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I picked up the HP 34401A from ebay. This unit was rejected by the cal lab with ‘failed 1000V adjustment’ sticker on it. It passed the self-test and all modes and ranges worked fine.
So I hooked it up to the calibrator and fed 500V. The overcurrent protection tripped immediately in my calibrator and multimeter went out completely.
I cycled the power and now it would not turn on at all. After digging quite a bit I found 5V VCC rail almost shorted to ground. Ok FLIR time and sure enough the U150 was about 170°C.
There was no more obviously bad or fried components or anything like that so I replaced the U150. The DMM worked again. I hooked it up to the calibrator once again and started increasing the voltage bit by bit.
At around 450V it happened again with exactly the same result – the U150 was dead. Then I started a complete and thorough check of the frontend and I found the culprit. This was the first time in my experience.
The K101 had a very small burnt spot on the case where it touched the E100 surge arrestor (with one side of it connected to the HI terminal). I checked the K101 relay and metal shield / housing had resistance to the coil about 14Ω.
So it was a double breakdown between the E100 and relay housing and also between relay housing and coil!
Perhaps the prime cause was too little clearance between the E100 and the K101 and some sort of vibration rubbed the paint of the relay housing and initial high voltage breakdown occurred.
The solution is to apply some Kapton tape between K101 and E100. Check your DMM!
Topic starter
Posted : 08/04/2024 10:13 am
Dropwire and Brian Wood reacted
I’m interested, did the Kapton tape work? They may have been touching since it was put into service and just the heat from the E100 has eventually melted through the relay case and caused a short. Anyway, I hope it’s working now for you.
Woodb180
Posted : 09/04/2024 2:12 am
@brian-wood Kapton tape remains stable up to +400 °C and I think it’s much better than nothing for sure.
Topic starter
Posted : 09/04/2024 9:04 am
Nice diagnosis, interesting find. That is why I always make sure components have room to move, or don’t touch at the least.
Radios + Tubes + Scopes + Cars= Nothing better!
Posted : 09/04/2024 9:48 am
hubequip.net reacted
I picked up the HP 34401A from ebay. This unit was rejected by the cal lab with ‘failed 1000V adjustment’ sticker on it. It passed the self-test and all modes and ranges worked fine.So I hooked it up to the calibrator and fed 500V. The overcurrent protection tripped immediately in my calibrator and multimeter went out completely.I cycled the power and now it would not turn on at all. After digging quite a bit I found 5V VCC rail almost shorted to ground. Ok FLIR time and sure enough the U150 was about 170°C.There was no more obviously bad or fried components or anything like that so I replaced the U150. The DMM worked again. I hooked it up to the calibrator once again and started increasing the voltage bit by bit.At around 450V it happened again with exactly the same result – the U150 was dead. Then I started a complete and thorough check of the frontend and I found the culprit. This was the first time in my experience.The K101 had a very small burnt spot on the case where it touched the E100 surge arrestor (with one side of it connected to the HI terminal). I checked the K101 relay and metal shield / housing had resistance to the coil about 14Ω.So it was a double breakdown between the E100 and relay housing and also between relay housing and coil!Perhaps the prime cause was too little clearance between the E100 and the K101 and some sort of vibration rubbed the paint of the relay housing and initial high voltage breakdown occurred.The solution is to apply some Kapton tape between K101 and E100. Check your DMM!
These are the missing photos from my original post.
Topic starter
Posted : 29/06/2024 7:52 pm