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.
A.C.
