HP 4145B Repair Attempt, live and neutral shorted - Page 1
Forum section: electronics / testgear

author: kodiop
author_rank: Newbie
timestamp: July 28, 2023, 03:59:15 am
content: Hello, this is my first time in this forum please bare with me.i am a student on a research project that will use the HP4145B. sadly the equipment is borked. just by plugging the power cable the equipment tripped our MCB. i have searched around to find the service manual, but only found the one for HP4145A. i have heard that it is more or less the same, just different drive? cmiiw (https://archive.org/details/HP4145ASemiconductorParameterAnalyzerOperationAndServiceManual)the professor gave us the green light to diagnose it. so i proceeded to tear down the equipment and extract the A11 module based on the troubleshooting tree below.i did the isolation check, but the short still existed (i left it measuring the resistance in the multimeter for a minute hoping any capacitor would be charged enough to show any resistance, still shorted), so i didn't plug it in.so i desoldered the fan to fully pull this module out, turns out there is a capacitor got blown to bits.here is the schematic for this parti first assumed that cap is fully shorted, the source of the problem, but the multimeter shows 0.4kohms(while the power button is off), while the c14 socket shows around 0 ohms, if the power button is toggled, it shows 0 ohmsbased on that, and the schematic, i determined that the EMI Filter might be shorted as well.soo thats the entire diagnosis journey, is my conclusion correct? or i missed some crucial steps?

author: AVGresponding
author_rank: Super Contributor
timestamp: July 28, 2023, 05:28:47 am
content: This, unfortunately, a very common failure point, and due to the brand of cap involved is often known as "RIFA madness". The line filters frequently use RIFA caps in them too...A good first step is to smell the DUT; if the magic smoke has been let out, you should be able to detect it. Then a thorough visual check, which usually reveals things like self-immolating RIFAs. My personal preference is to do these before trying to re-power the DUT after replacing the fuse; there's always a reason why the fuse has blown, and if it's because of a shorted component, you're down another fuse, at best.

author: wasedadoc
author_rank: Super Contributor
timestamp: July 28, 2023, 12:14:07 pm
content: 1. Remove C1. The PSU will function OK without it so you can test and troubleshoot without it. Of course you should fit a suitable replacement before putting the 4145B back into service after repair.2. With the fuse removed, test the resistance between the power input socket pins. If the short is there then the only cause would be the EMI filter and the wiring between it and the power socket. However a fault there would not have blown the fuse so I doubt the problem is there. With the fuse removed and C1 removed measure the resistance between the points where C1 was. It should be low (the transformer winding) but not zero.

author: kodiop
author_rank: Newbie
timestamp: August 04, 2023, 10:39:37 pm
content: only managed to access the equipment today, sorry. some additional details:1.the fuse is not blown, sorry i did not mention this earlier.2. this equipment was bought from Japan, which uses 100vac, based on the instruction, 100vac requires a 2.5A fuse. now, since we use 220V, it requires a 1.5A fuse, university overlooked that and the 2.5A has not been changed to the correct one.3.i took a drastic step to remove the entire EMI Filter, the short went away.checked where C1 used to be, it shows around 300ohms with the fuse not attachedwhat should i do next?

author: TimFox
author_rank: Super Contributor
timestamp: August 04, 2023, 10:55:56 pm
content: The higher UK line voltage uses a lower fuse current.However, assuming the transformer "voltage selection" switch on the rear panel, near the fuseholder, has been set correctly, the higher-current fuse used in Japan will not blow--it just doesn't protect against the expected overload current at 220 V.As mentioned above, the line filter is a well-known culprit in units of this vintage, especially in 220 V countries.

author: AVGresponding
author_rank: Super Contributor
timestamp: August 05, 2023, 07:30:56 am
content: Certainly looks like the filtered mains inlet is toast. Since the caps in that and C1 are all in parallel with the transformer, it shouldn't have caused any other damage down the line.300Ω is a plausible result for the primary of a relatively low power transformer like this. If you have a variac, you can try bringing the voltage up slowly, to make sure there's no unexpected power draw, but my guess is it's just the RIFAs that have let the magic smoke out, and it'll work fine with them removed, and replaced with more long term stable parts.

author: kodiop
author_rank: Newbie
timestamp: August 08, 2023, 12:44:13 pm
content: ordering a new line filter and C1. Now, how stupid it would be to just hookup the unit, and bypassing the problematic line filter?

author: alm
author_rank: Super Contributor
timestamp: August 08, 2023, 12:51:00 pm
content: Quote from: kodiop on August 08, 2023, 12:44:13 pmI don't see any problem with that as long as you hook it up safely and there's still an appropriate primary side fuse in the circuit.

author: kodiop
author_rank: Newbie
timestamp: August 08, 2023, 02:58:10 pm
content: thanks for reminding me of the fuse, best to wait i guess