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View Full Version : help with a bent Oscilloscope post/rod



Mcgyver
08-29-2008, 05:05 PM
Picked up a nice scope recently, a Tektronics 466 with the multi meter. Seems in good shape but two of the shafts coming out of rheostats have been bent. Not badly, but bad enough that there it binds and just isn't right. it would be a big job to get that particular board out, and i'm sure there's all kinds of opportunities for me to do damage.

anybody have any nifty ideas on how to straighten something like this in situ?

http://i20.photobucket.com/albums/b201/michael0100/bentscoperod.jpg

thanks

JoeFin
08-29-2008, 05:14 PM
Shim the gap - bend it back

its just Al.

sch
08-29-2008, 05:49 PM
If it binds on the front panel opening, you could try notching a steel
tube with 2-3 teeth on the end, diameter just a bit larger than needed
to enlarge the hole and long enough to fit over the shaft and use it
to enlarge the hole in situ. May have to devise a clamping mechanism
on the drill to keep it from wandering. A vacuum on the circuit board
side to suck the chips. You could use some of the other shafts as
indexers for the clamp.

Mcgyver
08-29-2008, 05:57 PM
thanks guys, hadn't thought of shimming the hole, simple is good. think i'lltry that, may be with a pin vise to get a grip on it. I don't think i want to drill the panel, being anal about it maybe, but i want the knob to turn straight not drunken and they have a nice solid feel when running in the their plastic bushing

biometrics
08-29-2008, 08:47 PM
Just because it is a piece of electronic equipment doesn't mean disassembly is all that complex. Take it one screw and one connector at a time, and take photos as you go to help with reassembly. When you get the assembly to slide out of the equipment case shell, then straighten the shaft and reassemble... You can do this.. You are a machinist!

Mcgyver
08-29-2008, 09:05 PM
i know i can do it, but it is complex, would take a lot of time, there's a lot to remove and when taking apart something unfamiliar there's a risk of messing something up. not knowing what you don't know is the worst quadrant from which to take apart a piece of older electronics equipment

There were two that were bent, the straight forward one was easy, but the other one was the double knob style, one rod inside another. Neither were AL either, everything steel in this puppy.

got both reasonably straight with pliers, the plastic knob around the AL piece with set screw was cracked, epoxying it now, will see....ultimate though i may have to replace the piece eventually...lots of old Tek parts around it seems. when the shaft is held in place with a bushing its still out enough that iit s moving the circuit board as it rotates; that can't be good for solder joints etc

Paul Alciatore
08-29-2008, 09:09 PM
I am an electronics type and have repaired Tek scopes. Frankly, I would think you stand more chance of causing big damage if you try to straighten it in place than by removing the board. Stress on the shaft in place could easily crack the circuit board and that would be an expensive repair. Often times the boards in these instruments are not the usual and almost indestructible fiberglass. Tek has used other, more fragile, materials for better electronic performance in many areas.

Tek scopes are usually not that hard to disassemble. If there are any connectors on a board, make a sketch of them and where they go first. If needed add temp numbers to the connectors or the cables behind them. Remove them carefully. If a lot of connectors have to be disconnected, make some kind of list so you don't miss one on the way back. Save all the screws in a ice cube tray, using the compartments in order for each successive step so the screws are separate and in the right order for reassembly. Don't laugh at this: I do it professionally and I use this technique regularly. No one can remember where each and every screw came from an hour or two later, much less after a week or two while waiting for parts to arrive.

I am not an expert auto mechanic and feel quite intimidated by the tangle of various lines under my hood. I have no idea what they all do. But with careful sketches I have successfully removed and rebuilt carburetors on more than one vehicle, replacing over a dozen lines in reassembly. It is the same thing. You don't have to completely understand the beast to take it apart and reassemble it PROPERLY. You just have to plan ahead and be careful.

Once it is apart, I would try to straighten the shaft with forces applied to the shaft only, defenitely not to the board. It is probably aluminum or brass so it should bend easily. If you must grip the body of the pot, try to make a (threaded?) bushing to grip it's mounting bushing or use a couple of matching nuts, one on each end of the threaded area. If you can not straighten it properly, you can obtain replacement parts from Tektronix. In the past, their prices have not been too steep and they will sell them in single part quantities to anyone.

This reminds me that I need to try to fix my scope. It has worse problems and I will probably need a scope to troubleshoot it. Chicken and egg thing.

portlandRon
08-29-2008, 09:46 PM
Could possible cut the shaft a 1/2" from the pot. Straighten out the piece you cut off. Get or make a tube that is a tight fit on the shaft. Supper glue on the shaft ends then slip the tube over the joint.

Mcgyver
08-30-2008, 12:15 AM
well with your guys encouragement, I got the board out, lots of pics along the way and an egg crate full of fasteners just begging for me to tip it over

solutions emerging on how to straighten, pics in the morning.

if i can't straighten, at least its out for easy replacement - and it was beautifully made back in the good old days so soldering in and out a replacement should be easy

Paul you can come borrow mine (the one I'm working on is my second but don't tell my bride that) but its a bit of drive

Mcgyver
08-30-2008, 11:30 AM
ok, next little bit of hand holding i need......

http://i20.photobucket.com/albums/b201/michael0100/dualpotcropped-medium.jpg

the offending pot. this dual pot assembly has 4 srews holding it together, seems like straightening would be easier apart - is there any reason that this is a bad idea; taking it apart that is?

oh yeah, also, should these things be cleaned or lubed while they're out???

thanks

Mcgyver
08-30-2008, 01:56 PM
decided to stop acting like an old lady on this, took it apart, springs did not explode across the room and i have bent shafts seperated. made two pieces from AL, both with 1/8 reamed holes and slits. didn't even bother to file out the tool marks lol. rectuangular piece starts at the .200" long remaining straight section (that was under the threaded end of the pot) and held in the vise. cylindical portion goes over the rod. (its drilled out the other end, only the last 3/8 or so is at .125. and gets clamped with a small vise grip

http://i20.photobucket.com/albums/b201/michael0100/DSC_4333-small.jpg

tap tap tap. get straight by eyeball, by square and by checking with a drill shank passing through the centre. finished with it straight enough that a drill (the dia of centre rod) doesn't bind at all.

did the small one usinga parallel as an anvil, held up the light, tap tap tap, rotate 90 degrees, repeat.

now i dont have to buy dual pot. thanks for the encouragement on this, i was wading into unfamilair waters and needed a push, like most things no big deal to figure out or fix once i got at it.

http://i20.photobucket.com/albums/b201/michael0100/DSC_4335-small.jpg

ps, the shafts were definitly steel

Paul Alciatore
08-30-2008, 10:23 PM
Glad to see you jumped in. No magic in electronics. The mechanical stuff like pots, switches, etc. is usually kept as simple as possible. Something to do with pricing out to four or five decimal places. The fewer parts, the more pennys you can cut off the 10K price. I am surprised the shaft was steel, but Tek did use good stuff. Most usually are brass or aluminum or now-a-days even plastic.

Only magic in electronics is the smoke. Whatever you do, don't let it out.

Do watch out for solder bridges when you resolder.

Mcgyver
08-31-2008, 03:11 PM
thanks Paul, lots in electronics confuses me but this was fairly mechanical.... all back together now and working......but it got worse before it got better. Some scoundrel got a hold this scope and it was an interesting fix so i took a bunch of pictures

after all the earlier posts, i noticed something wrong here....

http://i20.photobucket.com/albums/b201/michael0100/scope/somethingwronghere.jpg

It’s a double pot with only with knob on it. hmmmm. On closure inspection, it, like the one I straightened, is a dual control, one rod inside the other and someone has snipped of the inner rod (you can see the marks) and put on a single control knob. GRRRRR #$%@#$%@#$ this was done in two places. The one I straightened is the position/fine position control at the right.

http://i20.photobucket.com/albums/b201/michael0100/scope/someidiotcutofftheinnerrodtothepot.jpg

How to fix? I figured it’d get the pot all apart clean up the end and silver solder on an extension. The inner rod is only 5/64 (.078) and I had nothing like that in stock so I turned down some 1/8” steel and made a little fixture. Held at an angle in the mill, a cut a small V in the top then milled out a spot to do the SS.

http://i20.photobucket.com/albums/b201/michael0100/scope/silversolderingsetup.jpg

Ready to go

http://i20.photobucket.com/albums/b201/michael0100/scope/closeupsilversoldering.jpg

Mcgyver
08-31-2008, 03:12 PM
This worked well

http://i20.photobucket.com/albums/b201/michael0100/scope/completedsilversolder.jpg

I put the pots back together, put the scope back together and thank goodness everything works. Here’s the manual showing 20/22 & 26/27 – they were both single knobs when I got the scope.

http://i20.photobucket.com/albums/b201/michael0100/scope/466scopemanual.jpg

As it is now

http://i20.photobucket.com/albums/b201/michael0100/scope/finishedjob.jpg


Need to source some knobs, don’t have the service manual for this one, but there’s a local guy who might have them.

Paul Alciatore
08-31-2008, 04:05 PM
For parts:

Tektronix tech support: Phone: (800) 833-9200 Press 3

They will probably want the model and serial number. As I said before, in the past they have been reasonable on pricing.

Fasttrack
08-31-2008, 06:48 PM
A posistion knob? Whats that?

I got spoiled using a digital oscilliscope that had an auto-find feature. VERY handy! Also very expensive for a decent one...

I'm just curious though, what do you need two scopes for if you don't usually do electronic stuff?

Mcgyver
08-31-2008, 07:03 PM
A posistion knob? Whats that?


hehe, i went for new glasses recently. slightly stronger script for distance, slightly stronger reading. I told the doc i have hobbies where i need to see close up. Problem is, with distance now a wee bit stronger and reading focus now a wee bit closer (at my request), I'm just about blind as bat at normal screen distance and can't properly see what I've written. i messed up. and that's how an ill conceived ophthamological strategy has rendered me just about illiterate

bought the second on a whim - sort of provided liquidity to someone who had to move. Great price, thought the 466 with dmm and cart was an upgrade from the 465B i have. i'm long on scopes right now, one is going to be sold. or i can have one in the garage and one in the electronics shop in basement. maybe add a third for the kitchen in case something comes up :)

Fasttrack
09-01-2008, 12:44 AM
No, no - you didn't mess up. I actually had a typo with the extra "s". I was just kidding because the new digital scopes don't have a position knob. They have a period and amplitude adjustment but no "position". You just hit the auto find button! :)

Mcgyver
09-01-2008, 10:25 AM
well be nice to the poor old blind bugger :D I'll buy a top of the line digital scope too, 20 years from now when they're 1/50th of the new price :D

added a beckman function generator ($20) and an old fluke bench top meter ($10) this morning. the function generator is SWEET~~~~~~~ so precise and solid.

i am out of control and probably need help......my name is Mcgyver, and I buy tools....

nheng
09-01-2008, 03:55 PM
Mcgyver, Late arrival here but the only thing to be careful of with some Tek scopes is that in an effort to make some models compact, you will find that there are attenuator cables (with those little Tek coaxial pin plugs) and other goodies that attach to the heart of a board, some hidden from view. They can be damaged by pulling before locating and releasing them. Ask me how I know :D

Incidentally, a good source for Tek parts is: http://www.sphere.bc.ca/test/tek-parts/tek-info.html

I have a top of the line Tek DSO bought for pennies on the dollar. Only problem with them is that they are orphans. No schematics were ever published (to anyone), power supplies were purchased assemblies and it is possible that even Tek has no schematics. Only way to keep them alive is by cannibalism. Amazing the prices they pull on Ebay though even when no support is available. There are some third party repair facilities but for the most part I think the survive by the stockpile in the back room.

Den

Metalmelter
09-02-2008, 07:22 AM
When all else fails ...

http://www.sphere.bc.ca/test/tek-parts/tekpots.html

They got them and MUCH more too ;)


Cheers!

Previous post beat me too it, lol