Electronics > Repair
HP3458A Repair.
sprit:
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.
MathWizard:
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.
cruff:
Have you obtained a copy of the service manual? Should be downloadable PDFs available with a search, physical copies are probably also findable.
Swake:
--- Quote from: sprit on January 05, 2025, 05:05:48 pm --- I found an old HP 3458A
--- End quote ---
Careful, 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.
Swake:
--- Quote from: sprit on January 05, 2025, 05:05:48 pm ---U301(LT318 but It is labeled as LT1826-1265, I think it's a custom part or something like that. )
--- End quote ---
1826-1265 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'.
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