HP 412A issue - Page 1
Forum section: electronics / repair

author: bob91343
author_rank: Super Contributor
timestamp: December 19, 2020, 03:08:47 am
content: My 412A works great but when I go to Ohms, and adjust the rear pot for full scale, it doesn't reach full scale on all ranges.What I get is full scale (infinity) on most ranges but when I get down to Rx1 and Rx10 it won't go that high. On the Rx1 range the needle stops at about 50 on the scale, rather than infinity. It goes a little higher on x10.After perusing the diagram I can't see what could cause that. I checked the +150v and it's good, doesn't change when I switch ranges. All ranges have the same control to set full scale. And when I measure a resistor on the Rx1 range it reads a bit lower than it should, based on the reduced pointer swing.

author: m k
author_rank: Super Contributor
timestamp: December 19, 2020, 06:56:31 pm
content: Since the resistor network is a loop and those low ranges are always present their failure can't be affecting just them.That range selector is not exactly very tiny eighter, it has 6 planes.My guess is that the issue is mechanical, one range selector is not selecting as it should.Are other functions of same position ranges accurate?

author: bob91343
author_rank: Super Contributor
timestamp: December 19, 2020, 08:44:12 pm
content: m k, you are right. I checked it on volts and the same error obtains. So something is wrong with the range switch, not the Ohms circuit.What now?

author: m k
author_rank: Super Contributor
timestamp: December 20, 2020, 03:01:30 pm
content: With luck all selectors should be equally positioned and faulty one points elsewhere.Seems that function selector is finally a better candidate.Range selector plates E/F are volts/ohms out to amplifier and C/D(A) in from probes.(mA is B in and DE out)Function selector problem is much easier.One possibility is that one function selector is not moving at all or its conductor parts disoriented somehow.If amplifier(green) can be easily disconnected you can test the situation more.Since some of others go finally through volts resistors you can just measure how the thing goes there.(preliminary, use with caution).001 10M.003 10M (30mA).01 10M (100mA) (X1).03 30M (300mA).1 100M (1000mA) (X10)

author: bob91343
author_rank: Super Contributor
timestamp: December 20, 2020, 07:44:51 pm
content: m k I have difficulty understanding your comments. I will read them again but your English isn't good enough for me to get the meanings of these technical statements.

author: m k
author_rank: Super Contributor
timestamp: December 21, 2020, 11:12:31 am
content: I thought we are looking the same manual.https://www.manualslib.com/manual/1459321/Hp-412a.htmlThe manual is not exact about the actual appearences, maybe there are no parts like selector that is pointing.If you like you can name those selector parts and then I can use your words.

author: bob91343
author_rank: Super Contributor
timestamp: December 21, 2020, 10:34:03 pm
content: Okay the bottom line is - I am not going to try to repair it.My reason? I discovered the same error on the same two switch positions on volts, as well as Ohms. The amount of error is not that great, although it is out of specification. The problem is perhaps the switch, one or the other, but those are delicate and will be destroyed by many of the usual solvents. The risks when weighed against the potential benefits make it a bad decision to get into this otherwise wonderful instrument.So I closed it up and it's back on the bench. When I make measurements using those 'bad' ranges I will remember and compensate. It's only a few percent. And I have other instruments in case accuracy is important.In closing, I will thank you for the help and perhaps can return the favor one day.