74LS Replacements (Mos 7708, 7709)

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Zippy Zapp
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74LS Replacements (Mos 7708, 7709)

Post by Zippy Zapp »

Hi all. I am trying to troubleshoot and repair a C64 250425 board. I have checked all sockets ICs and they are good (SID, MPU, PLA, CIAs, ROMS, 8701, etc. I have started replacing RAM and Logic chips next. For reference it is a Blank screen problem. Voltages are fine and leads me to believe it is RAM or one of the logic chips.

From Ray Carlsen's description:

Code: Select all

U13/U25	74LS257APC or A or AN but not PC  (alt MOS7708) LOGIC
     Blank screen, less than 38911 bytes free at startup or "garbage"
screen. See U25  Note: MOS77xx series chips have a high failure rate.

U14	74LS258	LOGIC
     Blank screen.

U15	74LS139	LOGIC
     Blank screen
     
My U13/U25 are MOS 7708 so 74LS257 would work as sub.

Does anyone know what modern ones you can use to replace these? I see on digi-key and Jameco SN74LS257BN and 74LS258BN but can't seem to find A or AN or APC. Are these compatible replacements?

Is there a list someplace that shows modern equivalents? I thought the important parts was 74LS257, etc. And that the other letters like SN for TI and BN for package type etc didn't matter. But apparently it does.

Thanks!


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eslapion
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Re: 74LS Replacements (Mos 7708, 7709)

Post by eslapion »

Zippy Zapp wrote:Is there a list someplace that shows modern equivalents? I thought the important parts was 74LS257, etc. And that the other letters like SN for TI and BN for package type etc didn't matter. But apparently it does.
The extra letters are sometimes used to indicate the fabrication process (who cares...) and the type of package which can be surface mount SOIC, TSSOP or normal through hole DIP.

Obviously you want a through hole component no matter what letter is used.

Honestly, I just search on Digikey for 74LSxxx and then in the search options, I check "In stock" and click on "through hole" in the "mounting type" options field.

Usually what remains is the purchasing format which is either "tape and reel" in thousands of units or tubes which can contain 1 to 20 units.

I just did it on Digikey for the LS257 and the only result of this search was SN74LS257BN. At that point, whatever the letters are is insignificant.
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Zippy Zapp
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Re: 74LS Replacements (Mos 7708, 7709)

Post by Zippy Zapp »

Cool. I saw the same one. The letters at the end threw me off a bit so that is good to know.

Thanks!
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Re: 74LS Replacements (Mos 7708, 7709)

Post by eslapion »

@Zippy Zapp
Glad I could help 8-)
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
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