loosing input trace on several positions of Volts/Div switch on Tek 465 channel A

[SoulFetish]

I just got the scope a couple of weeks ago. It is in great cosmetic shape, and just got around to setting it up on my bench. I was getting ready to check phase response in the output stage in XY mode. I let it warm up for a while, pulled out two new 10x probes, and plugged the first one into channel B to compensate it. There was some noise in the Volts/Div switch, but nothing I was too concerned about.
But when I went to compensate the second probe, I couldn't get a trace really at all. Turned out that the signal drops out completely in all positions but the lowest settings of the Volts/Div rotary switch. (seriously, half my kingdom for a scope with working switches!!)..

So I had some time tonight and took it out to open it up and see if I could clean the contacts as a starting point. Here is where it gets shitty. I think someone at some point glued the set screws in place on the bottom part of the knob?? In fact I'm almost sure of it, and needed to drill them out to remove the knob. This is as far as I got tonight, and I'm left wondering if worst case scenario that the adhesive worked it's way down into the contacts basically ruining the vertical attenuator in this channel.
So I'm looking for a little advice –
First, I have the full service manual hard copy book, and Tek is pretty adamant about only using isopropyl alcohol to clean the contacts. But I was wondering if Deoxit is safe to use in this application as well? (I assume deoxit wasn't around at the time of the publication)
The other thing is that if the switch is ruined, is it worth replacing? Fortuitously, I happen to have second 465 I can use for parts. This kind of sucks, because I really didn't want a scope as a restore project. I need the scope to help me with other restore/repair projects. c'est la vie, I suppose

[drewl]

If it's not losing trigger on some volt settings then the DC balance adjust is out of whack
There are adjustments in the calibration procedure, try that first, you might get lucky.

[J M Fahey]

And yes, Deoxit is King, it simply didn´t exist way back then.

[mozz]

Do not over spray the contacts , use the minimum you can possibly use. Toothpick preferred. There's a big discussion on the antique radio forum about using that on any test equipment rotary contacts.

[nevetslab]

One of the unique aspect of Tektronix scopes of that era are their rotary cam-actuated leaf spring switches. IF the cam or it's intricate mechanics have broken, it may be a common part as used in some of their other scopes. I have a boneyard of 7000 series plug-ins, all using the rotary cams, and below the cam are the commonly used leaf springs. Replacing those is a challenge...I've only done it once on my R5031 Dual Beam Storage scope, and also had to service it's unique Time Base mechanics that offered 5 steps of magnification by pulling on the actuator to engage it, then rotating in the opposite direction. That was NO fun to service, but won the battle.

7000 series vertical plugs may be using the same rotary cam as well as support parts, and they're plentiful. I do have the service manual on hand for the 7A18 here in the shop (5mV to 5V/Div sensitivity, same as on the 465), if there are parts that need replacing.....I could look to see if that's also used in this plugin. In my Test Equipment Manual library in storage, I have the manuals for the 7A11, 7A13, 7A18A, 7A22, 7A24, 7A26, 7CT1, 7B53, 7B70, 7B71, 7B80, 7B85, 7B92.

[Leo_Gnardo]

Tektronix may have specified isopropyl only because some plastic components of those fancy switches may be made of materials that deteriorate immediately upon contact with other solvents such as acetone or xylene that may be found in some sprays. One awful day I had to deal with a pair of Neve preamps that had been sprayed with such stuff. Only way to fix 'em was to replace the pad switches entirely. What a costly and annoying project that was.

[drewl]

Yeah, I have some cans of cleaners from back in the day when pots and switches were made of much sturdier material that will just melt most of the newer plastics!

[SoulFetish]

Luckily, I have the exact replacement if I need it. But I does not look like an enviable job to undergo. In fact I don’t know how I would even gain access to the switch contacts in order to apply deoxit (with a toothpick) without some disassembly, or even removing the entire board.
...furthermore, what kind of butcher uses superglue on an oscilloscope switch?? I can only hope none of it migrated into the contacts area or it’s toast. Ill try and clean it and see what happens.

[loudthud]

The "Cam Switch" drums were usually specific to each instrument they were used in. They had a special machine in the Engineering department to make them as needed. The contacts on the PCB are gold plated as are the little lever contacts that get pushed down by little bumps (the cams) on the drum. Vertical attenuator assemblies are usually on a special little PCB about as big as the switch. Those PCBs are made of special material that doesn't absorb water so the scope can make almost perfect square waves when it's running right.

Sometimes the PCB warps under the cam so the contacts don't make contact with the PCB. You might be able to fix this with a tie-wrap if you are lucky. If you have the right stuff to gold plate the contacts on the PCB, that might work. Probably not available in USA for the last 40 Years due to environmental laws. If the gold plating is gone, deoxit might fix it for a while, but not long term. Deoxit may also screw up the HF response, don't be surprised if it does. Best course of action: replace the switch/PCB sub-assembly. You can find these scopes cheap when they don't work. I found a 465B for $5. Had a problem with the Horizontal amp. Trace was out of focus and skewed to one side. I found the problem in about 10 minutes (a broken trim pot). Blew the mind of my employees. Other than a bad power supply cap, the scope is still running 10 years later.

[Leo_Gnardo]

Yeah, I have some cans of cleaners from back in the day when pots and switches were made of much sturdier material that will just melt most of the newer plastics!

The point was, some of the older switches & pots can't handle solvents well. Caig's non-residue spray cleaner was one of those sprays that wrecked some switches in my workshop. Since then they reformulated it, leaving out the acetone it once had.

Had a Hughes/Kettner combo with a sticky power switch. While its owner watched, I hit it with the older Caig spray. That switch sprung apart, the rocker went flying across the room. At least he got some amusement out of it. After said customer stopped guffawing, "NOW what do we do?" Watch this I said, as I unassed the chassis from the cab. Snipped the leads to the switch, trimmed off a bit of insulation, twisted them together and stuck a wire nut on. "When you're plugged in, your ON. Until we get a new switch." Luckily a friendly local store was an HK dealer and ordered a switch. Couple of weeks later the new one was installed, amp back to normal.

Another time, late 70's, I sprayed Radio Shack cleaner into scratchy pots on a QSC crossover. After that, the knobs turned. And turned. And turned.... And pulled right out of the panel, with bits of potentiometer not far behind. Uh-oh! This time no friendly factory parts available. "You're not an authorized repair center." So I had to punt with a couple of crappy kit-pots from the electronic parts store. We had a couple back then. That was a miserable crossover anyway.

So, if Tektronix says "isopropyl" better find some 100%, or at worst 90% to treat those switches. Wouldn't trust grocery store 70% iso "rubbing" alcohol, 30% water for this task. You know what would happen, the alcohol will evaporate first leaving water inside your switch to help corrode it, phooey who needs that.