit
author_rank: Regular Contributor
timestamp: January 06, 2025, 12:33:53 pm
content: Hello everyone,wish you have a good day.In my case, Actually I have 2 3458s, I often use mine as a reference for each measurement, but I find it difficult to constantly use the measuring stick to check each test point one by one. The problem was pretty obvious when I got the other 3458a home and I could use my other 3458 as a reference, but since solving the -17VA voltage issue,The measurements I compared were almost the same, and you know, one work :blah:ed fine, the other kept Failing at error 202. I have considered the option of replacing all new DIP ICs but after checking on the parts selling sites, there are many IC codes that are no longer available or are sky-high priced...Can anyone tell me what I should do next? I tried replacing the A3 board from my other 3458 in there and everything worked fine, so I'm sure this A3 board is faulty. I also backed up the cal data to my computer,Looking forward to your suggestions. Thank you very much.Sincerely, Feng

author: coromonadalix
author_rank: Super Contributor
timestamp: January 06, 2025, 02:11:53 pm
content: well if you search about the a3 problems you will eventually find no ways to get a replacement unless you send the meter to keysight and shell tons of $ because they will calibrate it at the same timethe A3 board now is a searched one, many people seems to buy old 3458 / defectives one hoping they'll get a good A3 and resell dismantled part(s)

author: sprit
author_rank: Regular Contributor
timestamp: January 13, 2025, 12:39:05 pm
content: Update repair process. After digging around in this A3 circuit, I realized that pin 3 of U201 was not receiving any signal. I continued to trace the input and realized that the signal was missing at pin 2 of Q105, I should be getting something here. I took the Q105 out and tested it and confirmed it was working fine, I continued testing and confirmed pin 3 of the Q105 had no power. In the schematic I have, I notice that CR103 is pinned down to something that looks like 8V, and the other input to R114 is sitting at -9.515V, which is pretty weird. While pin 3 of Q105 I have 1.052.

author: sprit
author_rank: Regular Contributor
timestamp: January 13, 2025, 01:18:09 pm
content: On my other 3458, at pin 17 I still get the output although it's a bit noisy. At least I see the waveform I applied. I will try to continue replacing U201, Q202, Q203 to see if there is any progress. Damn, this A3 board is so annoying.

author: sprit
author_rank: Regular Contributor
timestamp: January 13, 2025, 02:24:44 pm
content: Update: After I tried replacing U201, the circuit worked, at least. Now when I press the AC DC button on the front, I hear the Relay beep. The output of DAC U301 has a signal, although the Even though the unclipped waveform isn't what I expected, at least I think I'm on the right track. As for Q202 and Q203, I will try removing it and replacing it to see if the error goes away.

author: sprit
author_rank: Regular Contributor
timestamp: January 13, 2025, 02:54:35 pm
content: Damm, im stucked. The Q202 &Q203 is fine... Now idk whre to digging next zzz