Horizontal Display switch shenanigans.
While pulling apart this 465 parts scope I tried removing the pushrods from the Horizontal Display switch on the A7 board. Working on the bottom "B DLY'D" pushrod the push button came out of the switch. I thought at first that it must be a simple mechanical assembly thing that I could put back together. When I had the chance to study it I realized that the plastic of the button had broken. I thought I must have done it trying to take the pushrod off, but then I noticed that the internal plastic button narrows to a very small "T" cross section. See the pictures in:
Notice the dirtiness of the cross section at the break. If the button was intact before I tried taking the pushrod off it must have been just by the top of the "T" and taken very little force to break it. I suspect it was already broken actually, and that having the board out of the scope allowed the pushrod to rotate down so that the button could come out of the switch assembly. When held horizontal the notches in the button catch on the ratchet mechanism of the switch, holding it in. When it rotated down it just came out with no force, nor did it make any noise. It just kind of fell out in my hand.
The back of the plastic internal button when complete has two channels that hold the metal spring contacts, which slide between the switch posts. I don't know if that's clear in the pictures or not. The point being, if this back piece with the contacts breaks off it ends up inevitably in the back of the switch. In the depressed position. I never turned this scope on - it wouldn't have worked with C1419 shorted anyway. I also didn't capture the back of the switch in any pictures nor notice if the button was visible from the back. They are when in the depressed position. So it's kind of lucky that I found this. If I'd tried the scope it would probably would be stuck in B DLY'D, or something worse with B DLY'D selected and whichever of the other buttons in the HORIZ DISPLAY group would be depressed. It's a pretty devious failure mode, and I have to wonder how common it is, given the fragility of that cross section.
Anyone else with experience with this kind of pushbutton failure?
But wait, there's more!
After finding this broken switch I looked it up by the Tektronix part number from the service manual I have - Fig 2 Circuit Boards, item 124: 260-1423-00, "SWITCH, pushbutton--HORIZ DISPLAY". I found a NOS listing from a company called "Talon Electronics LLC". I ordered it, but was surprised to find on my order after a day some notes from someone in the shipping department which read "cut pin", and submitted a refund for me. But then also shipped it anyway noting "might work in his application". Aw! How nice of them!
So the part came the other day. I unpacked it and compared to my A7/switch. Look at the last two pictures in the above album. Sure enough there's a "cut" pin, but note there's also a cut pin on the existing B DLY'D switch. But look closer: the new part has the wrong pin cut! The new switch in the picture is "on its back". It would be flipped over putting the cut pin at the front side of the board, not the back! Ahg! At least I got a refund. How the heck did this NOS part get in the system in the first place!? Bo-bo in the manufacturing process? Someone had too much to drink the night before and cut a whole batch of switches with the wrong leg cut?
So now what? I'm thinking what I'll do is cut the rearward pin of the new B DLY'd switch, and then cut the existing switch forward pin leaving it soldered into the board. Then when I mount the new switch I'll solder the new switch cut pin to the existing post from the old switch? Maybe place a piece of lead along the post and solder that?
Any other ideas or suggestions for this? I do see another one on eBay, but used. So no assurance that it's in good working order.
But what a convoluted situation! I wanted to share the failure of the switch. It seems like a hidden failure that someone might benefit from the experience. I'd also love to hear any ideas for "fixing" the new switch. Or should I just buy the used one on eBay?