[time-nuts] Low noise power supply recommendations
jfor at quik.com
Wed Jun 15 17:21:35 UTC 2011
> They used Lucite rods, I shouldn't have said tubes. The neon bulbs are
> about at the center of the PC board, and the CDS cells are at the edge, so
> to get the light to them, they used the rods.
More than that. They wanted to minimize capacitance coupled between the
neons and cells to reducing switching spikes.
> I think they did the same
> thing on the 845AB too, but I'm not looking at the manual right now to see
> for sure, as I'm not sure the 844 and 845 use the same PC board, just the
> same circuitry. The 844 was sold to other manufacturers, and it came in a
> plain aluminum box, with the switch shaft sticking out the front. The
> and pots were shipped seperate to be mounted in the customers equipment.
> did the same with the 419, and tacked another letter on the model number,
> as well as Keithly, which I know uses the same PC board in both versions.
> Keithly didn't use this type of chopper though (using LED's or lamps).
> I am going to locate the post on the HP forum and read about it. My guess
> is that the LED's brightness helped kill the CDS cells.
I don't think so. Apparently the CdS cells had aged into uselessness
before any LEDS were installed. Read the thread.
> I was thinking
> about trying an orange or yellow LED here, and dimming the LED with the
> series resistor, trying to make it as dim as the neon bulb, but I don't
> know if a LED can be dimmed down that low. On the old HP 412 VTVM, the
> incandescent bulbs in it were not very bright, and they were using CDS
> cells with them. Another instance of this, if I recall, was that Tektronix
> used a chopper like this in one of their older scope calibrators.
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