Feature #1299

Help sonicyang to repair HP 34401A

Added by tin about 7 years ago. Updated about 7 years ago.

Target version:
Start date:
Due date:
% Done:


Estimated time:
6.00 h


Meet sonic at Banciao MRT 1:30pm Jan.16

Meter Has following Issues
  • It's reporting Error 604.
  • DC measurement is extremely inaccurate.
  • Ohm measurement is non-working, but do reports ~ 0ohm if I short the test leads.
  • Conductive Test is Working as excepted.

As I tear it down, measuring the DC supplys, +18V + 15V and 5V, are all good
and Voltage refs, 7.5V, 5V, +-10V, are also good.

IC, Numbering U420, U105, U106, U312, U305, and voltage regulator are all hot as about 40~45 degrees Celsius, as other chips are 33~36 only.

And, I measured U102's resistance.
Everything is on spot except

Pin15 - Pin21 should be 100K measured 69K
Pin29 - Pin30 should be 30K measured 20K
Pin15 - Pin13 should be 100K measured 82K

I also had a 34401A with the Error 604 a while back.
This instrument had big problems in the OHMS reading and it turned out to be U201 that fixed it.

- The tantalum cap is well known ( any engineer still using those should be barred of ever designing anything ever again and branded on his forehead with the text 'Tantalum User'. Tantalums are a PEST) I lost the count of how many times i had to fix a machine or instrument and the cause was a fried tantalum. The only good Tantalum is one that is not fitted into anything i own.
- The swithc is also known
- the 2512 resistor ladder next to the front or rear jacks can crack loose in a range of serial numbers
- the cpu on the front panel can explode if tone of the VFD drivers calls it quits and commits harakiri in a fiery fireball.
There is another tantalum in the True RMS section that can kill that one off as well.

Useful threads:



1. Zero adjustment 4W short
2. -10V, +500VDC, 10mA@1kHz calibration if A/D , U101 or U505 RAM changed
3. 4W adjustment


34401-90013.pdf (4.16 MB) 34401-90013.pdf tin, 01/13/2016 04:37 AM
Accredited-Calibration-Certificate.pdf (5.02 MB) Accredited-Calibration-Certificate.pdf tin, 01/13/2016 04:56 AM

Updated by tin about 7 years ago

  • Status changed from New to Closed
  • % Done changed from 0 to 100

Fixed, worklog

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