HP 34401A Error 612

• HP 34401A Error 612
  Posted by BobH0001 on 16 Mar, 2024 00:56
• I have a HP34401A that will not read resistance correctly and is throwing Error 612. Attached are the readings from the current source circuit.
  U201 is suspect and I see that is a common failure. I just wondered about anything else to check before replacing U201. 
  Thank you all..
  
  Regards
  Bob H

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  HP-34401.pdf
• #1 Reply
  Posted by floobydust on 16 Mar, 2024 02:30
• Welcome to the forums
  
  Zener CR203-A appears to be dropping more than 5.1V (18V rail-5.1V=12.9V but you measure 6.942V) so I say the part is not working or maybe a bad connection. 1902-1565 is BZX84-C4V7 though, a 4.7V zener so I think it's typo to be 5.1V. Very early models there was a Service Note and change of the part I faintly recall.
  Regardless, you are showing a 11.06V drop. As a result Q202 is biased off. The +/-18V rails, are they OK? They are regulated so I'd expect them to be close.
  Usually the transistor string gets damaged or the JFET Q211 if the meter gets zapped on Ohms function.
• #2 Reply
  Posted by Dr. Frank on 16 Mar, 2024 22:41
• Hello,
  I also suggest to first check the +18V rail, and CR 203-A, before changing U201.
  It's most probable, that CR203 ist toast, as U201-B obviously outputs >14V, which means that its supply is 15V at least.
  
  If I remember correctly, these 5V1 of CR203 had been changed, (to 3V3 ), so that depends on the version of your 34401A.
  
  Do you measure reasonable values in the different 2W Ohm ranges?
  I expect , that not only the 10M range or higher is defective, like Err. 612 suggests.
  
  In the appended schematic, I have written down the nominal voltages for my working unit, 10M range with an external 10M resistor attached.
  The printed boxes show values for a defective U201.
  
  Frank
     

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• #3 Reply
  Posted by BobH0001 on 16 Mar, 2024 23:13
• Thank you both for the information.
  I am reading 18.9V supply.
  The forward drop for CR203 looked normal so I only replaced U201.
  After replacing U201, CR203 anode is now reading 13.7
  Selftest is now passing, auto-ranging is working and a few resistance readings I took look normal now.
  Tomorrow morning I will do additinal testing to confirm full functionality.
  I have never worked on one of these meters so I am grateful for your time! (and documented readings Frank!)
  Regards,
  BobH