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Archive for December 28th, 2016

Fordham FG-801 Function Generator Power Switch

The power switch in my trusty Fordham FG-801 Function Generator failed with an accumulation of oxidation / crud on the contacts. That’s fix-able, but the switch contained not one, but two powerful springs, and puked its guts all over the floor around the Squidwrench Operating Table. Even with (a preponderance of) the parts in hand, I couldn’t figure out how to reassemble the thing; the only way out was to replace the switch.

The OEM switch had a 0.360+ inch diameter pushbutton that fit into a ⅜ inch hole and, alas, my remaining stock of line-voltage switches had toggle levers and used ¼ inch holes. So I converted a bit of aluminum rod into a suitable bushing:

Fordham FG-801 Fn Gen - new switch hardware

Fordham FG-801 Fn Gen – new switch hardware

The lock washer in the middle started with a much wider tab that I filed down into a tooth for the dent from a #2 center drill. Protip: center drills don’t walk off like twist drills, even when you hand-hold the front panel at the drill press with all the electronics dangling below.

The bushing dimension doodle:

Fordham FG-801 Function Generator - Replacement Switch Bushing

Fordham FG-801 Function Generator – Replacement Switch Bushing

The internal wiring routes the 120 VAC line conductor to the switch, then to the fuse, then to the transformer. I don’t know whether it’s better to have an unfused switch or an unswitched fuse (surely there’s a UL spec for that), but I didn’t change anything. The new switch, being slightly smaller and mounting directly on the panel, required a new wire (the blue one) from the fuse:

Fordham FG-801 Fn Gen - power switch - installed

Fordham FG-801 Fn Gen – power switch – installed

The OEM switch mounted on two round brass standoffs and, wonder to tell, the new switch fit between them!

From the front, the new switch looks like it grew there:

Fordham FG-801 Fn Gen - switch in action

Fordham FG-801 Fn Gen – switch in action

The PCB mounts to the top of the case with one screw and four hexagonal brass standoffs. The standoffs have 6-32 tapped holes on one end and a 6-32 stud on the other; one of those stud had broken off. A 6-32 stainless steel screw secured in a clearance hole with a dab of epoxy solved that problem:

Fordham FG-801 Fn Gen - standoff stud

Fordham FG-801 Fn Gen – standoff stud

I stood it vertically and tweaked the screw to be perpendicular while the epoxy cured.

Memo to Self: The next time around, put a nut on the stud to make sure the answer comes out right. I didn’t do this time to avoid epoxying the nut to the standoff.

Done!

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