Diagnosing a faulty CoCo2

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robcfg
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Diagnosing a faulty CoCo2

Post by robcfg »

Hello everyone!

I'm trying to diagnose a 16Kb Korean made CoCo2 I got from a friend long ago, and as him before me, I'm quite stuck :lol:

So, here's the patient:
CoCo2_01.jpg
CoCo2_01.jpg (67.83 KiB) Viewed 1822 times
And here's the symptom:
CoCo2_02.jpg
CoCo2_02.jpg (52.67 KiB) Viewed 1822 times
The screen is grey because it's a NTSC machine on a PAL tv.

What do I know?

1) The machine boots up, and is able to load software. I type CLOAD, hear the relay clicking and when the program is finished loading, I hear the relay click again.

2) The SAM is working. I put it in a Dragon 32 and it works as expected.

3) The signals I poked with an oscilloscope looked nominal according to the service manual: https://colorcomputerarchive.com/repo/D ... Tandy).pdf

My only doubt is the clock signal from the MC1372 on the modulator, which looks like this:
CoCo2_05.jpg
CoCo2_05.jpg (59.31 KiB) Viewed 1822 times
4) No chips seem to be hotter than usual.
CoCo2_06.jpg
CoCo2_06.jpg (98.84 KiB) Viewed 1822 times
CoCo2_07.jpg
CoCo2_07.jpg (56.8 KiB) Viewed 1822 times
5) I tried to run a small basic program to switch to graphics mode and measure the |A/G pin, to determine if it's the PIA's fault or the 6847's, but it turns out that the 'R' key could be not working so I'm not able to run the program.

Has anyone encountered a similar fault, or knows hot to diagnose it?

My bet is on the 6847, because all |A/G, |A/S, GM0, GM1 and GM2 signals are low according to the multimeter, which is correct for the text mode screen, but as you can see on the tv picture, there is no border, which would indicate that the VDG is in graphics mode.

Cheers,
Rob
sorchard
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Re: Diagnosing a faulty CoCo2

Post by sorchard »

Hi Rob,

How are you connecting to the TV? The signal that normally comes out of the CoCo is not just a composite signal, it's a VHF broadcast signal, meaning it needs to go to an antenna input. (I think via an impedance matching switch box in North America).

Everything else you say seems to suggest the CoCo is fine. You say the signals on the scope match the service manual which I assume includes the signals coming from the 6847.

I could be wrong but it doesn't seem likely that a TV designed for one broadcast standard could tune in to something designed for another part of the world. For example I don't believe a UK TV can tune to a frequency anywhere near as low as the signal output by a CoCo. Even then the video and audio carriers are a different distance apart.

It's not the same issue as NTSC vs PAL. Those are video encoding schemes and many TVs can handle both via the composite input. Broadcast modulation schemes are something entirely different.
Stew
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robcfg
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Re: Diagnosing a faulty CoCo2

Post by robcfg »

Hi Stew!

It never crossed my mind that the VHF signal could be a problem.

Thing is, my TV set detects the signal, and I can clearly see when the computer is on or off because the screen is completely black or as I showed above.

Does the Tano Dragon have the RF output? If that's the case I could try it on my tv and see what happens.

Otherwise I'd need to get composite output in order to see what's happening.
sorchard
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Re: Diagnosing a faulty CoCo2

Post by sorchard »

robcfg wrote: Thu Apr 22, 2021 4:59 pm Does the Tano Dragon have the RF output?
I think it does. Not sure what channel. Worth a try to see what happens.

There are at least a couple of options for adding composite output to a CoCo 2. It's slightly more involved than a CoCo 1 but it has been done.
Stew
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robcfg
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Re: Diagnosing a faulty CoCo2

Post by robcfg »

Well, results are in!
CoCo2_08.jpg
CoCo2_08.jpg (79.18 KiB) Viewed 1765 times
The video via RF works enough to get a picture, so that's good news!

On the other hand, knowing that this works, and that signals from the PIA are correct as far as I know, the MC6847 must be damaged, so I guess it's time to get the desoldering braid and a spare socket.

While I was at it, I did a PMODE 4,1:SCREEN 1,1 and was presented with a pattern different than the usual one:
CoCo2_09.jpg
CoCo2_09.jpg (82.67 KiB) Viewed 1765 times
Have you seen a pattern like this one on other Dragon machines?
sixxie
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Re: Diagnosing a faulty CoCo2

Post by sixxie »

I've not seen that pattern specifically, but different chips come up with different patterns.

Check out CC:10 (page 18) in this Tandy document: https://colorcomputerarchive.com/repo/D ... Tandy).pdf
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robcfg
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Re: Diagnosing a faulty CoCo2

Post by robcfg »

Hi Ciaran!

Does CC:10 prevent the machine from booting correctly? Because I've managed to load software on the CoCo2, so my guess is that it initialises properly.

Here's the memory assortment on my CoCo2:
CoCo2_10.jpg
CoCo2_10.jpg (71.92 KiB) Viewed 1763 times
I see a chip different from the rest, but don't know if it's different enough :mrgreen:

Cheers,
Rob
sixxie
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Re: Diagnosing a faulty CoCo2

Post by sixxie »

robcfg wrote: Fri Apr 23, 2021 6:17 pm Does CC:10 prevent the machine from booting correctly? Because I've managed to load software on the CoCo2, so my guess is that it initialises properly.
Oh it just affects Wildcatting which does some weird tests for some reason. Don’t think anything else cares, was just a good example of DRAMs having known and differing startup patterns.
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robcfg
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Re: Diagnosing a faulty CoCo2

Post by robcfg »

Success!

I tried the CoCo 2's 6847 on one of my Dragon and the composite video output was dead.

Then I put the Dragon's 6847 in the CoCo 2 and lo and behold, we have a fully working CoCo 2!
CoCo2_11.jpg
CoCo2_11.jpg (47.71 KiB) Viewed 1593 times
CoCo2_12.jpg
CoCo2_12.jpg (70.17 KiB) Viewed 1593 times
Case closed! (Literally and metaphorically :mrgreen: )

Now, I need a new 6847. And I was thinking on maxing out it's specs (16 to 64KB of ram) and maybe changing the modulator for another solution to have composite output.

Thank you everyone for your kind help!
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