Time ago a friend of mine called mrc got a broken oscilloscope from our common friend fede. The oscilloscope was a Grundig GO 10, presumably from the 70s. The oscilloscope is two channels, rated 10MHz.
When turned on, the trace was in a very bad condition, showing complete absence of focus on the vertical direction, and weak focusing on the horizontal direction. The horizontal sweep circuitry seemed fine, as well as it was possible to obtain a sweeping with consistent timing. Also the trigger seemed ok.
So we did what we were most excited about: taking it apa(aaa)rt! From the top:
And the bottom:
The first place to look at was the high voltage section, with the plates focus control (we were suspecting some bad electrolytic cap).
Over there, a very nice flyback (?) transformer was found, with plenty of calibration trimmers. Nothing changed by tweaking those.
Another, even if unlikely culprit was the main big composite supply cap:
With the supply transformer:
But the measured supply voltage was stable and correct at many bias points (thanks to Radiomuseum for the schematic — you still can improve making the access to them less a mess, but still thanks).
The CRT seemed all fine, and the filament was glowing correctly:
Let us take a look at the fascinating all-discrete circuitry! Vertical amplifier stages:
Time basis generator (?) and frontend preamps:
And plate driver:
After a bit of scared testing (2kV around, even if our board was at about 190V) we found out that the BJT driver was fried! So we replaced it with a specialty of the lab — something looking mighty big (we had no direct substitute), a BUHxxx series BJT with huge absolute ratings. Here it is hanging in the air below the CRT driver board:
After replacing a spare BD115 — somehow expensive, the oscilloscope got back to life, with a perfectly focused trace! Thanks mrc for the fun in the repair.