Tek 2236 and the CTM Board


 

I'm trying to get in to a 2236 in order to lube or replace a noisy power supply fan, but I can't figure out how to disassemble the pushbutton extensions from the buttons. The service manual says:

"2. Remove the six buttons and extension shafts from the CTM switches by inserting a small screwdriver between the extension shaft and the switch shaft. Push down and forward until the extension shaft is disengaged and pull the shafts straight back through the front panel."

I'm perplexed by which way is up or forward in this paragraph. I have the scope sitting on it's belly (where the small rubber feet would be with the case on) and the front of the CRT facing me. Am I supposed to push "down" on the switch shafts from this position? Will they disengage from the extension shafts by sliding perpendicular to their normal direction of travel? I've tried to release the extension shafts several times with no success (and, fortunately, no damage to the switch shafts or extension shafts). I've tried different sizes of "small screwdriver" from a miniature screwdriver that would be suitable for adjusting the trace rotation or probe compensation, to a nylon spudger whose blade is about 6mm wide.

What am I doing wrong?

-- Jeff Dutky


satbeginner
 

Nope, put the screwdriver vertical behind the side of the knob, push down on the screwdriver and use it as a wedge to push the knob towards you.

Away from the front, in the same direction as the axle.

Better to use two screwdrivers to keep everything in line.

If you push down on the knob itself you will break things....

Saludos,

Leo


 

Leo,

there is no knob here (or, rather, the was a knob in step 1, but I got that out without any trouble). These are pushbuttons that have a short extension that pokes through the front panel. The extensions are clipped around the pushbutton's plunger. It's clear that they can be separated somehow, but not clear how that can be achieved.

I have created this album https://groups.io/g/TekScopes/album?id=257515 (titled "2236 CTM Pushbuttons Disassembly") to illustrate the process and the issue.

-- Jeff Dutky


 

Ah, I figured it out: by prying the right hand side of the extension away from the plunger, and then levering the left hand side with the blade of the spudger, and applying more force than I had thought safe, the extension lets go quite suddenly (and a bit distressingly). After you've done it a few times, however, you get used to the amount of force needed, and the proper angle to apply with the spudger. A diagram in the service manual would have been very helpful.

My apologies to Leo if that was what he was telling me to do. I misunderstood because of the use of the term "knob" rather than "button". I'm a terribly literal-minded sort, and it leads me astray all too often.

Next problem was removing the power supply cover, but that was merely a matter of find all the screws that held it in place (two recessed and one pan-head torx on the back, two pan-head torx on the bottom, one that holds the HV protective plastic cover in place, and one on the side), then wrestling it out of the case in close quarters (probably a lot easier if you've removed a the CTM board and the alternate sweep board). We will see if I'm able to get it back in place without first removing the CTM and alternate sweep boards (I kind of doubt it).

Finally it was a simple matter to remove the fan, peel off the hub sticker, and put some light oil on the end of the bearing shaft. We will see if that cures the loud fan noise, or if I have to order a new fan (a Matsushita PANAFLOW DC burshless, model FBP-06A12L, easily and inexpensively obtainable from at least one popular electronics retail site).

-- Jeff Dutky


 

I got the 2236 reassembled, but I had to unmount the CTM board in order to get the power supply cover back in place (the CTM board and its back hinge interfered with getting the power supply cover back in, and this was only made more difficult by the fact that the alt sweep board is not removable, at least not without unsoldering a couple dozen pins). The fan sounds a little better than it did, but I've ordered a replacement anyway, because I figure its going to fail eventually, and the replacement was only $5.

After buttoning everything up and playing around with all the toys (it's awfully nice to have a multi-meter/counter-timer that can read what you've got on the screen!) I realized that the x10 mag wasn't working. At all. So I unbuttoned everything (except the power supply) and had a good slow wander through the service manual again. It turns out that the x10 mag function is supported by a handful of passive components hanging off of one IC, and you can pretty much test them all because the x10 mag switch breaks the loop in the circuit. I found that the trim pot was reading 3x what its maximum value should be (it's a 100K pot, but was reading almost 300K). I figured that there was no harm in fiddling with it to see if it could be brought into trim, because I'd have to adjust it anyhow if I replaced it, but when I twiddled it just a little bit it came back into trim. I ginned up a signal that spanned 10 cycles across the reticule, pulled the 10x mag, and trimmed it to show exactly one cycle. Good enough for government work.

I also cross checked the counter-timer against two of my other DMMs that have a Hz measurement feature, and everyone agrees to within 1% so I'm feeling pretty confident in the 2236 CTM features.

So now I've got a 100MHz scope with multi-meter that I can use to check the 475 and 475A, which both still need some attention.

I really feel like I'm getting my sea legs now. I think I'm beginning to really understand a bunch of things that were only the vaguest theory to me before this (transistor circuits are just coming into focus, for example), and I'm getting a lot more confident in both my ability to diagnose problems, and manually perform the work to fix them. I'm also feeling confident enough in my abilities to tackle the two scopes that belonged to my dad without fear that I'll do something stupid and destroy one of them. It's a good feeling.

-- Jeff Dutky


greenboxmaven
 

Great job! As for the power supply cover, my scope (2230) was a totally dismantled basket case when I got it, so I had to figure out how to manuver the shield back in place, especially with all of the wiring in disarray. Two of the hinge halves mounted on the chassis for the top board were cracked and the screws could not be tightened. I removed them, super glued the cracks, and tried re-installing them. They broke again at moderate tension. I super glued them again, and began looking for another way to use them. I removed the two mating halves from the circuit board because they are all the same and in excellent condition. I countersunk the screw holes enough so the taper of the flathead screws holding them to the chassis would not bottom out and try to expand the holes. Now the halves on the chassis were secure. I took the repaired ones, drilled out the threads, and ground the heads of two flathead 4-40 bolts to a "D" shape, so they would go into the holes without strain and fit flat on the side tab of the hinges. I put a few drops of super glue on the circuit board, and immediately installed, aligned, and tightened the nuts I applied to the bolts. After a few minutes. I loosened the front chassis hinge half for clearance, installed the board, and tightened the chassis half. This was all before I made other repairs and got the scope working. After trying to install the power supply cover, I realized I had to remove the top board once again to get it in place, this time the hinges were fine and everything went back together after the cover was in place. Can you read the voltage of an input on yours with the digital multimeter while observing it's waveform? That could be a big help for troubleshooting an intermittent problem. The 22XX is light and feature packed, but I don't get the sense of physical ruggedness the older scopes had, even though mine was utterly filthy before I restored it. That has to count for something.

Bruce Gentry, KA2IVY

On 12/4/20 23:04, Jeff Dutky wrote:
I got the 2236 reassembled, but I had to unmount the CTM board in order to get the power supply cover back in place (the CTM board and its back hinge interfered with getting the power supply cover back in, and this was only made more difficult by the fact that the alt sweep board is not removable, at least not without unsoldering a couple dozen pins). The fan sounds a little better than it did, but I've ordered a replacement anyway, because I figure its going to fail eventually, and the replacement was only $5.

After buttoning everything up and playing around with all the toys (it's awfully nice to have a multi-meter/counter-timer that can read what you've got on the screen!) I realized that the x10 mag wasn't working. At all. So I unbuttoned everything (except the power supply) and had a good slow wander through the service manual again. It turns out that the x10 mag function is supported by a handful of passive components hanging off of one IC, and you can pretty much test them all because the x10 mag switch breaks the loop in the circuit. I found that the trim pot was reading 3x what its maximum value should be (it's a 100K pot, but was reading almost 300K). I figured that there was no harm in fiddling with it to see if it could be brought into trim, because I'd have to adjust it anyhow if I replaced it, but when I twiddled it just a little bit it came back into trim. I ginned up a signal that spanned 10 cycles across the reticule, pulled the 10x mag, and trimmed it to show exactly one cycle. Good enough for government work.

I also cross checked the counter-timer against two of my other DMMs that have a Hz measurement feature, and everyone agrees to within 1% so I'm feeling pretty confident in the 2236 CTM features.

So now I've got a 100MHz scope with multi-meter that I can use to check the 475 and 475A, which both still need some attention.

I really feel like I'm getting my sea legs now. I think I'm beginning to really understand a bunch of things that were only the vaguest theory to me before this (transistor circuits are just coming into focus, for example), and I'm getting a lot more confident in both my ability to diagnose problems, and manually perform the work to fix them. I'm also feeling confident enough in my abilities to tackle the two scopes that belonged to my dad without fear that I'll do something stupid and destroy one of them. It's a good feeling.

-- Jeff Dutky





tekscopegroup@...
 

Just for future reference, instead of using two screwdriver shafts there is another way of pushing the knobs off the shaft by using a pair of thin long nose pliers, by positioning each leg on opposite sides of the shaft and under the knob. Then slowly and gently start using leverage action against the front panel to push the knob off the shaft, moving it a little at a time of each stroke until it comes off. Be careful not to scratch the front panel, protect the area of contact with the pliers with a rag of a piece of thin cardboard.


 

greenboxmaven,

The 2230 looks like a very similar construction to the 2236, especially with the hinged top PCB. That sounds like impressive repair work you did to the hinges. I would have been tempted to build entirely new hinges, or repair with epoxy rather than cyanoacrylate, but that's because I really don't trust plastic parts once they starting to degrade.

Yes, I can read the voltage of a signal on channel 1, both DC V and AC V, depending on the coupling selected for the channel. The CTM system can also read frequency, period, width, delta time, and delay time, as well as something called "totalize" which I haven't fully understood yet. It also has external DMM probes that can be used for DC V, AC V, continuity, resistance and diode measurements, and temperature measurement, with the proper leads.

The 2200 series were aggressively cost reduced to compete with new scopes then available from Japanese manufacturers, so there is a certain flimsiness to them that was not present in the 400 series. Despite the cost reductions they are still excellent scopes and quite rugged. Some of the cost saving measures may have made the scopes more durable than their predecessors: very few components on the 2200 series scopes are socketed, which greatly reduces the tendency for things to come loose or oxidize their contacts, which is apparently a terrible problem for the 400 series, where every discrete transistor was in a socket.

-- Jeff Dutky