Tek 2215 Focus Issue?

[melling]

I'm working on a 2215 picked up at a radio rally a couple of weeks ago, and after replacing the MOSFET and a couple of BJTs on the A18 preregulator board I'm now able to get a trace, very happy with the result so thanks to those that replied on my last post.

I can only get the trace to (just about) fill the screen when I have AUTO INTENSITY fully CW and AUTO FOCUS fully CCW:



In this next pic I've rotated AUTO FOCUS about 1/4 turn CW:



Then if I move AUTO FOCUS fully CCW and rotate AUTO INTENSITY fully CW:



This is fairly consistent across all timebases.

I've ruled out the sweep generator as I can recreate this in XY mode.

Tracing through the AUTO FOCUS circuit showed that the 1M resistors were all within range. The auto focus pot is a bit jumpy and I haven't got round to pulling it yet to test if it really is 5M.

The horizontal output path it looks fine, test points match the service manual and I'm seeing 47V to each plate.

I've also gone through the horizontal adjustment procedure with no change.

I fear that it might be a bad tube, not the end of the world as I can still use the instrument but I don't want to accept that until I've ruled out everything else.

What am I missing?

[bd139]

Glad I missed that one. Dunstable?

Congratulations on getting that working. I've had preregulators in these sling more than one MOSFET at me which is why I probably walked past it

As for the focus, it's almost always the 510K resistors in these in the focus circuits. They go well out of tolerance. Replace with Vishay VR25 510K high voltage ones. Should be able to get them on RS or eBay.

Edit: just checked the SMM. They are 1M in the 2215.

Don't trust the measured values - they break down under high voltages.

[bdunham7]

I'm assuming that the horizontal position knob is not able to bring the trace back on screen.  Try this:  Put it in XY mode with the input coupling set to GND and both channels to 1V/div.  See if you can put the dot in the center of the screen, if not get it as close as you can.  Then focus it as best you can.  Then move it around vertically and horizontally to see how far it goes and what V-POS and H-POS inputs are required.  Also observe if the focus of the dot changes as it moves (don't readjust the focus).

The focus ladder is more reliable on these than the versions with the 510K/180K resistors, so I wouldn't do that first, at least without evidence.  Do you have an HV probe?

[strawberry]

broken tube

[melling]

Thanks all for the inputs.

Glad I missed that one. Dunstable?

Yes indeed! I was there with the club parking cars all morning. It should have been a giveaway that the 'scope was still on the stall after midday, alas I am a sucker for a project.

Don't trust the measured values - they break down under high voltages.

A good point, I will keep that up my sleeve.

I'm assuming that the horizontal position knob is not able to bring the trace back on screen.  Try this:  Put it in XY mode with the input coupling set to GND and both channels to 1V/div.  See if you can put the dot in the center of the screen, if not get it as close as you can.  Then focus it as best you can.  Then move it around vertically and horizontally to see how far it goes and what V-POS and H-POS inputs are required.  Also observe if the focus of the dot changes as it moves (don't readjust the focus).

Ok, starting off with a nice small dot in the center of the screen, auto intensity is fully CW, Auto focus ~1/4 turn CW.

Moving horizontal to the right will cause the dot to get more intense. Hard to quantify but 2 divisions to the right is noticeably more intense than at the center. It seems fairly linear, the dot at the right of the screen looks as bright as the dot in the center of the screen when I have intensity maxed out.

Moving horizontal to the left will cause the dot to get less intense. 2 divisions to the left of the center it will fade out completely. I can change this by reducing the auto focus so that it is still visible 3 divisions to the left, then 4 etc. The further right I go, the more intense the dot, because I have to change the focus setting to keep it visible on the left.

If auto focus is fully CCW I can just about get a feint dot at 5 divisions left of the center. Part of the center of the screen is lit up with a wide vertical band about 6 divisions wide (the sort of thing you see when the trace goes off screen).

Vertical position isn't really affected by this. If the dot fades out at 3 divisions from the left on the center line then it exhibits the same behavior at the same point horizontally in any vertical position.

Do you have an HV probe?

Don't own one myself but can get hold of one.

broken tube

 

Not found a great deal about failure modes of these tubes, one or two posts about anode connections degrading and breaking but nothing to convince me yet. I can get a dot on the left of the screen, it just requires a lot of twiddlin'.

[bdunham7]

OK, that is a problem I haven't seen or heard of.  The horizontal positioning is more or less OK, but the intensity is proportional to the horizontal position.   

Can you look over the circuit board in the area under the tube and the associated wiring to see if there is any indication of damage, poor rework, modification or ?  It looks like the Z-axis input  has become connected to the horizontal amplifier somehow.

[strawberry]

tube power consumption is brightness dependent. usually deflection supply and anode supply is on the same SMPS. any component associated to this powersupply could result in described failure mode
could be deflection amplifiers one of diff stages not working and loading HV supply....

''broken tube'' just kidding, me remember similar post

[TERRA Operative]

Check out my video on the 2235, including the links to my Google drive.

There is some overlap in these models, so you may find some useful info there.

youtube.com/watch?v=lrXCVg6T-ek

[m k]

Any intensity differences if beams are chopped with slow time/div?