I wasn't expecting this fault with this CoCo 1 motherboard

Time to continue working on the TRS-80 Model 3/4/Color Computer repack-mash-up. Next up after exploring it in Part 1 is to get the motherboard working.

Part 2: This part!

--- Video Links

Adrian's Digital Basement Merch store:

Adrian's Digital Basement ][ (Second Channel)

Support the channel on Patreon:

-- Tools

Deoxit D5:

O-Ring Pick Set: (I use these to lift chips off boards)

Elenco Electronics LP-560 Logic Probe:

Hakko FR301 Desoldering Iron:

Rigol DS1054Z Four Channel Oscilloscope:

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)

TS100 Soldering Iron:

EEVBlog 121GW Multimeter:

DSLogic Basic Logic Analyzer:

Magnetic Screw Holder:

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)

Plato (Clone) Side Cutters: (order five)

Heat Sinks:

Little squeezy bottles: (available elsewhere too)

--- Links

My GitHub repository:

Commodore Computer Club / Vancouver, WA – Portland, OR – PDX Commodore Users Group

--- Instructional videos

My video on damage-free chip removal:

--- Music

Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[chironbramberger]

This video was a great one! I'd love to know if the retro chip tester finds those RAM chips are actually fully dead. Or perhaps the CPU barfing out like over 6 volts fried all the RAM chips? Although the CPU output 6 volts seems really weird!?!?! Like what failure mode turns a CPU into a step-up converter? I mean, obviously not for real, but still... weird! Looking forward to part 3!

[CallardAndBowser]

I'm 55 and started out with the Radio Shack TRS computers when they first came out !
Fun video, brings back great memories !

[johanlaurasia]

When you're pressing enter, not only does it appear to crash, but you get the S in the top left AND a relay click, so it's not crashing, but it's somehow jumping to execute the cassette load function which puts an S in the top left of the screen and turns on the cassette motor (hence the relay click trying to allow current to turn on the cassette motor. Interesting...

[draggonhedd]

One thing ive learned in troubleshooting, be it computers or cars, is that the first thing you dismiss as "it cant be that" is the first thing you should test. Never rely on your gut feeling, always test the "known" goods before go swapping parts around. I had the exact same type of issue you had here with an old VW golf, where the first thing I dismissed because they looked new (spark plug wires), ended up being the problem, but i didnt realize it until i had tested and swapped every other possible part first, wasting a bunch of time and money. All because I assumed that i knew that one part "couldn't be bad". Its an easy mental trap to get into. So now when I think "It cant be that", I test it to be sure.

[YARC-]

Two other pieces of info you may find helpful in the future: First, inverted @'s on the display very often indicate problems with the DRAM. When you originally fired the board up, those were present amongst the other garbage on screen. It's a good indicator to look there first. If you'd have known, you could have possibly saved yourself some time. Second, you can find more listings for the SAM chip if you look it up under the 74LS783 identifier rather than SAM, Synchronous Address Multiplexer or MC6883.

[Dukefazon]

My first idea was that +6V overshoot was an indicator of a bad voltage regulator or something that could kill all the RAM chips. But then my hunch is that the RAM chips you are putting in are different from the ones that were already in and somehow they are incompatible. If you put all of them back the machine would work as before. Probably has something to do with the timing or some similar properties that can be different on a chip. While the C64 can accept different timing, maybe the COCO is more sensitive in that way.

[rudodejong]

33:30 It looks to me like the 6th RAM chip on the board is now replaced by the original 5th RAM chip on the desk. And right after 34:24 the 6th, "untested" RAM chip on the desk gets an X mark. Probably doesn't matter but I had to get this out of my system 😉

[boolfrog]

Hi Adrian, just a thought, did you check the speed rating of the DRAM chips you removed in case someone had fitted a complete set of slow chips? Might explain why it sort of worked when all of them were fitted together, but didn't when one was operating at a different speed to the rest. As someone else suggested a RAM tester would confirm they are indeed faulty. Jeremy

[Toby_Q]

28:38 - What if these chips were "bad" from the factory. They say 64k but maybe they are broken 64k chips but the first 16k works. So they popped them in and sold this unit as a 16k system. Someone else before you came along, saw they were 64k chips, put the jumper to 64k, and it didn't work. Have you tried setting it to 16k with those "bad" chips in there? I've heard stories of systems (probably from you!) being sold with bigger ram chips that were half dead, but worked in the system because the system didn't use the bad parts of the RAM.

[YARC-]

Other possibility, of course, is that some nozzle set the jumpers for 16K at some point and fried all the 64k chips with +12v. Then tried to go back to 64K, only to find that he'd killed his computer.

[WesleyNixon]

A guest! Unprecedented! Looking forward to it.

[KAPTKipper]

Might be an idea to run them thru your chip tester

[YARC-]

Occam's Razor. My bet is that the original Motorola 4164's were some of the "half-bad" RAM chips Tandy used to be able to sell "32K" CoCo's. Each one you put back in with the good MT & TI RAM introduced errors because they were known to have one or more errors in the top or bottom half of their address space. If just one of those Motorola chips' problem was with its address lines rather than the data storage or data lines, that could explain the addressing problems you were getting. I'd wager that if you put the Motorola chips back in and set the machine to 32K, you'd be able to get it to work. You might have to switch the jumper telling it to use the high or low bank, but I bet you'd get it to work. Whoever changed it to 64K probably didn't know about Tandy's little cost-cutting trick with the half-bad chips.

[DBB-KEDUO]

back in the day… pre Y2K… we used to modify [mix and match] all sorts of different computers together to make our own custom systems! (especially laptops for rugidizing)… this episode took me back to the future!!!

[ygstuff]

I loved this review/repair of a Coco1.
The Coco2 was my first computer, and I still have it. Everything Coco still has a warm place in my heart, so I loved that you were able to get this one working....and considering the outcome of the issue, it was similar to a problem I had after I did a 64K upgrade a few months ago.
Coco for ever.

[telemedic]

Another great video, thanks Adrian. As you stated, the coco is a very simple system. This makes repair easier than most, as the cpu will run regardless of the video system, and vice versa. The “crash” you were seeing looked like it jumped to a tape loading routine. If you type “cload” the screen clears and “S” it shown in the top left to show its searching the tape. Did we hear the relay click? It’s likely. You made some pretty accurate assumptions on the way the machine works. Fair play. Interestingly enough, the 6847 is used in the acorn atom with the same inverted video :)

[DefconSix]

I always look forward to your videos. You are to vintage computer repair as Bob Ross was to art.

[robertbewes]

Absolutely super video. One of the best retro repair videos I have seen.

[RacerX-]

Nice repair. Retro Chip Tester would help out testing those DRAMs. Also I think the BackBit Chip Tester has some Coco chips it can test too. I find the BacBit to be cool to keep om my workbench for it's size and the ability to test custom chips for Commodore, Atari, Amiga, Etc...

[brooknet]

Thank you for the effort that you put into finding that mischievous fault. I really like watching the process of narrowing-down a fault. I never would have guessed that all of the RAM had failed. I was thinking that perhaps a bit of wire fell into an edge connector socket and bridged two address lines. No.. I was wayy off!