Philips PM3267 100MHz Cathode Ray Oscilloscope
Slaeshjag and I picked up this CRO for free! It was broken and the previous owner, who had another scope, didn't have time to repair it. Acording to description, there was no picture on the tube, but everything else seemed to work fine, the LEDs on the front panel indicated that it trigged on signals, etc.
The fault was found in the high voltage driver circuit for the CRT; the switching transistor had failed (and taken its socket with it). A replacement transistor was found in the parts bin, and the tube sparked to life, altough the dot was about a centimeter in diameter and increasing in size to light up the entire display as the fillament was heating up, so something was still not quite right.
A high voltage voltmeter was built using 10 series 1 MΩ resistors and an analogue ampere meter, it was discovered that the voltage was much to high (or low rather); the cathode voltage was closer to -2000 V than the specified -1500 V. As a result, the anode voltage, which was derived from the cathode voltage using a voltage multiplier, was even more too high. Some probing around on the power supply board gave indications that an opamp used for controlling high voltage didn't behave as expected. A replacement (with different pinout, of course D: ) was found in the parts bin and soldered in using wires…
It works!! Sorry if the camera flash ruined some of the photos :(
Some crappy electrolytics were found as well and should be replaced, however, I didn't find any matching ones in the parts bin, so that will be taken care of later.
For those of you who have this scope and need to have a poke around inside it, the service manual is of great help and has all the circuit diagrams and component values for the unit. The PDF is scanned, and not searchable, but hey! It's better than nothing!