Tektronix 2215 with short trace

[ddeguire]

I recently bought a Tektronix 2215 off of eBay.  The seller said that he did not test every function, but he did connect the probe to each channel and applied a sine wave.  On the pictures the trace touched both sides of the screen.  When I tried out the oscilloscope, the trace does not touch both edges of the screen.  It goes from the middle of division 4 through division 8.  I've tried fiddling with all the knobs and switches, but nothing helps.  I downloaded the operator's manual and service manual from Tektronix, and went through the steps to get the initial baseline.  Unfortunately, I cannot fully calibrate the scope right now because I don't have a signal generator.

The serial number on the scope match the serial numbers of the scope in the eBay pictures, so I know it's the same scope.  That leaves two possibilities: 1) there's some knob or switch I haven't found yet that will fix the problem, or 2) something broke between the time he took the pictures and the time the scope got to my house.  Either way, has anyone seen this problem before and know what to do about it?

Thank you very much for your help!
Dan

[Paul Price]

Short Trace == Short Circuit, assuming you've twiddled all the knobs and buttons.

You can get a display somewhat like this in "A intensified by B" with the intensity control turned down to some odd setting on some scopes.

That is, unless the Beam Finder button is stuck in pushed down, always activated. Don't look like it is from your pictures.

Trace should always otherwise be min. >9 divisions when the display is triggered or free-running and not in X-Y mode.

What is even more frightening is how the front panel and the scope display changes color!!!

[tautech]

Check that it is not in dual/delayed timebase mode.
Quick check, turn the vernier knob and trace will widen or narrow.

[KSP]

Check that it is not in dual/delayed timebase mode.
Quick check, turn the vernier knob and trace will widen or narrow.

Agreed, it sort of looks like it is in delayed timebase mode. Cant remember this scope in particular, but in some Tek scopes you pull the timebase knob out to set the DTB. Have a fiddle with that

[ddeguire]

Thank you for your replies!

I have searched through the two manuals and the front panel for any mention of "A intensified by B", or timebase mode, but couldn't find anything.  There is a "B Delay Time Position" knob, but I can move that to every position between 0 and 9 with no affect on the trace.  I've included a picture of my panel in case I missed it.

The only knob that I can pull out is the x10 knob.  That does make the trace wider, but still not what I'd expect.

The beam finder button is not stuck.

I guess the next step is to void the warranty and see what I can find.

Dan

[w2aew]

You should rotate the timebase vernier clockwise until it clicks into the Cal position. It won't fix the width issue, but it is good practice.

If you pull the same timebase vernier knob out to engage the x10 function...
...and the width changes, then you probably have a horizontal oscillator issue
Or
...if the width doesn't change, then it may be a deflection/amp issue.

[kc9qvl]

The White mark on the "B" knob should line up between the 2 black lines on the "A" indicator. The red knob should be pushed in and turned clockwise until it clicks in the detent.

[Paul Price]

There is something very odd about the position and color of the Horiz Timebase knob. It doesn't tp match the other knobs.
You can also see that your intensity control is missing a knob. This happens when a scope is dropped upon its face or else with shipping without protective packaging.

See the attached to know what a 2215 and 2215A traces and front panel should look like.

[ddeguire]

I finally got around to voiding the warranty.  I didn't see anything obvious - no blown capacitors, brown spots, or any other evidence of escaping magic smoke.  I guess now I get to dig through the Service Manual.  It has a troubleshooting guide, but the first question was "Does the base line trace appear"?  The only options are YES and NO, but the real answer is "A LITTLE BIT".

[tautech]

As Paul Price mentioned but did not elaborate, low voltages will cause this sort of problem.

First thing to check is that all voltage test points on the PSU are within spec.

[ddeguire]

Thank you tautech!  That's where I'll start.

[David Hess]

Is the vertical sensitivity low by the same amount that the horizontal timebase is compressed?  Do the vertical position controls allow the traces to be moved through the full vertical range of the CRT?

If the display is compressed horizontally *and* vertically, then the CRT post deflection acceleration voltage could be high which points to a problem in the HV regulator which controls the cathode voltage or perhaps the low voltage power supply.

Another possibility is that the beam finder function is stuck on for the horizontal and vertical CRT amplifiers but not for the focus.  Maybe the switch is dirty.

[ddeguire]

Update:

I tested all the voltage test points, and they were all within tolerances.  I adjusted the the CRT Bias (as directed in the Service Manual), but it didn't help.  While I had the case off, I tweaked a few other pots, but nothing seemed to fix the problem.  The next step in the troubleshooting guide is to do a manual adjustment, which requires instruments I don't have.  I guess I need to find someone near me that I can borrow them from.

Dave:  Thank you for the advice, but the vertical displacement is working fine.

[Andy Watson]

I agree with many of the above replies - it looks like it is stuck in A-intensified B mode. The scope has obviously suffered some trauma to its front panel (i.e. intensity knob), can you rotate the time base knobs independently? And does the timebase knob pull-out freely and click back into place? - you may have to rotate the outer knob anticlockwise to pick-up the A time base setting. I believe there is a micro-switch that detects the inward movement of the timebse shaft - when it is out, the mirco switch puts the scope in A-B mode - can you hear the micro switch operate as you move the timebase knob kn and out? If the shafts are bent in anyway, they probably will not turn, or slide independently.

[Satbeginner]

Hi,

I had the very same thing :-)

It turned out to be a very easy repair:

On the bottom of the CRT one of the wires to the deflection plates us loose.

Put it back using tweezers, carefull not to break the pin on the CRT.

Saludos,

Leo

[David Hess]

Dave:  Thank you for the advice, but the vertical displacement is working fine.

That means that the CRT acceleration voltage is correct.  If the post deflection acceleration was missing, then it would result in lower deflection sensitivity in both directions.

[CaptDon]

Actually if post deflection acceleration is missing the trace grows in size, not shrink, and it will usually be dim!!!

[David Hess]

Actually if post deflection acceleration is missing the trace grows in size, not shrink, and it will usually be dim!!!

I have to disagree about its effects on deflection sensitivity because I have measured it under various conditions.

If the cathode acceleration is lower, than the defection sensitivity goes up because the elections spend more time between the deflection plates.  Oscilloscopes which support a reduced scan mode take advantage of this by doubling the acceleration voltage at *only* the cathode to halve the deflection sensitivity.

Post deflection acceleration is completely different and higher voltage here actually increases deflection sensitivity because of how it interacts with the expansion lens.  Oscilloscopes which lack scan expansion may be different but they usually (always?) lack post deflection acceleration anyway.

[CaptDon]

Look at your post where you mentioned POST DEFLECTION ACCELERATION. If POST deflection acceleration voltage goes down the trace will INDEED GROW! If the cathode voltage and corresponding voltages PRIOR to deflection are raised (made more negative with respect to deflection plates) the the trace will shrink as you mentioned. Raising the final anode (POST DEFLECTION) will shrink the trace, lowering it will expand the trace. There is one oddball here, the 5ABP1 or 5ADP1 or similar. As you raise the final anode voltage the deflection sensitivity decreases requiring more deflection differential voltage for the same width trace. At some point of raising final anode voltage the beam will hit the neck of the tube and you will find you can only get a 4 inch trace on a 5 inch tube. As you lower the final anode voltage the trace will grow and will require less differential voltage on the deflection plates. Some tubes like the 5UP1 don't use post deflection acceleration so I am not sure how they behave. Believe it, on post deflection acceleration electrostatic deflected CRT's lowering the final anode voltage will induce 'blooming', poor focus and poor brightness! I am probably certain raising the cathode voltage (making it more negative with respect to the deflection plates will cause decreased deflection sensitivity and a smaller trace which is the same effect as raising the final anode voltage (making it more positive) in a post deflection acceleration tube which indeed will shrink the trace. I may be wrong, I just do this for a living and have repaired more legacy scopes than ten people could count on all their fingers and toes. Incidentally, the beam finder changes the cathode voltage slightly to increase beam brightness and radically decreases the gain of the vertical and horizontal amplifiers on Tektronix electrostatic CRT analog scopes, they don't diddle cathode voltage to effect trace width!!
 

[David Hess]

Look at your post where you mentioned POST DEFLECTION ACCELERATION. If POST deflection acceleration voltage goes down the trace will INDEED GROW!

Like I said, I measured it and that is not the case when a scan expansion mesh is used, as is the case with the Tektronix 2215 and I believe every Tektronix oscilloscope faster than 50 MHz; the 50 MHz Tektronix 547 was their last fast oscilloscope without it which made it desirable for those who wanted the smallest possible spot size.  The Tektronix book on CRT design describes the differences between the CRTs you mention and what is going on although it does not mention how the post deflection acceleration voltage affects deflection sensitivity when a scan expansion mesh is used, which sort of makes sense because the way that a scan expansion mesh works relies on PDA and it will always have it.

I measured the effects by using different post deflection acceleration voltages which was easy enough to do because Tektronix used the same high voltage connector on all of their oscilloscopes.  There was some question at the time on the Tekscopes email list about how to diagnose failure of the high voltage multiplier.  It was assumed by me and others that the deflection would be increased significantly but there were too many reports of CRTs with about half the deflection sensitivity which did not make any sense, so I measured it and found that contrary to everybody's expectations, without PDA or at a lower PDA voltage, deflection sensitivity decreased.

Oddly enough, these CRTs also produced usably sharp and bright images without PDA, in line with low bandwidth CRTs which lack PDA, but of course they were much brighter and sharper with PDA.

[BrianHansell]

Short Trace == Short Circuit, assuming you've twiddled all the knobs and buttons.

You can get a display somewhat like this in "A intensified by B" with the intensity control turned down to some odd setting on some scopes.

That is, unless the Beam Finder button is stuck in pushed down, always activated. Don't look like it is from your pictures.

Trace should always otherwise be min. >9 divisions when the display is triggered or free-running and not in X-Y mode.

What is even more frightening is how the front panel and the scope display changes color!!!

I'm just wondering if anybody is still looking at this post. I too have a short scan on x1 but a full scan on x10.

My "short" scan is exactly nine divisions, as mentioned by Paul. So have I actually got a fault or not. All the base line voltages are correct, albeit with some really weird resistance readings at the test points.

I thought I'd try and get an answer to this question first before I started to rip it apart any further.

[David Hess]

I'm just wondering if anybody is still looking at this post. I too have a short scan on x1 but a full scan on x10.

My "short" scan is exactly nine divisions, as mentioned by Paul. So have I actually got a fault or not. All the base line voltages are correct, albeit with some really weird resistance readings at the test points.

I thought I'd try and get an answer to this question first before I started to rip it apart any further.

The trace is suppose to be longer than 10 divisions to provide at least a minimum amount of overscan.  If you only get 9, then something is definitely wrong.

[BrianHansell]

Thanks for the reply David, I'll now start delving. Wish me luck.

[David Hess]

Besides checking the power supply voltages and ripple, make sure the vertical deflection is correct.

If the sweep is 9 divisions, and the vertical deflection is only 90%, then the problem is likely with the CRT acceleration voltage which affects both equally.  If the vertical deflection is correct, then the problem is likely in either the horizontal sweep generator or horizontal CRT amplifier.  These can be distinguished by testing the horizontal deflection in XY mode which bypasses the horizontal sweep generator and drives the horizontal CRT amplifier directly.

[BrianHansell]

Once again David thankyou for a speedy and very helpful reply.

There are no problems at all with the vertical deflection side of the instrument, so that narrows it down. Thankyou for the tip about the sweep generator, that is a lot earlier in the chain than I had first envisaged.

I'll start there and see where the problems take me. Just as an aside, I did check for a loose connection on the Horizontal Deflection Plates, as in Satbegginer's post, but nah it wasn't that easy. I'll report back.

regards