Hello all,

I hope this newsgroup is still alive, I see too much spam lately

I'm repairing an old telequipment/tektronix D755 scope for a friend.
So far it started with no power supply voltages, turned out to be
some cold solder joints in the power supply PCB.
Then it was badly out of focus, tracked down to open high value
resistors on the EHT PCB. After mounting back the EHT pcb it developed
a short on the +105V rail (which wasn't there before). It turned out
to be two shorted transistors on the Z amplifier PCB.
One transistor is a BC207 and the other is reported on the manual as
FRB749/SPS5286 (which is unknown in the whole internet it seems). I'm
going to substitute it with an high voltage NPN (it sits right
between the 105V supply and ground).
What seems strange to me is these two transistors dying after
repairing the EHT board. However the EHT is now fully functional and I
have a clean focused track, however with full brightness until I
substitute the two transistors in the Z board.
I also set the -2500V cathode bias correctly in the mean time.
Now I have a full scan track on the CRT, time base seems to work
correctly and also track vertical position works, but I see a very
strange effect on the track:
with vertical inputs grounded I can move the horizontal sweep from the
bottom to the top of the CRT, however the track is perfectly
horizontal only on the half part of the CRT, when moving up it
develops an arc of circle near the right end of the sweep, the circle
become larger the more it goes up until it reach about the anode
connection where the track shows a couple of sinusoidal periods; going
further up the circular arc changes direction (track goes down instead
of up vertically) and behaves in a specular manner.
Now I don't know if there's something that needs tuning in the X or Y
drivers,
unfortunately this scope requires specialized calibration plugins to
go through all calibration steps, so I won't be able to follow the
exact calibration procedure.
However it seems to me that this kind of fault could be due to a bad
CRT, I'm not sure the Y voltage on the plaques should be variable
during a bare horizontal sweep, but also I'm not a great expert of
scopes.
Any hint is welcome.

Best regards
Francesco IZ8DWF