Electromechanical choppers - Page 1
Forum section: electronics / projects

author: munorc
author_rank: Regular Contributor
timestamp: February 21, 2025, 08:51:43 pm
content: I have some old components I would like to test for functionality.They are electromechanical choppers, Airpax A-175 specifically.https://w140.com/tekwiki/wiki/Airpax_A175They are designed to run off of a 6.3 VAC filament supply, so that is easy.

author: munorc
author_rank: Regular Contributor
timestamp: February 21, 2025, 09:00:02 pm
content: These were used in early vacuum tube operational amplifiers, which is why I have them.The Philbrick Archive has a pretty exhaustive overview.http://philbrickarchive.org/thecontactmodulator_ocr.pdfI assume I can test functionality based on the circuit shown, but does all the specifics of the phase angle etc. come in to play in their proper operation?I have 5 of these, one is NOS. The others are unknowns at this point.

author: jwet
author_rank: Frequent Contributor
timestamp: February 22, 2025, 05:21:50 pm
content: Those are really interesting! They are about 70 years old!The phase, etc is important in detailed applicatoin of these but you don't have to worry about it to test them. The drive coil is 155 ohms and wants 6.3v rms. In tubes days, this was a common filament voltage. If you can find a transformer that's close, you could put behind a variac or trim its output with a resistor. For example, if you found a 12v transformer, you would use a 150 ohm resistor would drop it down. The resistor will dissipate just shy of 1/4 watt, best to use a 1/2w. If you have access to a good signal generator with a 50 ohm output that could go down to 60 hz, just bring the voltage up slowly and monitor with an AC voltmeter.These are fascinating devices really. I'm a bit surprised at how imprecise this thing is really- 5 degrees is a lot. Those old amps had a lot of trims and could account for these issues with voltage trims.Post what you learn. You got me on a bit of tangent looking at old stuff. There is limited coverage of these circuits- Bob Pease worked at Philbrick a bit after this time and did some writing about the details of these AC modulated DC amps. Chopper amps using CMOS switches still exists but generally use a sort of sample and hold DC nulling techique. Look a the Max4208 for a good example.Have fun.

author: munorc
author_rank: Regular Contributor
timestamp: February 23, 2025, 12:05:56 am
content: I have a couple of power supplies intended for tube circuit work, 550v with separate 6.3v/12.6V filament supplies along with bias supplies. That will be handy to test.The GAP/R board needs +/- 300VDC regulated. That will be interesting when I get to that part. With the boards in the SD 3500, it has a dedicated 300V supply.

author: munorc
author_rank: Regular Contributor
timestamp: February 23, 2025, 12:12:26 am
content: This is what the intended application is. A Phibrick GAP/R USA-3 operational amplifier. Just a few years older than me.

author: munorc
author_rank: Regular Contributor
timestamp: February 23, 2025, 12:14:49 am
content: The other ones are part of a Systron Donner 3500 analog computer.

author: jpanhalt
author_rank: Super Contributor
timestamp: February 23, 2025, 12:24:07 am
content: As a student, I used both the Beckman DU in biochemistry and Beckman DK2 (w/ chopper). The Cary 12 was much better.

author: jwet
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:40:34 am
content: Very cool. I was in the post "hollow state" days, graduated college in 82. It was always around and I remember testing tubes at the drug store with my dad as a lad. I was a hobbyist as a kid and transistors were just newly available- Germanium, etc in the 60's. I had an EE professor that insisted on teaching us vacuum tube technology just so "we had some grounding" in it. I liked the simplicity and it helped me to learn semiconductors that had majority and minority carriers.That is museum or near museum stuff- fantastic. Philbrick was a brilliant engineer, he died unexpectedly at a fairly early age- 61? (IIRC). There is a lot of interest in this vintage stuff, old op-amps, analog computers, etc. Bob Pease who has also passed took his first job with Philbrick before a long career at National.Where the heck did you find this stuff- was it in the basement of a university- that's all I can imagine.Good luck.

author: munorc
author_rank: Regular Contributor
timestamp: February 23, 2025, 03:17:40 am
content: This is the unit the one board belongs to. It has 5 dual channel plug in amplifiers.It is a Systron Donner 3500.

author: jwet
author_rank: Frequent Contributor
timestamp: February 24, 2025, 07:51:33 pm
content: I had a EE controls lab at Univ of Illinois in the late 70's where we used old analog computers like this to solve differential equations and model systems. I remember getting little shocks from the banana leads (70v ?). My favorite was the bouncing ball on the scope- you could fool around with a few parameters to affect gravity, weight, etc. I should take a quad op-amp and put something like that together for fun. We have come a long long way!What was the source of this stuff, from a university?