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Radio1950 · 12th Oct 2021, 7:55 am

Interesting Vintage TTL Logic IC Problem

I was asked to design a circuit to provide a 5 volt pulse train at 3000 and 300 pulses per minute, to “calibrate” a counting device.
There was a possible later need for 1500 and 150 ppm, so I factored that in to the design.

In a whim of nostalgia, I decided to use vintage TTL, and to meander down familiar paths from the 1970s through to 2000 or so, where, probably like a lot of us, we had many equipments with heaps of TTL on boards.

I had lots of maintenance experience to board level, with repair, on Thomson MSSR radar processing equipment, etc, et alia, with hundreds of fully populated dual DIN41612 PCBS. Some 54xxN, but mostly 74xxN, and some 74LSxxN.
Never saw a single 76xxN.

I had virtually no design experience on TTL per se.

But ...
Should be easy.
What could go wrong?

So I used 12 V AC mains plug pack and a 5.1V zener to generate a 5V 50 Hz (3000 cycles per minute) drive signal, one lead at 0V the other 5Vpeak, with a Zs of 1000R or so.
Mains frequency was good enough accuracy for this project.

I then used a decade counter and divide by 2.
Rummage through my Logic IC bins to find some suitable TTL.
No 7490N to be seen there, so I had to use a few 74LS90N, a decade counter.

Wired up on veroboard, plenty of bypass caps, ICs on sockets, used a tight 5V DC supply.

SET and RESET lines to 0V; IC jumpers set to give divide by ten.

The 3000 cpm source signal went to the LS90 counter clock input via a 1000R “isolating” resistor.

Power up, check DC OK, check counter OP with CRO – zilch.

I expected to see a pulse train of period 20mS.
Occasionally, the OP went to HIGH, then to LOW, but mostly stayed LOW.

Many, many, hours later, I could not fault the circuit. New IC, all possible checks carried out.

Photo of project board and roughie circuit attached.

CRO photos of input signal and decade counter output attached. Probes are 10x.
Vert channels are DC coupled, 2V/cm, bottom trace is LS90 input, top is LS90 output.
Photo 3 Horiz is 10mS/cm, SYNC CH1.
Second photo is signal source with Horiz at 1mS/cm.

I thought the fault was so interesting, that I thought I might post it for veteran TTL users and designers.

I was not aware of the problem, for a good reason.

I have since fixed the underlying problem, but for fun and your intrigue, I ask for your possible solutions please.

A very nice Kewpie doll will be handed to the first person with complete correct (I think) answer, but must be picked up personally from my workshop.
I am currently impecuniously disadvantaged and cannot afford postage.

I hope that I am not the only person in the TTL world to know of this problem.
But have a feeling that I may even learn some more from your answers.

No clues yet.
Probably easy for experts.
Good luck.

12th Oct 2021, 7:55 am	#1
Radio1950 Hexode Join Date: Apr 2017 Location: Buderim, Queensland, Australia. Posts: 428	Interesting Vintage TTL Logic IC Problem *Interesting Vintage TTL Logic IC Problem* I was asked to design a circuit to provide a 5 volt pulse train at 3000 and 300 pulses per minute, to “calibrate” a counting device. There was a possible later need for 1500 and 150 ppm, so I factored that in to the design. In a whim of nostalgia, I decided to use vintage TTL, and to meander down familiar paths from the 1970s through to 2000 or so, where, probably like a lot of us, we had many equipments with heaps of TTL on boards. I had lots of maintenance experience to board level, with repair, on Thomson MSSR radar processing equipment, etc, et alia, with hundreds of fully populated dual DIN41612 PCBS. Some 54xxN, but mostly 74xxN, and some 74LSxxN. Never saw a single 76xxN. I had virtually no design experience on TTL per se. But ... Should be easy. What could go wrong? So I used 12 V AC mains plug pack and a 5.1V zener to generate a 5V 50 Hz (3000 cycles per minute) drive signal, one lead at 0V the other 5Vpeak, with a Zs of 1000R or so. Mains frequency was good enough accuracy for this project. I then used a decade counter and divide by 2. Rummage through my Logic IC bins to find some suitable TTL. No 7490N to be seen there, so I had to use a few 74LS90N, a decade counter. Wired up on veroboard, plenty of bypass caps, ICs on sockets, used a tight 5V DC supply. SET and RESET lines to 0V; IC jumpers set to give divide by ten. The 3000 cpm source signal went to the LS90 counter clock input via a 1000R “isolating” resistor. Power up, check DC OK, check counter OP with CRO – zilch. I expected to see a pulse train of period 20mS. Occasionally, the OP went to HIGH, then to LOW, but mostly stayed LOW. Many, many, hours later, I could not fault the circuit. New IC, all possible checks carried out. Photo of project board and roughie circuit attached. CRO photos of input signal and decade counter output attached. Probes are 10x. Vert channels are DC coupled, 2V/cm, bottom trace is LS90 input, top is LS90 output. Photo 3 Horiz is 10mS/cm, SYNC CH1. Second photo is signal source with Horiz at 1mS/cm. I thought the fault was so interesting, that I thought I might post it for veteran TTL users and designers. I was not aware of the problem, for a good reason. I have since fixed the underlying problem, but for fun and your intrigue, I ask for your possible solutions please. A very nice Kewpie doll will be handed to the first person with complete correct (I think) answer, but must be picked up personally from my workshop. I am currently impecuniously disadvantaged and cannot afford postage. I hope that I am not the only person in the TTL world to know of this problem. But have a feeling that I may even learn some more from your answers. No clues yet. Probably easy for experts. Good luck. Attached Thumbnails