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Hewlett-a-Sketch with a HP 7035B X-Y recorder (Patreon)

Published:
2022-05-31 03:24:47
Imported:
2023-03

Content

Disabled viewer Kevin Counihan had donated a HP 7035B plotter to the channel a while ago. I finally got to it, because I needed it to plot curves for an audio filter I made for the Apollo setup. 

It did not work at all at first. A shorted cap replacement later, and it worked for a while before stuff started to go south again. This device uses photo-chopper servo amplifiers. It's an early low DC-drift, low bandwidth precision op-amp made with transistors, neon bulbs and photoresistors. All that in the name of suppressing DC drift by chopping the near-DC input signal and making it AC for a while, before piecing it back together into near DC at the other end of the op-amp. Since the op-amp is AC coupled in the middle, there is no DC drift. It's a hat trick that was used in early electronic voltmeters and analog computers, dating back to the tube era.

The photo-choppers are the green tubes with light at the end I am pointing to. One pulsating neon at one end of a clear plastic rod, one photo-resistor at the other end. As you can see, one does not light up, the neon is kaput. 

 And also one of the photoresistors was not doing too well. It's this fellow here. Resistance should be super high (in the mega-Ohms) when in the dark, and should drop to a few 100 Ohms when illuminated. Quite an amazing device really.

After finding the right components (the correct neons are still available on DigiKey!), I got some action again. The mechanical adjustments were quite entertaining too, I had to use the blue strain gauge in the background.

It plots again! It's still a bit wobbly, but here are my Apollo 1kHz filter curves, measured by the HP 8903 Audio Analyzer underneath:

Of course I could not resist and make it an expensive HP Etch-A-Sketch. I named it a  Hewlett-A-Sketch. Eric (@TubeTimeUS) gave it a try, but he's got to practice before he is any competition to Princess Etch-A-Sketch (look her up on YouTube!).

But it's a load of fun. Thanks Kevin for the donation!

Marc




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Comments

Kevin Reardon

I have an HP DesignJet 430. It's only black and white, but its yours if you want it. I could be convinced to eBay it to a 450C if you can verify it works. I haven't used it in 10 years because I could not get a proper driver for Windows 10. There was a day when I did D size printing, but I changed jobs and now A size is just fine.

quintesseract

How does one begin to look for replacement neon bulbs? Old Macs used to use them on their "analogue" boards, for instance, but there's nothing (obviously) identifiable about them at a glance.

curiousmarc
>>85820596

On this one it was quite an adventure. I had the schematics, which referred to an HP part number, which was on my HP microfiches and had a standard reference number (A1C), which was in my Neon bulb book which confirmed what exact type of bulb specs it had, which was available on Digikey... But I also tried a run-of-the-mill NE-2 neon bulb and it worked just as well. The most important resource is the book, look up "using and understanding miniature neon lamps" by William G. Miller.