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.