Ed Scherer

As I'm wrapping things up on the PWM instrumentation lighting dimmer project, there's still one thing I'd really like to verify: the resistance on the dimmer potentiometer/rheostat that was used up until '88 or '89 models (and I'd like to know at what year the change-over occurred, too, as I see conflicting information in the various on-line sources I've looked at).

This is the one I'd like to have measured (photo from 928 International), the 928.613.121.01:



I'm looking for the maximum resistance, i.e., when the wiper is at the farthest end of the resistance wire coil. I guess I could just buy one, but I haven't found any used ones available and the new ones are rather expensive (> $60) and I'd have no further use for it aside from measuring its resistance.

I'm hoping it's close to the value I measured on my '90's 928.613.031.00: approximately 6 Ω. If it's significantly different (and I'd hope it's higher resistance if it is different), then I'll need to supply slightly different configurations of the PWM dimmer module.

upstate bob

that's a good question. I do not like driving at night but am occasionally forced to. No telling how many bulbs are out on my panel but cranking the stat full up gives me a faint view of the instruments. My first
idea is to just jumper the thing and see what i have.

mdkelly1

Be sure that it doesn't look like this when you measure it. Don't ask how I know

Ed Scherer

Yeah, good point.

A blast of contact cleaner and then some DeoxIT would be good, not only for the test, but for continued reliable operation.

Ed Scherer

OK, I wound up buying one of these 928.613.121.01 rheostats from 928 Int'l (not cheap!). Just got the shipment today.

It is about the same resistance as the newer-style 928.613.031.00; about 6 Ω. That's good news.

I'll probably sell this one off at a loss when I'm done with the PWM instrumentation lighting dimmer project.

An interesting observation about these rheostats: they're pretty damn unreliable. Right out of the sealed bag, I found it difficult to get a good resistance measurement; when rolling the wheel, I'd often find it going "high resistance" (i.e. more resistance than the 0 Ω to 6 Ω that it should have been). Using it with a bench testing setup (with my dimmer module and a bunch of LEDs), it caused a lot of nasty flickering as it was adjusted.

It seemed unlikely to me that the problem was where the wiper contacts the resistance coil wire, but rather that there was a bad connection at the wiper's center pivot point; there's a tension washer between the rivited-on arm holding the thumb wheel and the copper wiper arm that provides the electrical connection. The current path is through that arm, the tension washer, and then the wiper arm.

I applied some DeoxIT at that point (soaked the tension washer), and the rheostat performed quite nicely after that.

Moral of the story: when trying to resuscitate a poorly-performing rheostat, make sure to pay attention to all the moving contact points, not just the most visible wiper-to-resistance-coil contact area.

davek9

Ed as you know those old open units were prone to the issues you listed, it's no wonder the new systems use no moving parts (all solid state)

Can't wait for your new LED dimmer,

Thanks,
Dave