A1200 Composite output

[gazcbm]

Hello 1200 gurus!

I need some help getting a 1200 back up and running...

I have a 1200 board that outputs perfectly via the RGB port to a 1084s monitor but doesnt output anything through the neighbouring composite output. When I received the miggy it was in unknown state and so I recapped it (without testing it first - school boy error I know). Does anyone have any step by step suggestions on where I can start to trace amd repair this composite problem?

Not sure if this has any relevance but I have not yet tested the audio sockets and havent bothered with the RF output (does anyone else?)

 

[Zetr0]

Firstly check the RF out and see if you can get a picture - I suspect that this wont be the case.

The CXA1145 provides Chroma / Luma Y/C for the UHF modulator it also outputs Composite as well.

As you have done some repair in the area of the CXA1145 I suggest just tinning the legs of the chip before proceeding.


If you still dont get any joy, the its possible that either the colour encoder xtal is dead or the CXA1145 itself is dead... or both.

I would suggest getting a 4.43361875MHz (PAL colour carrier frequency) Oscilator replaced if thats no go, then you need to replace the CXA1145

These can get a little pricey on ebay, so worse case scenario find a broken Megadrive that could donar the part =)

 

[yuorz]

These can get a little pricey on ebay, so worse case scenario find a broken Megadrive that could donar the part =)

Ah, donor parts. I used a 74157 IC found in the Megadrive controllers for a Sega Saturn region mod. It seems the good old Megadrives have some useful parts that can be harvested.

 

[gazcbm]

So I finally got round to testing this....

First testing with RF tuning. The TV does not actually manage to zero in a specific channel, but as the auto search is going through I can see the purple screen and disk going in and out of tune around the same point in the search, albeit not quite full colour.

I tinned the legs of the CXA1145 (I assume this is the SMD component just below the caps on the left hand side of the board) and this has not made a difference.

If I have correctly identified the CXA1145 I don't believe the chip is in SMD format in the mega drive, so I think I may need to source an alternative.

One other question, could an incorrect or poorly replaced cap cause the same symptoms?

 

[gazcbm]

Also are there any voltages on the board I can measure to diagnose where it could be going wrong

 

[Zetr0]

@Gaz

it would seem the entire circuit based around the CXA1145 is at fault. seeing that you have a colour comming from the RF would suggest that the CCK circuit is okay, however I would look at stripping it down, clean the area's again - remove the caps and test all the pads - ensure they are connected correctly and there are no grounds / shorts.

From this I would re-populate the area, I would look at replacing that cxa1145 chip though, if you cannot grab one from a dead / donar megadrive / megadrive 2, you could source one from a dead A600 or even A1200 motherboard.

infact, if I get some time I will lift one for you, since the DAC is dead its not even being used by the system.