Tektronix 465 scope attenuator switches

[murphyv310]

Hi
I recently fell air to a well used Tektronix 465 scope. It's working ok bar the attenuator switches, channel 2 being the worst. On some ranges it's ok but on others jumps around and gives an impression that the bandwidth is seriously compromised. The switch is like a barrel organ with raised segments that press on leaf like contacts.
The question is will they just need cleaning or is it possible the drum is worn, also what cleaner is best with out damaging the plastic.
Picture is after respraying the very bad case. Need a cover for one side of the stand hinge too.

[Radio Wrangler]

Probably just cleaning needed. Give them some use and see if they come better. I think flexure of the contact strip is designed to make the contacts themselves wipe a little.

Try this before getting more invasive. Electrolube or IPA firt, I think.

I may have a spare 465 parts box... not sure. I moved onto 465Bs a few years ago.

David

[Craig Sawyers]

I have a 475A and I haven't had need to look at the switches. But assuming they are a cam-like drum operating leaf-like switches on the circuit board underneath, there is a standard way of cleaning the contacts.

You cut strips of printer paper slightly less than the contact spacing. Dip each one into IPA. Then operate the switch so that contact 1 (from either end) opens. Slip the paper into the open switch, the operate the switch so that contact closes.

Then gently drag the paper out. Do not under any circumstances push it in or scrub, because you will bend the contact and spoil your day. Just gently pull it out. The IPA and very slight abrasive quality of the paper is enough to clean the delicate contacts.

Work your way along all of the contacts in this way.

I've had to do this with many 7000 series oscilloscope plugins that use the same contact arrangement.

The only tricky thing is getting access sometime. You really need complete access.

I've tried the "easy" method of spraying the contacts with IPA. No dice. And totally avoid any contact cleaner. You will just end up with a dust attracting gummy mess. The contacts are much too delicate for anything like that.

Good luck!

Craig

[RogerEvans]

It is well worth getting the manual eg from w140.com/tekwiki.

If you look at the schematic for the attenuator you can see which of the attenuator sections are active for each V/div setting and this may help to spot which cam switches are most at fault.

The schematic also shows the first two sections X10 and X100 are switched in for three adjacent V/div settings. If you find that you have severe noise issues with these attenuator sections they have the added issue of being small plug-in modules with gold plated pins that still manage to get badly tarnished but are easily removed for cleaning of the pins.

If after cleaning you still have doubts over the plug in modules then swap the suspect between Ch1 and Ch2 (remembering how many times you have swapped them since it affects the transient compensation adjustments). The modules do sometimes get dry solder joints internally and often can be fixed with a fine point iron and a steady hand. Replacement plug in attenuators can be found but can easily be half the price of the scope!

Hope it all goes smoothly,

Roger