Request to compare two schematics

Thread Starter

metermannd

Joined Oct 25, 2020
343
Can someone do me a favor and compare the two schematics and verify the two are functionally equivalent?

It seems that for some reason the display board I assembled isn't lighting up at all, and I just want to rule out the possibility of a boneheaded design error (wouldn't be the first, frankly).

The schematic 'original' is what I started with, and the 'current' schematic (as fractured as it is - sorry... will redraw when I get a bit more time) is what I have now, and what I used when I laid out the display board.

Also, the original board had the 8279 on the same board as the displays. That IC was shifted down to the next lower board to make room for the keyboard. Did I err in doing so?
 

Attachments

Alec_t

Joined Sep 17, 2013
14,382
I haven't fully studied the original circuit, but note that the Pass and Fail LEDs have no apparent current-limiting. Does each LED have a series resistor?
 

Thread Starter

metermannd

Joined Oct 25, 2020
343
Think of the setup as sixteen 7-segment displays, five reduced to the 'A' and 'G' segments, and one consists of the 'G' segment only.

The LED anodes are wired to the 7-segment 'A' and 'G' segments and use their series resistors (the original uses a UDN2981A for the source driver, the new circuit uses a TBD62783A which should be a drop-in replacement?)

The common cathodes are fed through ULN2003A or ULN2803A drivers back to ground.
 

Alec_t

Joined Sep 17, 2013
14,382
1) The pinout for the UDN2981A in your original circuit does not match the datasheet.
2)
It seems that for some reason the display board I assembled isn't lighting up at all
In your current circuit the TBD62783A has a Vcc of 12V (why?). The LED segment current will be ~(12V-2.5V)/100Ω = 105mA. Perhaps you've already let the magic smoke out?
 

Thread Starter

metermannd

Joined Oct 25, 2020
343
Funny you should speak of letting the magic smoke out...

This morning when I had a bit of downtime, I set up to try and chase a few signals with my bench scope to try and figure out what's going on with the display board.

Somehow...

I made a wrong connection and the 5V and 12V supplies got shorted together for a few seconds. It was long enough to roast two glue logic ICs on the logic board (one merely went dead short and the other no longer has its smoke sealed within). I have to consider every single other IC throughout the unit suspect now...

i wanna cry :confused:
 

Thread Starter

metermannd

Joined Oct 25, 2020
343
So I went and looked at the nodelist I made of that board, and compared to the schematic.

Fixed the typo (no symbol for the 2981, and when I drew the schematic, I did not then realize the difference from the 2803).

Anyhow, I also fixed the layout a bit and hopefully this one is better.
 

Attachments

Last edited:

Alec_t

Joined Sep 17, 2013
14,382
Why do your 7-seg display drivers have a 12V supply and only 91Ω per segment to limit current? Are you trying to destroy the displays? The typical running current for MAN7A displays is <20mA per segment.
 

Thread Starter

metermannd

Joined Oct 25, 2020
343
The 91 ohms and 12V supply are what are on the original board, and even though the unit is >35 years old (albeit lightly used), the displays still work fine?

I am open to suggestions for a more appropriate dropping resistor value, and wouldn't mind just running everything off 5V as long as the SMPS module is up to the task.
 

Thread Starter

metermannd

Joined Oct 25, 2020
343
If you want I can take a picture as proof ;) I just went to take a look at the resistors - they're big 2W carbon composition resistors - bands: white, brown, black, gold, yellow...

So, anyway, if I assume an average of 25mA per LED segment, the 'all segments' check would pull over 3 amps (if using a 5V supply)? Would 150 ohms be a more suitable value?
 

Alec_t

Joined Sep 17, 2013
14,382
Since the high-side and low-side driver ICs each have Darlington output stages, between them at saturation they will drop about 2.5V (per their datasheets). So the LED current for 150Ω and a 5V supply would be ~ (5-2.2-2.5)V/150Ω = only 3mA.
22Ω would give you ~22mA, but with little headroom.
The LED current for 150Ω and a 12V supply would be ~ (12-2.2-2.5)V/150Ω = 50mA. 330Ω would give you ~23mA.

Edit:
I'm puzzled by your resistor markings. I can't find any standard colour code where the 5th band of 5 is yellow. Perhaps the value is 910Ω? If so, LED current would be <10mA, which is more plausible than 100mA.
 
Last edited:

Thread Starter

metermannd

Joined Oct 25, 2020
343
OK. I've tweaked the resistor values accordingly.

I did take the original unit apart as I realized I'd never taken pictures of the boards - here's the original board:

And I did measure the resistors, and they do measure between 91-98 ohms (so it seems it's true that carbon composition resistors DO drift upward in value over time).
 

Attachments

Top