Tek 2225 fault in X/Y-mode

[RAPo]

Turned on my Tek-2225 yesterday and it is displaying only a "half-circle" instead of a "full-circle".
Input 50mVpp 1 kHZ with a 90-degree phase difference from a Juntek PSG9080 (wave/freq/amplitude synchronized on both channels).

X and Y channels are set to 10 mVpp, time base to X-Y-mode.

The same signal on a Tek 2445 works fine, see image.

Any idea what the problem could be?

[jdragoset]

Check the two x plate connectors at the CRT neck.
one may have fallen off.
Be very careful not to stress the thru-glass pins.

[RAPo]

Thanks for the quick response.
All other modes are working perfectly so maybe.
Will look into it tomorrow and report back.

Check the two x plate connectors at the CRT neck.
one may have fallen off.
Be very careful not to stress the thru-glass pins.

[RAPo]

I did have a look and couldn't find anything strange at the neck of the crt gun.
Is there something special I have to look for?

Did the checks as mentioned in the service manual, no problems there.

[jdragoset]

Got the 2225 set up with a normal XY display
Will remove cover for voltage comparison checks.

[bdunham7]

Can you move the display left and right with the position knob?

What do you get if you just use the normal mode to display both channels?  Can you post a photo of that?

I wouldn't go tearing the scope apart just yet until you do more tests and reason through it. 

[RAPo]

Agree totally that poking around without a good plan is not a good idea .

As for your questions:
1) I can position the half-circle to the right edge by turning the coarse/fine adjust maximally to the right
2) Idem for moving to the left, but I can move it totally off-screen on the left side (it is not possible to shift the half circle of to the right beyond the screen dimensions)
3) See the attached image that shows both waves (in normal/alt-mode).

[CaptDon]

Doesn't that X-Y Invert switch need to be switched to the right X-Y?

[bdunham7]

OK, that's good and weird...

As an experiment, can you try putting 20mV and 50mV DC biases, both positive and negative, on the CH1/X channel?  I don't know whether your AWG allows you to add a DC bias in simple fashion.

[tekguy]

i have always seen the deflection plate connections about halfway up the crt not on the neck!

[jdragoset]

Try raising the output voltages to 1 V p-p and adjust the vertical scope attenuators to the same.

[CaptDon]

The deflection pins are usually about 2/3 of the way along the neck heading toward the funnel / bell. Anyway, he checked them and they are fine. It looks like somewhere the signal is not transitioning through the negative half on the 'X' function. Strangely enough however he can move the trace fully to the left and has normal 'X' axis sweep in the standard triggered sweep mode. Looks like a problem in the signal path where channel 2 (X) is being routed to the horizontal amplifier. Obviously the horizontal amplifier is capable of providing a full sweep. I don't fault the signal generator because it can produce a normal circle on his other scope.

[jdragoset]

Ch 1 (X in XY mode) is picked off before Ch 1 vertical position since that control is not functional in XY mode.
The only pick-off before the vertical position is in the vertical preamp to the Ch 1 trigger amplifier, on to the trigger amplifier, which has an X-axis output to the horizontal preamp, all controlled by XY and 1/XY signals generated by one closed contact of the S/div switch in the XY position.
XY and 1/XY have quite numerous points of function.
If higher amplitude signal still results in half moon display, I would suspect a component problem in this train.
Schematic Sheets 2, 3 and 5.

[jdragoset]

Check for Ch 1/Ch 2 trigger anomalies in normal sweep mode, same signal into both channels.

[CaptDon]

Good catch on Ch1 being the 'X' axis in XY mode. I missed that. On my older tek analog scopes CH1 remains as the vertical 'Y' input and the 'X' input is from the Ext Trig / H Amp BNC connector. Although not marked on my TDS2004B, I see CH1 is 'X' and CH2 is 'Y'. I seldom use XY on my TDS2004B or my TDS644B and forgot that the channel functions are different from the older Tek analog stuff. I haven't had the digital scopes powered up in over 6 months. My goto test gear is a TM506 with Power Supply PS503A with the adjustable 5vdc mod, DM502A Multimeter, SC504 80mhz Analog Scope, DC503A Counter/Timer and SG503 Signal Generator. My digital stuff is reserved for advanced triggering events or capture of transient events. The 5441 Storage scope is in good health and is more of a toy at this point. To do XY on the 5441 you yank out the Timebase Module and put one of the Vertical Modules in its place! Interesting way to do XY and very handy as there is a huge assortment of Vertical Modules to choose from. See, you guys got me knocking the dust off my toys again!! Cheers!!!

[RAPo]

Yes, my AWG can do that. See the results in the images. Overall effect: full circle shows up with positive offset, circle collapses totally for negative offset.

[RAPo]

I've checked the whole tube. No signs of missing pieces.

[RAPo]

Changing Ch2 invert shifts the semicircle a bit up.

[RAPo]

I'm studying the schematics now ;-)

[RAPo]

At your request

[RAPo]

Good question. Triggering on X or Y no problems, trigger mode set to 'vertical' mode gives a flickering image.

[RAPo]

problem persits, so we have to delve deeper.

[bdunham7]

Yes, my AWG can do that. See the results in the images. Overall effect: full circle shows up with positive offset, circle collapses totally for negative offset.

So somewhere in the circuit there is the equivalent of a diode across the channel that is not allowing the amplifier to go negative.  Or, alternatively and probably more likely, one side of a bipolar power supply has collapsed.  And it has to be in a section that is unique to the XY operation since this doesn't appear to be an issue in normal operation.  Very possibly something in the channel switching logic, I'll take a look at a schematic when I get a few moments.  Fortunately you have another scope so you can trace the CH1/X signal through the scope.

Edit:  Have a look at the excerpt here from the schematic sheet "XY Amplifier and Horizontal".  Check all the voltages as shown.  I would suspect and check Q736, but I haven't thoroughly thought it through.  U380 in the trigger amplifier could be an issue as well, but I'd expect the trigger function to be compromised to a degree.  Perhaps it is?

[jdragoset]

In XY mode look at "X-Axis" J-90, pin 5 with positive then negative offset.

[RAPo]

Thanks for the pointers, will look into them and report back.