HP4156A Semiconductor Parameter Analyser VSU1,2/VMU1,2 Error 10031 - Page 1
Forum section: electronics / repair

author: JohnPi
author_rank: Regular Contributor
timestamp: March 05, 2018, 12:08:47 am
content: I have an HP4156A Semiconductor Parameter Analyser. The VMU's on it give an error 10031 on power-up. When I remove them from the instrument, there are no other errors. I removed the board, and there is no obvious damage.Any hints on repair, or searching for the fault ?p.s. -- does anyone know if the NewScope-5 - type LCD will work in this unit ?

author: JohnPi
author_rank: Regular Contributor
timestamp: March 10, 2018, 07:12:30 pm
content: Looking at the board, the material around U501 7815CT is discolored (nearly black on the back of the board). I'll replace it and see what happens -- nothing else is discolored.The board slots into the machine -- can't debug with access to any of the parts or traces. If it failed with a shorted output on the board, I may not be able to find it; if it failed input/output short, it may have damaged other components :-(. There are 2 of most other parts of the board (2 channels), but only 1 of these -- it's likely on the motherboard side of whatever isolation is used.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: March 10, 2018, 07:29:46 pm
content: Darkened PCB underneath a 78xx-IC can be quite normal. They are specified to 125°C, some can take even higher temperatures, and it seems designers are ok with those temperatures.In this case it looks like the PCB is supposed to be the heatsink.Can you measure if the IC is regulating? Voltage at the Output should be 15V. If its less, the load is too high, or the supply-voltage at the input is too low.If it is higher, the IC is toast.

author: JohnPi
author_rank: Regular Contributor
timestamp: March 12, 2018, 12:40:54 am
content: It's burned under the regulator. I removed it, and on the PCB the output appears shorted to GND. I can't power the board up and probe it (it's embedded in the instrument when connected. I think I'll force a current from output to GND and trace the I*R drop through the PCB, or see if a thermal camera can pick anything up.

author: fenugrec
author_rank: Frequent Contributor
timestamp: March 13, 2018, 04:34:38 pm
content: Quote from: JohnPi on March 12, 2018, 12:40:54 amWhy not ? can't you solder up some thin wires to your test points and route them out to an accessible location ?

author: JohnPi
author_rank: Regular Contributor
timestamp: March 14, 2018, 02:22:09 am
content: The board plugs into the instrument with a ~ 60 pin connector. As you say, I could just wire out the 15 V regulator node, but it's not that which is bad - it's shorted to GND somewhere.Edit -- I fixed it. There was a short on the PCB somewhere which shorted the 15 V regulator output to GND. It has a thermal shutdown, but must have eventually given up. Removing it still shows a short. I applied ~ 1 A between the +15 V and GND pins, then traced the I*R drop along the PCB -- if there was a branch where it stopped dropping, then that wasn't the current path. Eventually I found a small 100 nF ceramic capacitor that appeared shorted -- it didn't look discolored or anything, but removing it removed the short. I soldered in a new 7815 regulator and now the 4156A powers up without any errors !

author: KrudyZ
author_rank: Frequent Contributor
timestamp: March 14, 2018, 04:04:22 pm
content: Don't you love it when you can fix stuff like that.Hard enough, that you can feel smart about it, but not so hard that it takes forever...

author: JohnPi
author_rank: Regular Contributor
timestamp: May 06, 2018, 10:35:05 pm
content: Same instrument -- it seems its battery backup (clock) isn't working. Does anyone have a service manual, or some idea of where that battery is buried in the instrument -- I don't want to take the whole thing apart if I don't have to.