Help sonicyang to repair HP 34401A
Meet sonic at Banciao MRT 1:30pm Jan.16Meter 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.
1. Zero adjustment 4W short
2. -10V, +500VDC, 10mA@1kHz calibration if A/D , U101 or U505 RAM changed
3. 4W adjustment