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Videopac G7000 with no video output

Started by CRG, November 03, 2020, 09:58:37 AM

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CRG

Firstly let me say hello.  I'm Glen, I go by CRG online.  I'm a collector of all things vintage / retro  when it comes to computers and consoles and recently I stumbled across a G7000 on eBay spares or repairs for a price I couldn't refuse. 

The system itself was sold as not turning on but thats not really accurate.  I've been able to test that the system does in fact power up and it seems to be running the games OK since if I connect a speaker to the audio output from the main board I can hear firstly the startup then the games running.  The problem is the video, theres nothing. 

I've been trying to follow the service manual but I'm not having much luck, hopefully one of you lot can help. 

https://ia803206.us.archive.org/14/items/G7000_Service_Manual_198x_Philips_M6/G7000_Service_Manual_198x_Philips_M6.pdf

Part of the issue I think is that my board is different to what is described in the manual, for example none of my ICs are labelled as per the schematic meaning I have to physically trace out all the connections.  Not impossible but it takes forever. 

Using the flow chart at the back of the manual I've been able to confirm all the voltages and mhz signals as it describes. 

Going round the PAL encoder (which unfortunately seems undocumented), pins 2, 3, 4 which handle R-Y, B-Y and Blank the signals are very weak.  There is no output on pin 5 which should be the composite signal.  I can see the 4.4mhz signal at these 3 pins but its very low, less than 100mv but I admit I don't know what level these should be at.  Tracing these back to where they are generated, at the ICs I see a signal level of about 3.3v.  After it passes through the resistors and caps the levels drop significantly.  None of these caps are shorted to ground etc.

I'm not by any means an expert when it comes to electronics (rather the opposite), I do this just as a hobby and while i probably could bypass this section of the video output by going with an RGB mod, in the first instance I'd rather try and get it working. 

Any ideas why I'm not getting a composite out of the pal encoder.  Is it something to do with those very weak R-Y, B-Y and blank signals or perhaps its just faulty? I half tempted to just change all the 74LS logic down where the above mentioned signals are generated.  Its not expensive to do but since that 4.4mhz signal is there I'm not convinced that's the problem. 

Sorry for the long winded first post but any help would be appreciated. 

CRG

If someone could even measure for voltage on pins 2,3,4 of their pal chip it would go some way to helping me understand if I'm missing something. 

Xuul

Could be a faulty PAL-Encoder chip ( IC679 - TEA1002 ).
I've swapped 2 of them myself so far.

Sorry for necroing, but this IC croaks pretty often, so this thread could help someone  :)
Programmers don't die.
They just gosub without return.