Back-lit switches

Tek introduced back-lit pushbutton switches at the end of the 1960s in the 7000 scope family.

The design is described in detail in US Patent 3,584,174.

Lamps

There is one miniature 5 V incandescent lamp per interlocking group of buttons, with clear plastic light guides reflecting the light to the backs of the transparent plastic buttons. The lamp is powered all the time. In the Off state of the switch, a small metal flap covers the un-lit button end, whereas in the On state, this flap is pushed open.

The lamps are not supplied from the standard +5 V (logic supply) rail but from a separate lamp supply rail which is not present in all mainframes (e.g. missing from 7xx3 series scopes; see modification 040-0686-01 adding this supply).

In mainframes that do supply the lamps, the lamp voltage (and therefore lamp intensity) can often be varied, e.g. through a 3-way (off/mid/high) switch in the back of the instrument. In some early 7000-series scopes like the 7503, 7504, or 7904 (non-A), this is a front panel control.

Contacts

The switch consists of a mechanical interlock with sliding plungers that is attached on top of a PCB. This plastic part is screwed to the front panel (under the printed aluminum cover plate).

The contact surfaces are gold-plated PCB traces, the switch contacts are small metal see-saw style springs point-welded to a center support pin that is soldered into the PCB.

The contacts can break off from the support, giving no or intermittent conduction. It is possible to solder the contact to the support if care is taken to apply solder only to the center, or to transplant an unused contact (see example photo).

Repairing sticky ganged switches

While Tek documentation states that Series 71 switches are not repairable, this is not strictly true. If the problem is a gang of interlocked switches where it is difficult to change which switch is depressed, this can be resolved without disassembly.

On switches using a metal light guide, a raised rectangle exists on the housing. Cutting through this with an x-acto knife will expose the mechanical interlock. Dispense electrical contact lube into this hole and work the switches. I had best luck alternating lube and alcohol; I believe the latter helped to transport the lube through the switch. See photos in gallery below.

It should be possible to lubricate switches with acrylic light pipe similarly, though they lack the handy raised mark.

History

From On The History and Environment of Tektronix:

So, one day, Howard said, "the [7000-series] plug-ins will be 5¼ inches in height."
Oliver Dalton recalls the decision to reduce the plug-in height as a traumatic one:
"The decision set us back at least one year for the height decision required three major electro-mechanical component efforts in addition to the IC read-out effort: 1) new lit push-buttons ... we needed 25 on a panel; 2) cam switches - these were Howard's ideas ... we needed them to be small and reasonably cheap; 3) relays ... we needed them to be small and reliable. We then largely had to make do with other available parts, like potentiometers.
[...]
Howard Vollum was the person who started the push-button effort because he wanted small buttons. Some of the key inputs on the design came from the Model Shop, such as from John Winkelman who came up with a latch mechanism.

Photos

Dual DPDT switch interior (mV / mρ mode switch from a 7S12)
Sticky switch, before repair
Cutting a hole for lubrication
Injecting lubricant