Suggestions on Plus 4 problem?

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Gyro Gearloose
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Re: Suggestions on Plus 4 problem?

Post by Gyro Gearloose »

Well it amuses me to no end that I can get TTL chips from behind the Iron Curtain!

https://www.ebay.ca/itm/K555KP11-SN74LS ... rk:29:pf:0

LOL I'm going to give these a try. They'll probably be on 2.5mm centers instead of 2.54mm. No big deal on low pin counts.


This is fun!


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Zippy Zapp
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Re: Suggestions on Plus 4 problem?

Post by Zippy Zapp »

Cool. Sounds like you are on the right track. The MOS versions of the 74XX chips are some of the worst chips in any C64/Plus/4. According to Ray's website they are a frequent failure. Those and the TED and CPU are probably the top 3 worst chips in the Plus/4 - C16.
Commodore - Changing the world 8 bits at a time.
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Gyro Gearloose
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Re: Suggestions on Plus 4 problem?

Post by Gyro Gearloose »

I'll hook up the 8708 chips on a breadboard and take a look at how they work or rather don't work.
My place is a mess, so later.
In the meantime I noticed I was actually missing a lot of 74LS parts used in C= machines so I ordered a bunch from Futurlec. I don't know about buying 74LS parts on eBay.
I have many 74LS245s for some reason.
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Gyro Gearloose
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Re: Suggestions on Plus 4 problem?

Post by Gyro Gearloose »

I tossed in a 157 instead of a 257 and now I have the full 60K free on power up. The 257 has three-state outputs but in the Plus 4 the OE is always low so the hi-z output is never even used so a 157 does the job.
It's been a long time since I fooled around with discrete logic... But I got it to work in the end.
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Gyro Gearloose
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Re: Suggestions on Plus 4 problem?

Post by Gyro Gearloose »

Well there we go. I received my device programmer, the TL866, and it has a logic IC test function.
Image
There's the problem with the 8708.
Honestly I bought that programmer since it was recommended over at EEVBlog, I didn't check all its features. I was pretty happy to see the chip test feature, certainly saves me the trouble of lashing up a breadboard.
In the meantime I found two vintage 74LS157s in my bin from 1981, CSF-Thompson branded. I have more parts than I know!

And oddly even though I've been tootling around with electronics for decades, I never had a general purpose programmer before. I feel like the chip test function is the coolest thing ever. It can also test SRAM. It claims to do DRAM but I don't see any familiar part numbers.
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Re: Suggestions on Plus 4 problem?

Post by Gyro Gearloose »

update: I ordered some 74LS257 parts from Futurlec and they also fail in the same way, one out of 4 logic units fail, all the same.
guess I can't cheap out on these.
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Re: Suggestions on Plus 4 problem?

Post by Zippy Zapp »

I had no idear that the TL866 has a logic test for 74x chips. That is pretty cool. I think I would pick one up for that alone as it is still cheaper then the dedicated logic ic testers I have seen. I have a GQ-4x but it is an older revision and the latest software only works with the later revision which is just dumb. It doesn't have an IC tester either.
Commodore - Changing the world 8 bits at a time.
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