HP AC-4A Decade Counter
© Brooke Clarke, N6GCE |
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One of the first digital counters was the HP 521A. It's a rack size instrument using tube technology and single sided printed circuit boards. Made in the mid to late 1950s. Each decade consisted of an AC-4() Decade counter module. These modules use 4 each dual triode tubes and 10 each NE-2 Neon bulbs to indicate the digits 0 through 9. There's an output that's the input count divided by 10 to drive the next stage.
The AC-4A is specified to count up to 120 kc and the AC-4B up to 220 kc.
The bottom of the module has a standard 8 pin octal tube base and so the main 521A counter chassis just uses a standard tube sockets for the modules. This particular module has a date stamp of 11 17 59. The manual sugests that other manufacturers also made a plug and play compatabile counter module. If you know of these let me know.
For some reason that I've now forgotten I ended up with a number of these decade counter modules and miscellaneous parts that went into them such as the string of NE-2 bulbs in the late 1950s or early 1960s. Maybe a friend of our family worked for HP, Palo Alto which is about a 10 minute drive from Mountain View.Special Version
This particular module is not a standard AC-4A although it's marked as an AC-4A. On the octal plug pin 7 is now a special staircase output, maybe coded 4221 and the filament connection that used to be on pin 7 has been moved to ground pin 6. The other pins have the same functions. If you know what this is about please let me know.
The Manual schematic for the AC-4A is dated Nov. 11, 1957 and the AC-4B schematic is dated 18 Apr. 1958. This special version has a newer date stamp11 17 59 than either of these, yet is labeled as an AC-4A and the PCB is marked AC-4A.
Phantastron
The HP 521 used a Phantastron circuit to divide the power line frequency down to the gate time. This circuit combines a couple of concepts.
First is the Miller Integrator. A tube circuit typically used to generate a linear ramp voltage for scope horizontal axis time sweeps. If you use an RC charging circuit the left or starting part of the sweep is fairly linear but as the cap charges the sweep becomes more non linear. What's needed for a good linear sweep is a current source to charge the cap instead of a resistor. The Miller Integrator uses a pentode tube where the current in one of the grids looks like a current source. I found a patent class 328/128 that's for the Miller integrator using Google, but there is no top class number 328 on the USPTO web pages, it's been consolidated into 327/131 ramp generators.
The phantastron adds an input diode that couples negative going pulses so that the Miller Integrator is started with a pulse and the ramp time is set to be a little short of the desired output period so that the "nth" input pulse resets the circuit. This circuit can only divide a known frequency by some relatively small number like 10 or 12. The HP 521 has a pot in each phantastron circuit to tweak the ramp time so the the divisor number is correct. If the pot is turned full one way or the other the count may be off by 1 or 2 counts.
The 31 Jan 1950 patent 2495726 by Houghton is an improved phantastron circuit. I've not found a patent on the 1941 phantastron.
HP could have used circuits like the AC4A to get the gate period from the line frequency, but the AC4A is much more complicated than the phantastron.
All four tubes are the same dual triode 5963. The CV3900 is a direct replacement. The 12AU7, 12AU7A, 12AU7WA, 5814, 5814A, 6067, 6189, 6680, 6CC40, 7489, 7730, B329, B749, CK5814, CV10323, CV10666, CV4003, CV4016, CV491, CV8155, CV8221, CV9092, E2163, E82CC, ECC802, ECC802S, ECC82, M8136 may also work.
Many patents related to electronic counters are found in Class 377/109.
The Flory 1946 patents:
2404047 Electronic Computing Device, July 16, 1946 708/627 ; 327/356 and
2410156 Electronic Timing Device, Oct 29, 1946, 368/58; 327/118; 340/332; 968/564; 968/946; 968/DIG.1
cover the concept of an electronic counting device that uses pairs of triode tubes in what was then called a "trigger circuit", later called a flip-flop. The patent has a discussion of the advantages of the binary number system and a reference to a book about binary (it was not a common concept then). Also the concept of using feedback so that when the count reached 64 it would set the third stage from the input i.e. set the counter to a count of 4 so there would be 60 counts until the sequence started over. Uses a neon bulb on each Flip-Flop to show in binary the count. Robert Flory has a web page with more of his grandfather's patents but they are not linked.
Note that in modern micro controllers if you want to have a counter with a period of 60 it's easy to start with a count of 4 and reset when the bit for 128 turns on. Very similar to the above method.
The 1950 patent 2521788 by Grosdoff uses 4 flip-flops and a "dual diode tube" as the feedback device to automatically reset the decade counter. In addition the 10 Neon indicating lamps are present as in the AC-4A.
The 1951 patent 2538122 by J. T. Potter eliminates the dual diode tube so only 4 tubes are required for a decade counter, but it uses 4 Neon lamps as the binary indicators.
The 1956 patent 2762915 by A. S. Bagley assigned to H.P. is for an improvement to the AC-4() type counter that increases it's speed to over 1 MC.
I think that staircase generators already existed but don't know if there is a patent on generating a staircase using a 4 stage decade counter. None of the counter patents includes the staircase output.
Has a manual test schematic circuit that uses a push button to cycle the counter and a related troubleshooting table indicating the probable cause for a number of counting sequences.
I have made a pdf version of the manual on CD-ROM and added a number of color photos of my special AC-4A, including ones of the left, top, bottom and right side with all the parts identified. Also includes a marked up schematic showing this special version AC-4A with the second staircase output. Includes schematics for the AC-4A and AC-4B and mechanical mounting drawing.
See the Product page for ordering information.
Description
HP AC-4A Manual
& Patents:
2404047
2521788
2538122
2762915
This is the 03081 hits since May 04 2007 page created 16 October 2003.