hardware repair 7800 - No Video with 7800 carts, 2600 carts works perfectly

[billlagr8]

Hi All

I have a PAL 7800 with composite mod, and it has a weird problem. When a 7800 cartridge is in (and also with no cartridge!) I have no video, but I do have audio. No video is no signal at all, audio plays as normal. With a 2600 cartridge inserted, the system behaves as it should - all video and audio works correctly, and games are playable. Anyone have any suggestions as to where I should start looking?
Thanks

 

[+Mitch]

Mostly likely something is wrong with the video part of the composite mod on the 7800 side. Does it work if you use the RF connection?

 

Mitch

[billlagr8]

24 minutes ago, Mitch said:

Mostly likely something is wrong with the video part of the composite mod on the 7800 side. Does it work if you use the RF connection?

 

Mitch

The modulator was removed, so I can't test that, along with some other components. If the composite mod was faulty, then I'd see nothing even with a 2600 cart? The video is taken from the pcb where the modulator originally took it from

 

[Trebor]

7 hours ago, billlagr8 said:

If the composite mod was faulty, then I'd see nothing even with a 2600 cart?

Not necessarily.  The part of the mod where the connection is being fed to it from the TIA chip (Video and audio for 2600 games, audio only for many 7800 games) is evidently fine; however, the part of the mod where the connection is being fed to it from the MARIA chip (Video for 7800 games) is possibly where there is an issue.

 

[Stephen Moss]

Trebor is correct, in the 7800 the TIA is responsible for generating video for the 2600 games and Maria for the 7800 games.

PAL 7800 are supposed to have Asteroids built into the ROM that loads if no cartridge is detected so if you are getting Asteroid sounds but no video with no cartridge inserted then that combined with 2600 carts playing correctly would suggest that the cartridge detection circuit is probably OK.

I am not sure what would happen video wise if the TIA video was active and MARIA not, but I would expect that you would get something even if it was rubbish so that fact you have no video at all suggests the TIA video output is being disabled as it should for 7800 games.

 

Consequently, I sounds to me like there could be a problem with MARIA, either faulty or not getting power/a required signal to make it operate.

@billlagr8Try looking at the input pins of U3 with a logic probe of multi-meter. With  2600 cart inserted I would expect pins 2, 5 ,10 & 12 to be a steady 0V (logic Low) and pins 1, 4, 9, & 13 to be switching between 5V and 0V as the video signal changes (on a Multi-meter you will read and intermediate voltage.

With 2 a 7800 cart or no cart inserted I would expect the TIA pins (1, 4, 9 ,& 13) to be 0V and the MARIA pins (2, 5, 10 & 12) to be switching if MARIA is outputting video. 

On 3/14/2022 at 5:35 AM, Mitch said:

Mostly likely something is wrong with the video part of the composite mod on the 7800 side.

I don't know where the video for composite mod is being taken from but I would have though that rather than take the signal from TIA and MARIA from separate points that it would be taken from a single point on the output of U3 (which logically ORs the TIA and MARIA input signals). If that is the case then if 2600 video is OK, 7800 video should be as well assuming MARAI is generating it.

[billlagr8]

32 minutes ago, Stephen Moss said:

 

@billlagr8Try looking at the input pins of U3 with a logic probe of multi-meter. With  2600 cart inserted I would expect pins 2, 5 ,10 & 12 to be a steady 0V (logic Low) and pins 1, 4, 9, & 13 to be switching between 5V and 0V as the video signal changes (on a Multi-meter you will read and intermediate voltage.

With 2 a 7800 cart or no cart inserted I would expect the TIA pins (1, 4, 9 ,& 13) to be 0V and the MARIA pins (2, 5, 10 & 12) to be switching if MARIA is outputting video. 

I don't know where the video for composite mod is being taken from but I would have though that rather than take the signal from TIA and MARIA from separate points that it would be taken from a single point on the output of U3 (which logically ORs the TIA and MARIA input signals). If that is the case then if 2600 video is OK, 7800 video should be as well assuming MARAI is generating it.

Thanks for that - you answered my next question, where should I start looking at MARIA! I also have a cheapo scope I bought to learn how to use, would that be any use? If it is where/what should I be looking at? 

The video for the composite mod is taken from where the input to the modulator was - the modulator has been removed and the composite mod video in is taken from that point. (Along with ground and +5v that the modulator previously also used). 

[marauder666]

The colour pot for the Maria might be out of adjustment, try giving it a little turn. 

It's the one nearest the cartridge port.

 

[+Mitch]

I would still check the video mod first. There are a lot of low quality ones out there and have a higher failure rate then the Maria chip.

 

Mitch

[marauder666]

Post a picture of the insides, might spot something.

[Stephen Moss]

22 hours ago, billlagr8 said:

Thanks for that - you answered my next question, where should I start looking at MARIA! I also have a cheapo scope I bought to learn how to use, would that be any use? If it is where/what should I be looking at? 

Well a scope is more useful than a mutli-meter when looking at changing signals such as Clock, Address/Data lines and video as with changing signals a multi-meter will only give an indication of whether they are changing or not (fixed 0 or 5, or something in between) whereas you can actually see it with a Scope but is more useful if you have a refence signal to compare it to such as you might in a service manual.

Personally, I would still start with the U3 inputs I mentioned earlier to see if MARIA is outputting any video data. At least that would give some indication as to whether the problem may lie with MARIA/the system preventing correct MARIA operation or the composite mod, with 2600 games outputting video through the composite mod you could assume (at least for now) that that the Video mode is working and not causing any problem like killing the video signal.

[billlagr8]

So..thanks to @Stephen Moss's advice and my cheap USB oscilloscope, I have it all up and working! I tested the inputs to U3 and found they were ok, and worked as expected (2600 cart inserted/7800 cart inserted /No cart). I then checked to see if there was a signal at the video input of the composite mod and there was, with any or no cart inserted. What I found however, is that when a 2600 cart was in, there was a nice strong signal from the video out of the composite mod, but when there was a 7800 cart in, there was a signal as well but it was extremely weak. I then found that the composite mod was only receiving 1.8V from the 5V connection. When a 2600 cart is in, there was still a strong enough signal to display despite the composite mod being underpowered, but with a 7800 cart, there was not. I took the 5V from a different point, and now all works beautifully. Thanks everyone for your advice!

[marauder666]

Where did you take the original 5v feed from?

[billlagr8]

3 hours ago, marauder666 said:

Where did you take the original 5v feed from?

Where it originally fed the modulator from

[marauder666]

I always use that on a pal machine. Never had a voltage drop. Thats weird

[billlagr8]

1 hour ago, marauder666 said:

I always use that on a pal machine. Never had a voltage drop. Thats weird

I know, I was baffled by it for a while, which is why it took me so long to find the problem - it just never occurred to me that the 5v wasn't 5v. I just took it from a different spot and it's all good now