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Internal Drive Mechanism for 7B53N (et., al.) Sweep Speed Cam
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The rear cam in my 7B53N does not turn when the sweep speed knob is rotated. I have disassembled the entire cam assembly but whatever mechanism causes the shaft to be able to rotate the cam is virtually inaccessible. From what I can tell, the shaft and drive mechanism were assembled and then the end caps were glued onto the cam.
I presume that in order to allow the shaft to both rotate the cam and be pulled so that the rear cam can be rotated independently of the front cam, there may be a spline inside the cam but I can't see that. From what I can tell, the cam in the 7B53N is different from the other 7B53 variants so finding a replacement may be a challenge. Does anyone know if this might be repairable? I'm not sure I can get the end cap off without destroying the cam but maybe that's possible but, even if I can get it apart, I'm not sure what this looks like inside and whether it could be repaired. Thanks, Barry - N4BUQ |
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On Sun, Dec 12, 2021 at 08:18 AM, n4buq wrote:
The rear cam in my 7B53N does not turn when the sweep speed knob is rotated. IThis sounds like the same problem that I've fixed on a number of 465 sweep drum switches. The rear drum rides on a plastic drive dog that was overmolded onto the 1/8" hollow shaft. With time and possibly abuse, that plastic dog can shatter,. While it may look like the drum was glued together after the shaft w/dog was inserted, that's not the case. Note the elongated hole at one end of the drum. The drive dog and it's spring was able to slip thru that hole, then a tinnerman washer was pushed onto the shaft from the other end, limiting the amount of axial excursion available to the shaft. The hard part was finding someone to make me some brass drive dogs to replace the plastic one. I could slip the brass dog into position, marked by the knurling that Tek put on the shaft to key the overmolded part, and solder it to the SST shaft using some flux that was slightly more aggressive than rosin. This makes for a repair that's better than new. The trick is finding someone who can make the brass replacement dogs. They are quite small, and I had to finish the final shape to make sharp inside corners where a 1/8" end mill left too large a radius. The other trick is getting EVERYTHING assembled correctly when you're done. This can be quite fussy. And, of course, you have to be very careful of the spring contact fingers that the drums control. It's possible that the OE Tek parts consisted of a shaft and a plastic drive dog that was pressed onto the knurling. It's impossible to tell, once the dog fails, because all that's left of the dogs to examine is a small collection of little plastic bits. |
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Hi Jim,
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I'll have to disassemble the cams once more; however, I don't recall seeing an elongated hole in either end of that drum. The hollow shaft isn't constrained for axial excursion much at all and in order to get the knob back on, it's necessary to insert something in the back end of the rear drum to provide some pressure against that shaft to keep it from pushing towards the rear. I can push it all the way to the end where the larger hole is and can see a disk that's attached to the hollow shaft but I can't make much out about it. If I pull on the hollow shaft, there's definitely spring tension as it should be, but when I rotate it with it pulled forward, all I can feel is it grinding/catching against something in the forward part of the drum. I presume that's the broken dog (or what's left of it) grinding against whatever the dog used to latch into. Regarding those leaf fingers, apparently I must have accidentally moved the front cam axially just slightly before it was fully seated. Things weren't working at all correctly for sweep timing so I disassembled it only to find two of the leaf fingers floating around inside the housing. It isn't easy, but I managed to solder those back in place and, from what I can tell, those are functioning properly once again. I REALLY hated that I'd done that and I'm now EXTREMELY careful to seat that assembly down to the board ONLY when it is fully positioned axially. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jim Adney" <jadney@...> On Sun, Dec 12, 2021 at 08:18 AM, n4buq wrote:The rear cam in my 7B53N does not turn when the sweep speed knob is rotated. IThis sounds like the same problem that I've fixed on a number of 465 sweep drum |
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On Mon, Dec 13, 2021 at 09:55 AM, n4buq wrote:
If I pull on the hollow shaft, there's definitely spring tension as it should be, butIt's been a couple decades since I worked on one of these. Thinking about it, I suspect that the drive dogs should drive that drum in normal operation, but when the shaft is pulled the dog disengages and lets that drum stay in place while the front drum still rotates. If that's the case, the rear drum controls the main sweep while the front drum controls the faster sweep. The drive dog body is a cylinder about 1/4" in diameter, with two ~1/8" protrusions, 180 apart, that stick out from the main body. The hole in one drum end plate is 1/8" to clear the bare shaft; the other end plate has a 1/4" hole with two protrusions to clear the dogs. To assemble, slide the spring on the shaft, then the drum on the shaft. The drive dog body slips in the larger hole, the spring is compressed, and then that little retaining washer slides on the other end, so the dog body can't come back out. The position and orientation of everything is important. If you have everything apart, the shaft may not want to come out the small hole end because of the knurling on the shaft. In that case, carefully work the retaining washer off the shaft so everything can slide out the other end. At that point, you should be able to see exactly what you are up against. I MAY still have one of the brass replacement dog bodies. |
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Hi Jim,
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I'm reasonably sure that the forward drum is the primary sweep control and the rear is the delayed sweep control in this plugin. I may need to open my 465B and compare the rear drum. From what I can tell, there's not much way to get the shaft and the disc that's holding it in place out of rear hole without some modifications. The disk has either a retaining/grip ring or is crimped onto the shaft. Either way, if I get the disc off of the shaft, the shaft will probably come out the front end of the drum but I'm pretty sure the disc will not come out of the rear hole (and, certainly, not the smaller front hole). I'll keep looking at possibilities but it's not looking real good at this point. Thanks again, Barry - N4BUQ ----- Original Message -----
From: "Jim Adney" <jadney@...> On Mon, Dec 13, 2021 at 09:55 AM, n4buq wrote:If I pull on the hollow shaft, there's definitely spring tension as it shouldIt's been a couple decades since I worked on one of these. Thinking about it, I |
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On Fri, Dec 17, 2021 at 03:06 PM, n4buq wrote:
The inside shaft reaches through the hollow haft to the rear drum and is controlled by the front grey knob; the B sweep. The outer, hollow shaft, only reaches to the front drum and is controlled by the clear plastic disk with protrusion which sits behind the gray knob; the A sweep. There is no other way this could be arranged. Raymond |
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Yes, it's clearly been a long time since I've had to work on one of these. I was thinking that pulling the drive dog forward would disengage the drive dogs from the rear drum, but that bothered me, as something in the back of my mind kept saying, "A Sweep in front, B Sweep in back."
So the interlocking is done by the knobs and the drive dogs continue to turn the rear drum even when its shaft is pulled forward. Thanks, that makes more sense. For the original poster: If the front knob does not turn the B drum, does it turn the 1/8" B drum's shaft? If it does, but the B shaft does not turn the B drum, then the drive dog has either shattered (most likely) or come loose from the shaft. You'll have to remove that retaining washer from the shaft to get it apart. That can be tricky, or you can destroy the washer and buy a new one at your local hardware store. Does anyone have a better name for that kind of retaining washer? It looks like an internal star lockwasher, but the inside spikes flare in one direction so that it can be pushed on but can't be easily pulled off. McMaster-Carr calls them Push-On External Retaining Rings. |
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I will have to try removing that retaining ring and go from there. I'm afraid that even if I can do that and release the tubing so that it comes out the front, I still may not be able to get the disk out of the larger hole in the back of the drum but I don't have anything to lose at this point so I'm going to give it a try.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jim Adney" <jadney@...> Yes, it's clearly been a long time since I've had to work on one of these. I was |
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On Fri, Dec 17, 2021 at 09:05 PM, n4buq wrote:
I will have to try removing that retaining ring and go from there.Yes, that's the only way to get to the bottom of this. Once the retaining ring is gone, the shaft should slide out the other end easily. Then you'll be left with a plastic dog that's either cracked into 2 parts, or busted into a dozen pieces. It will take some effort to get everything out of there, including the spring that won't be helping matters. Try to get an idea of the orientation of the dog as you get the pieces out. The dog body has a long and short end; that is, the actual drive dogs are closer to one end of the body. If the dog is simply broken into 2 halves, I suppose it might be possible to glue it back together on the shaft, but I hate to resort to fragile repairs on something so difficult to get to. That's why I went with the brass replacements. |
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On Fri, Dec 17, 2021 at 10:44 PM, Jim Adney wrote:
This all looks very much like a rather frequent problem with 465- and 475-scopes, which have similar construction. A lot has been written about this on this Group, including the tricky - but successful for all who tried, AFAIK - glue method that I suggested. Look around in the Group's messages; I have no pointer nor the time currently to look them up, unfortunately. Raymond |
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I figured out how to get to the broken parts and here are some pictures:
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/g/TekScopes/album?id=270912 It turns out that the front part of the drum cam was indeed glued to the drum body. I took a razor knife and worked it around that seam and the front hub popped right off. As you can see, the two sections inside that comprise the drive dogs and the hub were broken. I suspect a hard hit to the shaft with the gray knob removed but can't say for sure. Jim - I presume these are the "dogs" you referred to? Raymond - was this the part to which you were referring when you mentioned the "glue method"? In any case, a bit if glue and this should be back together and working once again. If the tunnel diode I ordered works (I should get it in a few days), I may have a fully-functioning 7B53N. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Raymond Domp Frank" <hewpatek@...> On Fri, Dec 17, 2021 at 10:44 PM, Jim Adney wrote:This all looks very much like a rather frequent problem with 465- and |
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On Sun, Dec 19, 2021 at 03:37 AM, n4buq wrote:
That looks quite promising, Barry! No it is not what I meant with the "glue method". The glue method is meant to repair a situation where a plastic part that normally is fixed on a corrugated shaft inside the drum assembly has broken loose, allowing complete extraction of the shaft through the front. The repair is done by carefully greasing most of the shaft, leaving just the part that is to be glued open and putting some cyanoacrylate glue on that part. Next, the shaft is pushed in through the tube (outer shaft) into the plastic part. Surprisingly(!), this has worked for quite a lot of people. I remember at least three successful operations and no failures (like glueing everything together... Raymond |
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I was kind of surprised that this came apart as easily as it did. I began to realize that the hub appeared to have a slightly different color than the drum and that started me down the road of using the razor knife. I should've realized that long ago but kept thinking that end of the drum was one molded piece so kept looking elsewhere.
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Thanks again, Barry - N4BUQ ----- Original Message -----
From: "Raymond Domp Frank" <hewpatek@...> On Sun, Dec 19, 2021 at 03:37 AM, n4buq wrote:That looks quite promising, Barry! |
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On Sat, Dec 18, 2021 at 08:37 PM, n4buq wrote:
/g/TekScopes/album?id=270912I don't see anything in your photos that resembles the drive dog that I'm familiar with. In my opinion, you would have been MUCH better off simply removing the retaining washer and not damaging the drum. Now you have 2 things to fix. |
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Removing the retaining washer would not have allowed a fix. It was necessary to remove the front hub from the drum to get to those parts and repair them. I can only presume the drive mechanism in this drum is different from the one(s) you worked on.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jim Adney" <jadney@...> On Sat, Dec 18, 2021 at 08:37 PM, n4buq wrote:/g/TekScopes/album?id=270912I don't see anything in your photos that resembles the drive dog that I'm |
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On Sun, Dec 19, 2021 at 05:32 PM, Jim Adney wrote:
I don't see anything in your photos that resembles the drive dog that I'm familiar with.I may have been too quick to respond here. There's something with a ~1/4" diameter pushed right up against the retaining washer. Your photos don't show what the other end looks like, but it may be the drive dog that I expected. Yours appears to be clear plastic, while the ones I've seen were the same material as the drums. Does that dog body rotate with the shaft? If so, does it still have dogs that engage the slots in the hub? Something seems wrong about the parts in your photo, because those parts should simply slide out of that hub right now. Perhaps someone has glued them all together, so the dog body is glued to the hub and the shaft just slides in the body. Or perhaps the dog body is rotated so that the dogs are no longer aligned with the slots in the hub. I was not familiar with the "glue method" mentioned earlier. That could work if the dog body was simply loose on the shaft, but it would have been much simpler to pull everything out and glue the body to the knurled portion of the shaft. The dog body will slide out the end if the dogs are aligned with the notches in the larger hole. I'm quite sure that Tek did NOT assemble that part and then glue the drum around it. If you look at the parts list, Tek shows only complete drums and complete shafts w/dog bodies. Neither would have been useful if the drums were glued together after inserting the shafts. The 2 slots on the sides of the larger hole in the hub, visible in one of your phots, are also evidence that this was designed to clear the drive dog body. If the drum was expected to be glued around the dog body, the holes on both ends could have been simple 1/8" round holes. My impression is that the retaining washer should have been on the other end of the drum. It looks to me like someone has been in here ahead of you. That would explain a lot. My prior experience is a couple decades old, so forgive me if my memory is imperfect. |
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The way this works for this particular rear drum is that the two splines drive the drum, regardless of whether it's in delayed sweep or normal sweep mode. Those splines also allow the shaft to be pulled forward such that when the knob is pulled far enough to disengage the pin that ordinarily locks that knob it to the larger ringed knob (the one that rotates the forward drum for the A or MAIN sweep), rotating the smaller, gray knob still causes the rear drum to rotate independently of the front drum thus allowing two sweep speeds.
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I'm reasonably sure no one has been into that rear drum since it left Tektronix. If they had been, I doubt they would have glued it back together without gluing the broken drive mechanism parts too. I looked around a bit on eBay from other examples and I think I see where the style of drive mechanism to which you're referring is shown. I'm letting the glue cure where I glued the end cap to the drum. After that cures, I'll reassemble the entire cam assembly and report back. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Jim Adney" <jadney@...> On Sun, Dec 19, 2021 at 05:32 PM, Jim Adney wrote:I don't see anything in your photos that resembles the drive dog that I'mI may have been too quick to respond here. There's something with a ~1/4" |
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On Mon, Dec 20, 2021 at 04:44 PM, Jim Adney wrote:
The glue method, which (AFAIK) I introduced and performed successfully in several cases, has been successfully performed by others, some of who reported back in this Group. It is meant for those who can't take the 'scope apart, don't dare or don't think it is worth the trouble. It works in those cases where the drum, which is permanently fixed on the knurled part of the shaft during the molding procedure, breaks loose. The shaft then turns freely inside the drum. If the shaft can be pulled out through the front of the 'scope without any further disassembly than removal of the "var" knob, this is what has happened. Applying the correct procedure, a small amount of cyanoacrylate glue is applied to the knurled end of the shaft, with the remainder of the shaft being covered with a very, very thin layer of grease. Next, the shaft is reinserted into the 'scope from the front, into the drum. With the right amount of glue (not too much!), this turns out to be a seemingly tricky but successful procedure. I haven't experienced nor heard of, any failures. Because of the (tiny amount of) grease left in the tube around the shaft, the procedure cannot easily be repeated if unsuccessful. Raymond |
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I'm happy to report that the cams are reassembled in the plugin and they are working exactly as they should. Thanks to all who replied.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Barry" <n4buq@...> The way this works for this particular rear drum is that the two splines drive |
Raymond Domp Frank
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