Atari STE - no monitor output, RF fine

[Hellraiseruk]

Hi Steve,

Thanks for the reply but his problem is different to mine - he gets composite output but no RGB, whereas I get no composite video output at all via the monitor jack, I do get an RGB signal when connected to a CRT TV, but it rolls all over the place with black bars. I know for a fact that the TV I am using works with an STE as I had one previously which I used on both the CRT and also the LCD TV I use for testing and it gave a perfect RGB picture.

As RGB relies on the composite signal for the sync, and the fact I have a sync issue, and the fact that it appears my composite output is shorted to ground somehow, it looks like an internal ST fault somewhere - however I can't see any obvious sign of anything that would cause this kind of issue whilst still providing a perfect signal via RF, so composite is obviously making its way to the modulator.

[rubber_jonnie]

Hi Steve,

Thanks for the reply but his problem is different to mine - he gets composite output but no RGB, whereas I get no composite video output at all via the monitor jack, I do get an RGB signal when connected to a CRT TV, but it rolls all over the place with black bars. I know for a fact that the TV I am using works with an STE as I had one previously which I used on both the CRT and also the LCD TV I use for testing and it gave a perfect RGB picture.

As RGB relies on the composite signal for the sync, and the fact I have a sync issue, and the fact that it appears my composite output is shorted to ground somehow, it looks like an internal ST fault somewhere - however I can't see any obvious sign of anything that would cause this kind of issue whilst still providing a perfect signal via RF, so composite is obviously making its way to the modulator.

My suggestion here is to grab the STE schematic (I'd advocate the use of @sporniket's excellent remade ones) and trace the pins back from the socket all the way through the circuit to see where all the signals get to and where they potentially stop.

I had to do a similar trace with my OSI 600 cassette circuit, I traced through from each point to the next until I found where the signals were stopping, it's the same theory. You don't have to work backwards, you can do it the other way too, I just prefer to work back.

I'm presuming (I haven't looked back through) that at this point you have checked the solder joints for cracks etc.

[Hellraiseruk]

Cheers for that, if only it were an open circuit I was tracing, that would be so much easier! I found a schematic but it was poor quality, so I'll have a look at the one you've recommended.

From what I can see the comp input to the modulator is fine, no sign of any short and as it's coming from the same place there's not much that it can be. I'm leaning towards a physical issue with the socket but will look a little closer once I get the time. Thanks for your help :)

Oh, and checked for anything weird on the board, no obvious signs of anything.

[rubber_jonnie]

Cheers for that, if only it were an open circuit I was tracing, that would be so much easier! I found a schematic but it was poor quality, so I'll have a look at the one you've recommended.

From what I can see the comp input to the modulator is fine, no sign of any short and as it's coming from the same place there's not much that it can be. I'm leaning towards a physical issue with the socket but will look a little closer once I get the time. Thanks for your help :)

Oh, and checked for anything weird on the board, no obvious signs of anything.

Ideally you need an Oscilloscope for tracing the live signals, but I do advise folks to stay away from the cheapo portable ones.

There are some things you just can't do without a 'scope.

[Hellraiseruk]

Yup, got one of those cheapo ones - the DSO1511G I think it is, little yellow lump of goodness. Seems to work pretty well, had it measuring up to 32Mhz without too many problems. That's how I found out I had virtually no signal on pin 2, so might have a look at the composite in on the modulator and see what the signal is supposed to look like. A part of me is tempted just to patch a fly lead across to pin 2 on the monitor socket and see what happens :)

[Hellraiseruk]

The picture in the first post of this thread is exactly what I get on my CRT over the monitor port, so it is definitely sync related:-

viewtopic.php?f=23&t=7036

Going to remove some components and try to locate the short at the weekend, will report back.

[Hellraiseruk]

OK, latest instalment....

It looks like the socket itself has an internal short. I desoldered the end of R439 which is basically the last component before it goes off to pin 2 of the socket and I have a perfect composite signal on there as confirmed by the oscilloscope. Pin 2 of the video connector still shows a short to ground/pin 13 of the video socket with R439 lifted.

I've checked the board and can't see any solder whiskers between any of the pins on the underside of the socket, nor can I see any obvious traces of anything on there so I have a feeling the socket itself has a fault internally. Has anyone experienced this before?

I'll take a couple of pics of the top and bottom of the board, if anyone can see anything obvious I've missed that would be great. Would also be cool if someone could confirm that pin 2 is not supposed to read zero resistance between itself and pin 13/ground, and that I've not been going on a wild goose chase :)

Cheers for your help

[stephen_usher]

One way to confirm this would be to desolder the socket.

[Hellraiseruk]

That's what I am thinking, however it looks like a bit of a pain to desolder manually, might have to wait till I order my desoldering gun. I assume no-one else is reading a short between those 2 pins, and I've definitely found the cause? Will have to have a look in the Exxos store for a new socket.

[stephen_usher]

Waiting for a desoldering station is an excellent idea.

If you've got a high pressure air duster you may try blowing it between the pins of the socket on the motherboard in case a little bit of wire or solder has got in and is causing the short.