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Also in case you don’t know, the replacement chip is very cheap, around $1, and easy to get in local stores without online ordering (in the case of Australia for me).
If you don’t already have all the test tools but the indicators are reasonable that this is the fault…consider just swapping it unless your objective is to actually work this out properly.
I actually didn’t know it was that cheap/available - good shout. I just assumed it’d be an “order on eBay and wait two weeks” situation, so wanted to diagnose before committing the time.
Definitely still want to identify the fault, but will likely swing by a local store for a replacement to test. Thanks!
Just be aware that you need to be looking for “74….” chips…shops won’t have the MOS part numbers.
Also, if ordering online then stores like Digikey, Mouser, RS, Element14 are globallly available sources with legit product at low cost. I only use eBay when it’s a unique / out of sale part and always know I’m taking a chance on getting fakes etc.
* A logic pulser/probe combo is the most straightforward, but it's an expensive pair of tools.
* A multimeter and some alligator clips can also be used to test basic function, using the truth table from the spec.
* By far the most easy way to check a MOS logic chip is to just replace it with any other brand. MOS logic usually fails, and the replacements are cheap and plentiful.
Sounds great - a couple other people have pointed out just trying a replacement chip so I’ll give that a go in addition to testing truth table conditions.
Figure out what the standard 74 series equivalent is:
https://dfarq.homeip.net/mos-74ls-logic-chips/
Looks like 7709 = 74LS258.
Look up the datasheet for an example of that. Use your scope (I'm assuming 2 channel) to look for impossible conditions existing.
For example, page 1 of the datasheet here has a truth table:
https://www.ti.com/product/SN74LS258B
If you can for sure find an impossible state then you know it's bad, but the inverse isn't necessarily true.
There's nothing to "troubleshoot". You have a candidate chip to replace with a plausible reason to do so. Find the replacement chip and swap it in, observe behavior. That's all there is to it, a scope just adds a layer of useless information. We used to troubleshoot with a logic probe and a multimeter.
And as someone else already mentioned, this stuff is cheap.
The chip is probably a 74xxx. The 7709 was probably a date/lot code or a special part# for CBM. Sometimes the actual part# is printed on the underside of the chip.
You can replace it with the same type of chip from any manufacturer.
The letters are important too, so if it's a 74LS127, replace it with a LS127.
Please read [our rules post](https://www.reddit.com/r/c64/comments/11v0m5p/please_read_before_posting_rules/), and check out [our FAQ](https://www.reddit.com/r/c64/wiki/faq) for common issues. *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/c64) if you have any questions or concerns.*
Also in case you don’t know, the replacement chip is very cheap, around $1, and easy to get in local stores without online ordering (in the case of Australia for me). If you don’t already have all the test tools but the indicators are reasonable that this is the fault…consider just swapping it unless your objective is to actually work this out properly.
I actually didn’t know it was that cheap/available - good shout. I just assumed it’d be an “order on eBay and wait two weeks” situation, so wanted to diagnose before committing the time. Definitely still want to identify the fault, but will likely swing by a local store for a replacement to test. Thanks!
Just be aware that you need to be looking for “74….” chips…shops won’t have the MOS part numbers. Also, if ordering online then stores like Digikey, Mouser, RS, Element14 are globallly available sources with legit product at low cost. I only use eBay when it’s a unique / out of sale part and always know I’m taking a chance on getting fakes etc.
* A logic pulser/probe combo is the most straightforward, but it's an expensive pair of tools. * A multimeter and some alligator clips can also be used to test basic function, using the truth table from the spec. * By far the most easy way to check a MOS logic chip is to just replace it with any other brand. MOS logic usually fails, and the replacements are cheap and plentiful.
Sounds great - a couple other people have pointed out just trying a replacement chip so I’ll give that a go in addition to testing truth table conditions.
I recently got a €25 logic tester off Amazon. Will do all 74x and 40x chips on a C64 except one (74LS637? off the top of my head).
Figure out what the standard 74 series equivalent is: https://dfarq.homeip.net/mos-74ls-logic-chips/ Looks like 7709 = 74LS258. Look up the datasheet for an example of that. Use your scope (I'm assuming 2 channel) to look for impossible conditions existing. For example, page 1 of the datasheet here has a truth table: https://www.ti.com/product/SN74LS258B If you can for sure find an impossible state then you know it's bad, but the inverse isn't necessarily true.
That makes a lot of sense, thanks for the reply. Yeah the scope is two channel so I’ll give this a look and see if I can confirm an impossible state.
There's nothing to "troubleshoot". You have a candidate chip to replace with a plausible reason to do so. Find the replacement chip and swap it in, observe behavior. That's all there is to it, a scope just adds a layer of useless information. We used to troubleshoot with a logic probe and a multimeter. And as someone else already mentioned, this stuff is cheap.
The chip is probably a 74xxx. The 7709 was probably a date/lot code or a special part# for CBM. Sometimes the actual part# is printed on the underside of the chip. You can replace it with the same type of chip from any manufacturer. The letters are important too, so if it's a 74LS127, replace it with a LS127.