HP3456A: strange behaviour (solved)

The 1000V, 100V, 10V and 1V DCV ranges didn’t work right. The Ohms ranges didn’t work either. The self test didn’t pass (error #4).

After reading the service manual I was expecting some problem in the Input Amplifier (a open, shorted or leaky FET). But as the service manual suggested, I first checked the logic.

A CD4555BF wasn’t working right. An output was always high and the end result was that a couple of FETs were always turned ON even when they weren’t supposed to.

The inputs toggled right so I was kind of happy as replacing the CD4555 was less worrisome than replacing even a single FET in the Input Amplifier. It would have been easier too.

Well… I decided to test if the CD4555 was the only problem and so I put the voltmeter in the right range and pulled down the offending output with a resistor, thinking that at worst I would end up blowing the IC output but I was going to replace it anyway.

It worked fine. In fact it worked so fine that the CD4555 started to work in every range when I removed the extra pulldown.

I am going to probably replace it anyway but I am rather curious to understand what happened.


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