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Series 71 Switch Repiar
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I've got a 7B92 where the EXT÷10 push-button switch in the TRIGGER SOURCE column is stuck in. Has anyone successfully repaired one of these switches before?
I've read the "Repairing Tek-Made Board-Mounted Push-Button Switches document, but it simply says that these are unrepairable and must be replaced as a unit (and I don't have a replacement part, or even a parts mule from which a replacement could be harvested). I see that the switch body is "heat staked" to the PCB, and I understand that cutting those heat stakes is a one-time deal, but I expect that I could reattach the switch body to the PCB by other means after the repair is complete. I'm wondering, however, what awaits me inside the switch itself. -- Jeff Dutky |
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Harvey White
I have a 214 scope that had such a switch on a PC board. You can, and you can't..
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When you cut off the plastic stakes, about the best you can do is to drill out the posts with some #0 (or smaller) screws. Didn't work all that well, but I may not have been careful enough. If you cut the part that was melted over, or you can pry it up, and press it into more or less the same diameter, then you preserve the material. You might be able to keep the plastic and preserve the housing. Tricky, though. Try Q-serivce? Harvey On 10/12/2021 6:59 PM, Jeff Dutky wrote:
I've got a 7B92 where the EXT÷10 push-button switch in the TRIGGER SOURCE column is stuck in. Has anyone successfully repaired one of these switches before? |
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Harvey,
So those four buttons on the face of the 214 are series 71 switches? I would not have guessed it by looking at them from the outside (or even from the inside, the housing looks a bit different than the ones I've seen in 7k plug-ins). I'll give Qservice a try, but I'm not too hopeful. I already searched for the part number (670-1589-00) to no avail, and Qservice is pretty good about having their stock listed on eBay (I also went directly to their web page, but no luck there either). If worse comes to worst I guess I will just have to buy a second 7B92, but that could take a while. They're not nearly as common as the 7B92A. -- Jeff Dutky |
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Mark Vincent
Jeff,
It is possible lubricating can free the switch. I have had these switches get sticky and silicon spray was the only thing that worked. The contact cleaner helped with contacts and the switch for a little while. The spray kept them working right. What is in that switch, I do not know. Maybe a piece of plastic that is broken and can be glued back. If you get so stuck you cannot repair it, I have a parts unit with this in it I can send you. These switches work and the 683 lamp is good. Mark |
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Mark Vincent
Jeff,
I see you replied about it being a plain version. The A version switches are the same. What you are talking about will be the four switches in the upper right of the front. That is where I see X/10 at the bottom of the Source under the Main Triggering. The parts unit I have is the A version. Mark |
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Harvey White
I won't say that they are, but they do seem to be similar. I tried to repair them, but ended up buying a replacement switch simply because it worked better. May well have been some other problems in the switch.
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The staked in part is quite similar, though. IIRC, the actual switching pads are on the PC board. I may have something from a 7B92A, but I don't think a 7B92. When I had a chance at the 7B92, they went to someone who wanted the 7B92 (don't remember why), and I liked the 7B92A better. If you can send me the exact part number, I can look to see what I had. I *had* to scrap some 7A18/7A26, 7B53? and 7B92 (I think) plugins. They weren't mine, and I got to save some boards and some parts for the aluminum. I kept some of the frames in exchange for some old floppy drive chassis (minus the electronics). I guess you had to be there, but it was the best I could do with the choices I had. For all I know, I did take a 7B92 apart, but I'll need the Tek part number. Harvey On 10/12/2021 8:12 PM, Jeff Dutky wrote:
Harvey, |
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Jim Ford
Hmmm, I will have to try silicone spray on the sticky switch in my HP 11720A pulse modulator. Not that I really use it, and if I did, I'd probably just buy another 11720A on the famous auction website for $50 or less. But I do have silicone spray for my garage door anyway, so what the heck? Thanks for the idea! Jim Ford Sent from my Verizon, Samsung Galaxy smartphone
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-------- Original message --------From: Mark Vincent <orangeglowaudio@...> Date: 10/12/21 5:14 PM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Series 71 Switch Repiar Jeff,It is possible lubricating can free the switch. I have had these switches get sticky and silicon spray was the only thing that worked. The contact cleaner helped with contacts and the switch for a little while. The spray kept them working right. What is in that switch, I do not know. Maybe a piece of plastic that is broken and can be glued back. If you get so stuck you cannot repair it, I have a parts unit with this in it I can send you. These switches work and the 683 lamp is good.Mark
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Harvey,
The exact part number is 670-1589-00 and the board has silk screening on it that indicates it is the "SOURCE SW" for the "TYPE 7B92" (and part number "670-1589") so it would be hard to mistake for anything else. The other two boards (TRIGGER MODE and TRIGGER COUPLING) have different part numbers (670-1588 and 670-1587 respectively, but I can't see any silk screen markings on those boards other than "TYPE 7B92"). I've already compared it against a 7B92A and the boards are quite different (they have their inter-board connectors oriented completely differently), so I can't use a 7B92A as a parts mule for this. There's just enough space is the instrument to replace the heat staked pins with some very small screws and nuts, like the ones used in AB MOD pots, assuming that there's nothing inside the switch body that would interfere, though I was hoping that I could just epoxy the whole thing down afterwards. Screws would be better for providing downward force on the switch body, of course. -- Jeff Dutky |
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Harvey White
Sorry, I just looked. Almost everything I have is from a 7B70 or a 7B71, with some possible parts from a 7A18 and 7A26. Apparently I either didn't take apart any 7B92's or I saved the front panel as an entire unit (possible, but I don't know where they are).
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Harvey On 10/12/2021 9:08 PM, Jeff Dutky wrote:
Harvey, |
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Harvey,
Thanks for looking. I jumped on this 7B92 because I knew that they were rare, and the price was comparatively good ($45 including shipping). It was not advertised as working, so I'm merely inconvenienced. There was another 7B92 that was more expensive, and I should have jumped on it, but I was trying to play the odds. Now I get to play the waiting game. Meanwhile, I get to repair an unrepairable part. I guess this is what makes this hobby "fun" -- Jeff Dutky |
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Harvey White
For certain limited and obscure definitions of "fun".
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Harvey On 10/13/2021 1:02 AM, Jeff Dutky wrote:
Harvey, |
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Any recommendations on what silicone lubricant to use?
There are many to choose from: two kinds of WD-40 "specialist" (blue and white can vs. black can), 3M (dry and wet versions), DuPont (squeeze bottle), CRC 05074, Permatex, etc. -- Jeff Dutky |
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I am nothing if not obscure and limited.
-- Jeff Dutky |
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Bruce Atwood
If you are ever going to adhesively bond (aka glue) any part of the assembly I would not use any silicone product. They are almost impossible to remove and adhesives don't stick to them.
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Bruce,
Good to know, for several reasons: first, my original plan was to epoxy the switch body back in place after cutting the heat staked pins, and trying to free the stuck switch with silicone would probably foreclose that option. I'm leaning toward reattaching the switch body with screws and nuts, however, because it will give me better control of downward pressure. Second (off topic), I've been experimenting with casting replacement knobs, and I've been very unhappy with the mold release compound that I've been using (my epoxy casts have been sticking to the molds so that the only way to remove them has been to destroy the mold), and silicone lubricant may be a solution to that problem. -- Jeff Dutky |
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I managed to free the EXT÷10 switch through the generous application of WD-40 and moderate physical force. I can now use the other three SOURCE selections, which was my goal. The EXT÷10 switch remains extremely stiff and will probably jam again if selected, but I'm calling this a win.
There's a lot more to do on the 7B92: a stiff-to-frozen pot for the TIME/DIV VARIABLE control, and some kind of white residue on the trigger boards and the sweep selector board, spots of soot on the Peltola cables (but no evidence that anything in the 7B92 incinerated itself), some screws that need attention (one loose, the other rusted), and a broken rear frame plate to replace. All this because I want a time base that supports ALT horizontal mode, and I like the lighted push-buttons for the delayed time base SLOPE, COUPLING, and SOURCE controls. -- Jeff Dutky |
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Since I managed to free the stuck EXT÷10 switch I am no longer planning to try to disassemble the switch, but I would like to try cleaning the switch in the hope of freeing EXT÷10 push-button (which is still stiff, and would stick again if I actuated it).
While I can live without the EXT÷10 feature, it would be nice to have the plug-in in full working order. Would it be safe to soak the switch assembly/PCB in IPA? -- Jeff Dutky |
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Well, I guess I don't need to try to clean the switch now. I noticed that the sticking switch gave a tiny amount when another switch in the group was pressed, and a tiny clicking sound could be heard, as if the latch was trying to release the stuck switch. I was then able to pull the stuck switch to the "out" position with a pair of needle nosed pliers. Repeating this cycle (press in to stuck, press other switch, pull stuck switch with pliers) several times loosened up the stuck switch enough that it would fully release when another switch is pressed.
There is still slight stickiness, and the switch will still stick if pressed in just the wrong way, but it is otherwise working, and I expect it to get better with use. I'd still like to know what is going on inside the switch, but not enough to risk destroying it in the investigation. The current difference in behavior is subtle but noticeable: when most Series 71 switches are depressed they latch firmly and have almost no outward motion once latched. When the sticky switches are depressed, however, there is noticeable outward motion of the switch as you pull your finger away. The switch does not sit out any further than other switches in the latched position, but I think that it is actually sticking for a very short time longer in the fully depressed position than the other switches. This implies that there is something gummy inside the switch that is holding the switch in when depressed. I may try flushing it with IPA and see if that makes a difference. It's already been flushed with WD-40, which hasn't made as much difference as I would have expected. -- Jeff Dutky |
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David Holland
On Sat, Oct 16, 2021 at 2:15 AM Jeff Dutky <jeff.dutky@...> wrote:
<Snip> . I may try flushing it with IPA and see if that makes a difference. It's already been flushed with WD-40, which hasn't made as much difference as I would have expected.I'd recommend that anyway. If it's the stereotypical Blue/Yellow WD40 here in the states (https://en.wikipedia.org/wiki/WD-40) I've never heard anything good about leaving it in place. It turns sticky/gooey over time. Might not be a problem if you're using it to get a nut off your car's exhaust. Somewhat more so in a piece of electronic equipment. David |
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