Problems booting a light sixer

Hi all!

First of all, sorry for my english - I'm from Spain. I entered in this forum 6 years ago now, but I wasn't really active.

Recently I got interest again about Atari games, and purchased a light sixer Atari, but seems I'm having trouble getting it to work.

When I turn it on, it keeps an stable image for about 2 seconds. In those 2 seconds the cartridge inserted is working correctly - joystick response, sound and image are ok. But after this, it hangs up, usually showing a black screen or 2 coloured-vertical lines screen, sometimes with a "piiiiip", sometimes silent. After turning it on and off some times, this working time get shortened, from those 2 seconds to less than a half.

I tried all my cartridges, cleaned contacts and checked bad solder points, so I discarded a bad contact when inserting a cart.

I've opened the console, and I checked if power after the 7805 regulator is ok. It gives 5.07 volts, completely stable.

Any idea? This is my first "original" Atari, I always had Jr ones and clones, and this one really looks beautiful :-)

Thank you!