IBM 5100 Keyboard Issue

[Hugo Holden]

I'll try to see where the trace to C goes. Since it's an input, the issue probably lies on the other end (if I'm not an idiot).

Hardly an idiot, like I said, more like a quick study. You have already picked up on one of the major clues in fault finding logic circuits, an aberrant logic level. And you have deduced the likely cause. Well done !

Generally the output of TTL circuits are good at sinking current and poor at sourcing it, and the input pins source current around 0.36mA for 73LS TTL's and about 1.6mA for standard 74 types.
If a signal is not going low enough or high enough, it generally represents a failure of the TTL output stage from the chip driving the line, rather than the input of a chip sourcing or sinking too much current. (but there are always odd exceptions)

If you ever get suspicious though, that the input pin is the problem, one trick is to solder suck just that pin. Usually by moving the pin back and forth a little in the plated trough hole, you can get it to disconnect (check with the meter) then re-test it. This avoids doing awful things like cutting tracks. One other less favorable method (but still less destructive than a track cut) if the line of interest passes through a via, you can run a tiny just over-sized drill through the via to open circuit it, and re-connect it later with a wire passing through the via and soldering on both sides. Obviously if the IC's are in sockets its easy to bend a pin out a little and re-fit the IC.

Make sure to keep the scope on DC coupling at all times so that you know exactly where the zero volt baseline is.

 

[Hugo Holden]

Jackpot! The IC labeled 760505 is the one with the weird low signal. C connects to the top-right pin (when the hole is on top). Checking all the others, they all have 5V, constant or oscillating. I think this is the culprit, but we'd need a circuit diagram to make sure.

Here's the IC in question:
View attachment 1270492

Here's what the top right pin was reading (same as C):

View attachment 1270493

I'll wait to hear from the pros before I do anything else. Thanks, as always.

The pin you have measured doesn't seem to have the low level pulses that you measured on C, so double check you have traced the C input pin back to the correct pin on the chip.

The hole or dent or dot in the chip identifies pin 1. If you have the chip in front of you, its long axis on the horizontal meridian, the dot should be on the lower left. Also, the writing should be up the right way too.

Then the pin number increases from left to right and pin 7 (for a 14 pin IC) is then on the lower right. Pin 8 is above that on top of the IC and pin 14 is on the top left above pin 1.

If you browse the TTL data book you will see that as a proportion there are far more IC's with an input on pin 1, vs an output. If it really is an output on pin 1 that will significantly narrow it down.

The first thing with the unknown IC, check the pwoer supply connections, and look to see if there are any pin linkages and if any pins are tied low, or high and start drawing a diagram to create some clues.

Another clue, if there are pullup resistors, then it narrows it to an open collector part.

 

[Chruus]

The pin you have measured doesn't seem to have the low level pulses that you measured on C, so double check you have traced the C input pin back to the correct pin on the chip.

The hole or dent or dot in the chip identifies pin 1. If you have the chip in front of you, its long axis on the horizontal meridian, the dot should be on the lower left. Also, the writing should be up the right way too.

Then the pin number increases from left to right and pin 7 (for a 14 pin IC) is then on the lower right. Pin 8 is above that on top of the IC and pin 14 is on the top left above pin 1.

If you browse the TTL data book you will see that as a proportion there are far more IC's with an input on pin 1, vs an output. If it really is an output on pin 1 that will significantly narrow it down.

The first thing with the unknown IC, check the pwoer supply connections, and look to see if there are any pin linkages and if any pins are tied low, or high and start drawing a diagram to create some clues.

Another clue, if there are pullup resistors, then it narrows it to an open collector part.

Sorry, I think I was unclear. If the hole is pin 1, then the output pin that connects to C is pin 14. I didn’t check continuity, but it did match the pattern as C. The reason it looks different in the image is because I didn’t zoom in on the scope (On the 74154 I used a scale of 1V/50uS, but on the mystery IC I used a scale of 5V/1mS).

I’ll go through the TTL book to see which it could be. I’ll also send the values of each of the pins on the mystery IC.

EDIT: It looks like all these circuits have their highest pin as VCC. Maybe these two chips share VCC and it’s an eroded trace? I already had to repair one, so it’s definitely not out of the question. Maybe it would be beneficial to desolder the IC, if not to replace it then to check the traces underneath.

 

[Hugo Holden]

Sorry, I think I was unclear. If the hole is pin 1, then the output pin that connects to C is pin 14. I didn’t check continuity, but it did match the pattern as C. The reason it looks different in the image is because I didn’t zoom in on the scope (On the 74154 I used a scale of 1V/50uS, but on the mystery IC I used a scale of 5V/1mS).

I’ll go through the TTL book to see which it could be. I’ll also send the values of each of the pins on the mystery IC.

EDIT: It looks like all these circuits have their highest pin as VCC. Maybe these two chips share VCC and it’s an eroded trace? I already had to repair one, so it’s definitely not out of the question. Maybe it would be beneficial to desolder the IC, if not to replace it then to check the traces underneath.

I wouldn't de-solder an IC, until you have:

1) identified it with certainty.

2) tested it in circuit, and,

3) concluded it is defective and not obeying its logic table.

I doubt if there are too many eroded traces, generally the pcb you have looks pretty good.

One of the traps to fall into repairing something, is jumping to conclusions and replacing random parts, without any solid scientific evidence that they are defective. It generally leads to disaster and failure to repair.