I don't normally like cross-posting but I thought maybe some of you guys might be interested in my restoration of a Laser 128.

For reference, I started the thread over here: https://forum.vcfed.org/index.php?threads/next-project-laser-128-with-invalid-voltages.1240428/

Anyway, the computer isn't supplying the required 5V to the IC's. Instead, it's only generating about 2.9V.

One thing that really caught my attention is how extremely hot a 74HCTLS244N is getting. It's so hot that it would burn my fingers if I left them on there. But what really struck me as unusual is that I didn't know they made an HCTLS version of that chip (or any chip). I guess it would work with just about any other chip!

Mouser doesn't stock that chip. I have some HCT versions and some LS versions. So I'm wondering if you guys think that using an HCT would work?

I suppose I could just try and see what happens. But I first have to de-solder the existing chip.
If you think I should really use the HCTLS version, I may see if I can source some on eBay or somewhere else.

Thanks for any suggestions.

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Cat; the other white meat.

I've never heard of HCTLS before and a bit of googling turned up... surprisingly little.

A bit more googling turned up the Samsung AHCT / HCTLS Databook. An 800 page monstrosity that looks pretty informative.

But long story short I think it might be better to use an AHCT replacement for an HCTLS chip rather than an HCT. This all comes with the caveat that my knowledge of HCTLS is about 5 minutes old and I made be completely wrong.

(The datasheet for the 74HCTLS245 is on page 620 but I'd imagine its performance characteristics are pretty similar to the '244. The 74AHCT245 is on page 252)

The hot chip is a conspicuous clue regarding the +5 supply rail being low (only 2.9V). The fault is almost certainly NOT the power supply. Instead, the load (the motherboard/whatever) is drawing excessive current.

Various factors could cause this. The 74HCTLS244N may have an internal defect (in which case removing the chip will restore the +5).

Or the 74HCTLS244N may be attempting to drive a bus one of whose lines is shorted high or low. Removing the chip will restore the +5, but of course the shorted lines still needs to be identified and fixed.

Or -- and perhaps most likely -- the 74HCTLS244N may be attempting to drive a bus which is also being driven by another chip (ie, bus contention). In this case you'll have two chips getting abnormally (although perhaps not equally) warm, and removing either chip will restore the +5. But of course you'll need to determine why both chips are attempting to drive the bus. It could be some sort of fault in the glue logic. Or perhaps this other chip is defective, and ignores the input telling it when to release the bus.

ETA:I think it would. As for the "HCTLS" designation, I suspect it's a relic of the times, dating back to the days when the HCT family was new, and folks perhaps needed reassurance that it was indeed LS compatible.

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

UPDATE!

So, I pulled the suspected 74HCTLS244N out and still getting nothing. 2.5V this time!
Then I noticed there is another 74HCTLS244N in a socket on the other side of the board. It started getting hot. Since it was socketed (the original was not), I pulled it out and now I get 5V!

In fact, I get some video of black/white squares.

Funny thing is that the socket appears to be factory but the other chips around it are not socketed.

I'm going to dig around for shorts and see what I can find.

Thanks!


UPDATE 2

I put a socket into the original 74HCTLS244N and I put in a new HCT version and I get video and ROM message! So it's definitely working. I don't have the keyboard attached and the video looks corrupted (and a little wavy) but real progress.

_________________
Cat; the other white meat.

Well that looks like a step in the right direction!

(I'm totally unfamiliar with the Laser 128 though. I vaguely remember it being some sort of Apple knock-off or something).

The Laser 128 was a clone of the Apple IIc. At least one model (128EX) offered even better specs. There are a number of videos on them on YouTube, including several from The 8-Bit Guy and Adrian's Digital Basement.

Yeah, it's a pretty awesome little machine. I still like the IIc better (my IIc actually has an internal Mockingboard clone) but the Laser 128 is probably a better Apple II than the IIc is.

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Cat; the other white meat.

Cool, Apple IIs were not something we got here. No Apples really that I remember. Whether that due to sanctions just a lack of marketing I don't know. The first Apple II ever saw was probably in 1995, I played some game about a rabbit on it for maybe 5 mins before getting kicked off and going back to a DOS based PC.

I think I might have seen an advert for a Laser but this is going back a long time so who knows... My knowledge there is even worse than my Apple knowledge.