Hi,
I managed to repair the CRT circuit of my TEK 475. The screen intensity control and focus, is currently working as I should.
I now can create nice, sharp horizontal and vertical lines in the XY mode.
Screen Intensity Control, Focus and astigmatism do all work.
The CRT is NOT defective as I had feared.
The fault in the end was still the rather familiar (intermittently) shorted Tantalum Capacitor in this case C 1387 in the +15V power rail.
I had measured all components around the CRT with Power Off; All OK, also the Restorer diodes.
But whilst measuring "dry" on these circuits, the short circuit in the +15 V returned again!.
By coincidence I found the cause (i.e. the offending tantalum C1387 ( wrong labelled C126 in the photo Fig. 7.19) , 2.2 uF, 20V in the +15V power rail), next to the ceramic plate.
Previously I had observed that the short in the15V rail vanished by pushing/pulling on the Interface PCB.
It reacted strongly in the vicinity of the back of the scope, but I could not really locate it.
(I thought the short circuit occurred at the backside of the board against the chassis.
In the end, I could get a stable (61.2 Ohm vs GND : i.e. No Short Circuit) on +15V rail, if a removed the LH Bottom pillar of the Safety protection cover.
I thought I had solved the problem, see previous mails.
But only now I found out that removing this pillar (intermittently) cuts out the trace of the earth rail of the +15V (on whichthe offending tantalum C1387 2.2 uF, only 20V! was located.
So, the +15 V rail was still shorted All Right at the end of the rail (and I had only unintentionally disconnected (part of) the GND rail).
Bij measuring the resistance between the + 15V and that shorted part the GND Rail, while scanning around this GND rail with a sensitive Ohm meter, I could clearly measure that the short was at the C1387 location
Removing this capacitor, indeed removed the short.
When I powered up the scope, the INTENSITY, FOCUS and ASTIGMATISM all worked OK.
The unfortunate thing is that I somehow lost the Horizontal sweep (A en B).
(In the XY mode the scope works as it should), but the sweep is dead.
The vertical amplifiers are AllRight and I can move lines horizontally and vertically in XY mode and in "Beam finder" mode.
So the fault is located more "upstream".
O well, back to the drawing board to diagnose why the Horizontal Sweep did quit (I have not touched it all and was extremely careful, both mechanically and electronically).
(Maybe the "floating 15V GND section did destroy something.)
But many thanks to you all with helping me with (understanding/troubleshooting) the CRT circuit.
Will be continued ( I will start a new topic, if I have any questions).
Martin
