> John, > > 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 > meter > and pots were shipped seperate to be mounted in the customers equipment. > HP > 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. > > Thanks, > > Will Good luck, -John ================== _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
