- 2023
- Dec
- 22
Stabilizing the oscillator in the IM-1212 Meter
One of the problems with the Heathkit IM-1212 / Bell&Howell IMD-202-2 meter is the oscillator is made of crap-tier parts, and has a problem drifting all over the place. 10% carbon comp resistors and who knows what capacitors make up the parts complement. While that was probably fine for the age, a few dollars more could have made this at least a decent instrument for it’s time.
The main problem here is the parts are not temperature stable. For example, the meter I just recently acquired (see https://wereboar.com … ter-for-some-reason/) came in to the shop cold from being in a delivery van. It was allowed to warm up a little and plugged in. Oscillator was sitting at 94. It drifted down to 79 as it warmed up, and then back up to 88 - all in the course of an hour. That’s not good when you’re relying on that signal to generate the timing for your counts!
There’s not much to the oscillator. A couple of transistors for the oscillator itself, an inhibit transistor to start and stop the counter, and the passives that make up the oscillator circuit itself.
Here’s what we’re interested in:
There are two 3.3k 5% resistors, a 15k 10% resistor, an 8.2k 5% resistor, and two 0.0022uF capacitors. What’s the PPM drift for temperature on any of these parts? “Yes.”
So, off to Mouser we go!
Are these parts overkill? You bet. These are “Parts exceed value of device.” But they’re not really that expensive these days. So…why not? It’s mostly to satisfy my own curiosity to see if it really does make a difference in how the unit operates. If not…oh well, I learn something either way.
(The 15k pot may get replaced with something better - I was thinking of bringing it out to the back panel somehow, since it’s the main pain point.)
I do have some lesser quality (but still better than what’s in there!) parts, if I get ambitious I may replace some stuff before the actual parts arrive. Stay tuned!