Oscilloscope Tektronix 2235 Short Unstable Horizontal Trace

Could be as simple (to diagnose ,not necessarily to rectify) as a switch or board interconnect corrossion problem , try waggling/pulling/pushing them

=A0or

Thank you for the tip. Tonight I will open the scope and try to replug everything possible.

I was looking at the schematics, and realize the horizontal sweep output amplifiers (Q789 and Q779, two PNP transistors) depend on

+100V. The +100V power supply is just a full wave rectifiers from a center tap secondary (from the switching power supply), filtered by a single 33uF (C954). If this cap is bad or going bad, hell may appear at the horizontal sweep.

But, I remember the problem started at the right half of the sweep on screen, while the left was working nice. When I saw it happening, immediately suspected about the amplifier of the CRT plate that controls the right side.

If the +100V was bad, the problem would not appear only at the right side.

Right now, the problem appears both sides simultaneously.

At the schematics, page [6], scope points 34 and 35 is the output of the horizontal sweep chip, respectively left and right, that will drive the output transistors. May be I could scope (I have other scope, a 2246) those points and see if anything different is happening.

Of course, there is a connector from those points to the output amplifiers... may be your are right, some bad connection in there, may be a simple replug will fix it. This scope was stored vertically in my room, and from time to time it was moved a little bit to the right, left, under a desk, etc. This moving vibration (dragging the back feet in the floor) may cause some bad contacts.

I will report any news.

Thanks again for the tip.

Wagner

If the position control will move what trace you do have to the right, that might eliminate the output stage. Look for it on the print, in some scopes it is a dual pot wired in llike an "X" formation and may feed the CRT plates "directly", but I don't think Tektronix does this. (they have good reason on an engineering level)

I would definitely check it in XY the very next thing. If it works properly in XY you've eliminated the entire amplifier section and can proceed directly to the oscillator. However if whatever you try to display compresses on the right, you know it's that amp, and whether the position control moves the flattened portion or not is the next test. If the whole squished trace moves, it is before the position control, if the trace squishes (love those technical terms) without reaching the right side then it is after the position control.

If the problem is one of the output stages to the plates, the non linear portion of the trace whould exhibit defocussing or astigmatism, I think. It may not with certain CRT designs. To go any farther I'll have to get the print, but this should be a good wham bam eliminate a bunch of stuff. In other words, quichly find out what it isn't.

JURB

You are correct, Jeff.

If the Horizontal Position adjust (R726) can move the "squishing" trace to the right or left, beyond the dead zone, the amplifier is working and has enough voltage to show the trace in that region. Perfect debugging thought. Will do it tonight.

If the XY works all over the screen, for example, a 60Hz sine generating an almost perfect ring on screen, it eliminates voltage, amplifier, plates, etc. Will be around the trace sweep generator, or something like that.

I already identified all the scope points on the diagram and their position on the board printout, so it will be easier to follow this thing.

The only place in the diagram where I can see different circuits (bipolar) for left or right side of the screen (half trace) is the output of the U760, the guy who does the sweep controls, and it is called "Horizontal Pre-amp". It has +SWP and -SWP outputs that drive directly the output transistors. The input of U760 is not separated, it comes from one linear ramp from Q742 that represents the whole screen trace, feeding the U760 "A SWP IN".

So, the fact that the right half of the screen is more "damaged" and "squishing", lead me to think about connections between those boards, or something at the output transistors driving the right plate. As far as I remember, the right side failing was not steady, but as if there is a lot of noise in the right side sweep ramp.

I need to find it out... it seems easy. The first test was to make sure +100Vdc is steady and Okay. Then the Horizontal Position knob to make sure the "sqhished" trace can be moved to the left or to the right of the screen. Next, the XY test, to eliminate forward or backward circuits in the path. There is also a -5V regulator (U750) specifically feeding the U760, and I guess this negative rail is only used to generate the negative sweep ramp to control the output transistors for the right plate.

It will be nice to have this 2235 back to life, even with the Tek2246 available, I always choose the 2235, it is smaller, lighter, faster to operate and I can transport it anywhere I need it, without pulling a muscle in my back.

Thanks, will report.

Wagner.

Look at a signal. Does the trace stay the same when it squishes? Or does the trace get truncated? That should tell you whether it's a sweep timing issue or a gain issue.

o

Another good tip Mike, will look at it. Thanks a bunch. Wagner

Problem Solved, and it was easier than it could be.

When the trace was intermitently shorten (squished), I tested XY, and it was solid steady, so I was relieved to know that everything from U760 (the sweep horizontal control chip) up to the CRT was okay, since XY is a secondary input to this chip, along the A Sweep Input. If the A Sweep input was shaking the screen, and the XY was not, the problem was before that chip, and at the A Sweep circuitry.

I followed the sawtooth waveform from the horizontal "Miller Sweep" generator, from page and it was perfect. The output of the generator being okay eliminated a bunch of problems, all the rotary switches, etc.

Scoping probe point [31] (entry of the U760 chop) the image was deformed. Scoping probe point [30] (generator output) it was okay. In between both points there is R740, a 1k trimpot.

This trimpot can be seen on page of the foldouts (schematics), grid F4, it is physically located at the edge of the timing board (A4), it controls the "gain" of the horizontal trace. The "gain" is the adjustment of the trace width on screen, and obviously, the width of the image. Changing the adjustment of this pot will mess with the accuracy between the trace (image) and the acrylic marks, so you might read a wrong period of time from a waveform.

Before the R740 the sawtooth was neat, after the trimpot, it was deformed and shaking... hmmm... using a small screwdriver I just rotated the trimpot to one side and another, several times, as "to clean the trimpot contacts over the carbon film", it solved the problem for few seconds, but always returned, until the trace collapsed completely. Suddenly the trimpot was showing more than 2M Ohms. As the trimpot is at the edge of the board, I was able to replace it without disassembling anything. Used a different multiturn

1k trimpot. Now it is working nicelly.

Of course this trimpot should be adjusted with a very precise waveform and coincide the waveform with the acrylic marks (graticule). I adjusted using a crystal 10MHz squarewave (verified with a HP frequency counter).

Later I opened the tiny trimpot and found that one of the end-of- course lead was making bad contact with the carbon film metallic termination. Unfortunately, if you grab one trimpot lead with a pliers and turn few degrees it will loose the good contact with the termination. Looking at the trimpot, it seems the Tektronix assembly required the trimpot lead to be rearranjed with a pliers, to fit the pcb holes, it then reveal the problem was "planted" at the production line. Probably the pin broke solid connection with the termination, years later and temperature changes increase oxidation in the bad contact. So, rotating the trimpot adjustment back and forth will not reveal the problem, since the bad-contact was not between the carbon film and moving contact. It was not super easy to pinpoint the problem, but evident when the trimpot presented 2M Ohms of resistance.

On the same page , grid F6, there is another exactly the same trimpot, R730, 1k, for the B Sweep Gain. B sweep is when you expand part of the actual trace. This R730 adjust the width of the B sweep. Testing and comparing, R730 is actually also presenting some problems, just touching it with a pen makes the image shakes and squishes. As it is somehow stable, I let it be quiet.

Will try to find two better brand trimpots that might fit well in the space and replace both later. In real it doesn't need to be a high precision trimpot, it only need to be small and top adjusted. Even a plastic body will do it. Found some replacements at Jameco for less than $2 each.

The trimpot Tektronix p/n is 311-0635-00, the manufacturer p/n is

82P-6-2-102k, manufacturer code is 73138. It is a NonWir 1k 10%, 0.5W round, metalic top, very small trimpot (0.25" diameter). It seems to manufactured by Bourns, even that part/number doesn't match: 27

I wonder, there are several other similar trimpots all over the scope boards... hmmm... new problems soon?

Used the opportunity to clean the interior of the scope with compressed air. It is a nice scope returning working to the bench.

Thank you for the help.

Wagner Orlando Florida

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If it ain't broke, don't fix it.