About 12 years ago, I bought my first Tek 475 scope. I only had a 2,245A at the time, and this unit upgraded my workbench's bandwidth to 240MHz. Not bad for a scope rated at 200MHz. In fact, the 475A was rated at 200MHz but one step less vertical sensitivity. So mine has the best of both models. It also had the DM44 on top. Unfortunately, the display on the meter had dopey segment issues.
I got a manual for the DM44 and traced the problem to the DMM 2 chipset. I used the 475 scope function to look at the digital lines and finally found one that was trying to go high but couldn't. It looked similar to a runt pulse. I checked leakage between the pin and VCC and GND. The VCC path was open. I did a visual inspection looking for anything unusual.
I didn't find any obvious faults so I pondered my next move. If it can't go high enough to drive its neighbor, then maybe I could help it a bit. I grabbed a 10K resistor and got the signal to go a little higher. I figured I was on the right track, so I wired in a pot and dialed it.
The seven segment displays lit up right away and gave me a reading. I unsoldered the pot and tacked in a fixed resistor of the same value as the pot (it was a 5.1K, if I remember correctly). The DM44 is still working to this day. I tried to find a replacement for the DMM chip, but it has long been obsolete. I went to work and described the situation to our engineering manager.
His response: "You did not fix the scope by adding a pull-up resistor. I gave up the debate right there. I had not been detailed enough in my description. He was thinking design flaw. The actual cause was an open upper totem pole output transistor, which is a part failure. I just changed the output from a class B to a class A linear stage. Thankfully, the timing on these early DMM chips is slow, so no worries on interface.
This entry was submitted by Steve Lindberg and edited by Rob Spiegel.
Steve Lindberg has loved electronics since he received his first Weller soldering gun when he was 12. He has 35 years of experience in test, debug, and design.
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