Thomson TO8 - no picture - Update 13.08.16 - broken PSU

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TrainingForUtopia
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Thomson TO8 - no picture - Update 13.08.16 - broken PSU

Message par TrainingForUtopia »

Hi,

well, i repaired some of my other Thomsons and now i got a TO8 with a cut power cord.

Ok, i replaced the power cable and the suppression capacitor.

Image Image Image

I powered on the TO8 and and he brings up the normale red light, but i only get this noisy picture ...

Image Image

Strange, 'cause i don't know, where to start searching for the failure.

I little help would be great.

Thanks & regards

Tom
Dernière modification par TrainingForUtopia le 13 août 2016 21:37, modifié 2 fois.
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

How did you connect your TO8 to the display ? This looks like a cheap SCART cable missing the RGB signals or your TV not supporting the RGB on scart.

try the CAPS LOCK key on the keyboard, if the LED lights on and off when you press the key, your TO8 is essentially in working condition (because this is managed by software).

Also pay attention to the blue capacitor next to the fuse, it often explodes with white smelly smoke. Better replace it asap.
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

I connected some of my other Thomsons (TO7, TO8D ...) with the same scart cable and i always got a great picture.

I think there's a bigger problem, 'cause CAPS LOCK led is always on.

And i already replaced the blue suppression capacitor as i mentioned in my first post (i replaced some of them in my Thomsons - two of them in my TO9 some weeks ago).

Before:

Image

After:

Image

Maybe i need a description of all components of the TO8's mainboard.
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

okay, excellent. The next thing to check is, does your TV react when you power up the TO8 ? I guess yes, since the screenshot you sent is not all black. Which means the switch signal is OK.

The next thing to check is the clock. There's a 16 MHZ oscillator on the board with a cold boot hardware mechanism which sometimes fails. In that case, the CPU stays in RESET forever and the screen remains black. You should check the clock and the E, Q and RST signals with an oscilloscope on the 6809E.
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

Thanks for your answer ...

Ok, when i power up the TO8, i can see a very short "flicker", so yes ... it reacts.

Where is the oscillator on the board?

Image

I don't have a oscilloscope anway.
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

I suppose you've checked the +5V and +12V from the power supply, right ?

From now on, it won't be possible to diagnose without either an oscilloscope or a logic analyzer. You can find the latter on ebay or ali express for 10 bucks.

The full service manual of the machine is found here : http://dcmoto.free.fr/documentation/docto8/docto8.djvu
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

How about replacing the 16 Mhz O. with one of another Thomson?
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

Well, and i think we both mean different things ...

This is what i call "oscilloscope":

Image

And this is a "voltage detector" which i surely have for my repairs.

Image

So what do you mean?
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

The problem is probably not with the oscillator. there's a whole set of subclocks derivated fromt the 16 MHz signal which may fail independently + the cold boot as I mentioned earlier.

Besides that, trying random things is not a way to troubleshoot. Connect a logic analyzer and look at the result, it takes half a second and you will know what's wrong. Without a minimum of equipment, you should not try to fix digital devices. A logic analyzer is really unexpensive nowadays and it will save you hours and prevent you from trying stupid/dangerous things (I have myself made a lot of stupid things when fixing computers, everybody does).

The oscilloscope is even better, but I agree it's still quite expensive. Here is one logic analyzer which should be perfect for troubleshooting 8-bit computers :

http://www.ebay.com/itm/saleae-logic-an ... SwwpdW89FI

Image

It's a clone of the real Saleae (http://www.saleae.com), but I have one such clone myself that I still use from time to time and it works great for anything under 24 MHz.

One thing you can do still is check the voltage with your voltage detector on the reset signal (pin 37 of the 6809E). it should be 5V stable. and if you press the reset button on the side of the computer, it should be 0V stable.

oh, and you did not answer, are the 5V and 12V ok ?

If the scart cable is not to blame (seems so), the black screen on a TO8 either means that the CPU has not started (for the reasons I mentioned before, clock etc.) or that the D/A converter for the video is dead (often linked to 12V supply issue). Since your CAPS LOCK key does not work properly, I would vote for the CPU not starting for some reason. That's where the logic analyzer really comes in handy. Maybe one of the capacitors on the reset and clock lines has a problem, or the cold boot (combo resistor + diode) is the culprit.

Let me know.

Edit: or there's a buggy solder joint somewhere on these lines or on the chipset (the square IC on the back of the mainboard).
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

Ok, thanks again ... i'll pull out the mainboard later and check some things.

So are there any pics with detailed mainboard component information (alle the ics and so on)?
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

as mentioned earlier, everything is in the service manual : http://dcmoto.free.fr/documentation/docto8/docto8.djvu
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

BTW, I can guide you through the schematics if you want, I know them almost by heart.
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

Well, i will be back tomorrow i think ...

But one more question ... What 5, 4 and 6 stands for? Why the numbers 5,4 and 6?
Pièces jointes
Unbenannt 1.jpg
Unbenannt 1.jpg (63.26 Kio) Consulté 5147 fois
Fool-DupleX
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Re: Thomson TO8 - no picture - tried to repair

Message par Fool-DupleX »

Not much I guess. These chips are the ROMs and the machine used to be built on the same assembly line than the TO8D and TO9+, maybe it could be something related to which ROM goes where on the assembly line.
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TrainingForUtopia
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Re: Thomson TO8 - no picture - tried to repair

Message par TrainingForUtopia »

One thing you can do still is check the voltage with your voltage detector on the reset signal (pin 37 of the 6809E). it should be 5V stable. and if you press the reset button on the side of the computer, it should be 0V stable.
It is as it should be ...

The 5V and 12V are ok too ...

I'll keep the TO8 for spares or as a sample, no time to get deeper into that stuff.

Thanks anway.

DONE!
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