Some time ago my friend mrc got a broken oscilloscope from our friend fede: a Grundig GO 10, probably from the 70s, two channels, 10 MHz.
When turned on, the trace had no focus at all in the vertical direction and weak focus in the horizontal one. The horizontal sweep worked, with consistent timing, and the trigger seemed OK too.
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 was the high voltage section with the focus control (we suspected a bad electrolytic cap).
There we found a very nice flyback (?) transformer with plenty of calibration trimmers. Tweaking them changed nothing.
Another, less likely, culprit was the big multi-section supply cap:
With the supply transformer:
But the supply voltages were stable and correct at many points (thanks to Radiomuseum for the schematic, even if getting it is a bit of a mess).
The CRT seemed fine, and the filament was glowing:
Now the all-discrete circuitry! Vertical amplifier stages:
Timebase generator (?) and front-end preamps:
And plate driver:
After some scared testing (2 kV around, even if our board was at about 190 V) we found that the driver BJT was fried! We had no direct substitute, so we put in a lab specialty: a mighty big BUHxxx BJT with huge ratings. Here it is, hanging in the air below the CRT driver board:
Later we fitted a spare BD115 (somewhat expensive), and the oscilloscope came back to life with a perfectly focused trace! Thanks mrc for the fun repair.