Description
This page describes an experience with an old oscilloscope that I tried to repair to full form. The device in question is a Philips PM3230: a dual trace 10MHz oscilloscope, based on a hybrid valve/transistor circuit. It was originally produced in 1971 in The Netherlands.
The oscilloscope was initially non-operational, showing no trace on the CRT. After replacing some damaged components, mainly a few transistors that were broken due to corrosion, it came back to life.
Malfunction
After working with it for a few days, the signal starting losing focus for no apparent reason, causing the trace to have a height of at least 1 cm. Fiddling with the focus knob had no effect. Adjusting the astigmatism calibration potentiometer inside the oscilloscope also had little effect.
The Fix
The problem pointed to the possibility of damaged component linked to the focus knob. After following the circuit, I noticed that a 5.6MΩ resistor connecting from third pin of the focus potentiometer to the ground had ∞ resistance. Surprisingly this was the case for both potentiometers on channel A and B.
The malfunction was fixed by replacing the 5.6MΩ resistor. In this case I had to use pair of resistors in series, since I didn't have 5.6MΩ resistors at the time. After doing so, the trace was back to normal (see Photo 7 and 8).
The question remains: how did both resistors fail all of the sudden? It could have been due to corrosion or degradation of the materials since this equipment wasn't been used for over 20 years.
Photos Gallery (Click to Enlarge)
Written in
2013-12-13