So I made a massive blunder in my attempt to copy a piece of test equipment.

I made an assumption.

The display board I copied uses the obsolete MC14547BCP for driving the anodes on the 7-segment displays.

I ....assumed.... ....without checking the datasheets.... that the 74LS47 would be a drop-in replacement.

The pinouts for the connected pins match (pins 3 and 5 differ but as neither is used, moot).

What I COMPLETELY FAILED to notice is that the polarity of the outputs on both are opposite!

For example on the MC14547BCP, to display an 8, all the 7-segment outputs are high. On the 74LS47, they are all low.

Now what? The MC14547BCP is definitely well past the NRND stage...

 

SN7448 and SN74LS48 are active high outputs.

 

Not much luck finding sources for that one... what of the CD4511?

 

The CD4xxx series generally can only source/sink a few mA. What current does your display draw, per segment?

 

The output of the 7-segment driver is fed directly into the input of a source buffer (UDN2981 / TBD62783A) - it doesn't drive the displays directly.

 

It’s almost pin compatible with the 4511. The 4511 has some extra features which are n/c on the 4547.
Both 4547 and 4511 have plenty of output current because they both have built-in emitter followers on the outputs.

 

So I just tie off the extra inputs so they don't come into play (because CMOS) and I should be good to use the 4511 in its place?

 

Seems the TBD62783 has an inverting version - the the TBD62785.

That should accomplish the inversion I need without having to go back and and add the extra traces needed to accommodate the change to the 4511, right?

 

IT LIVES.

(sorta)

No keyboard and horrendous flicker... I think I blew out the 8279 when trying to troubleshoot the previous version of the display board.
I have a spare set I'll have to swap in later tonight.

But hey - LIGHTS!!!

Update: Swapped in the backup set of ICs and the 8 readout displays are steady 0, but the test numbers and LEDs are out and still no keyboard.

Edit: The above condition was due to several critical flaws in the redesigned circuit. To do: Order parts for next updated version of CPU board.