Atari 400 power intermittent

[+slx]

I have a 400 with a B-key type keyboard and installed UAV. Somehow it works intermittently only. It works fine in Memo Pad mode when I turn it on without a cartridge after having been switched off for a couple of hours. Once I open the cartridge door and insert a cartridge operation it sometimes boots the cartridge and sometimes it doesn't and the power light remains out. The power LED remains off even when I remove the cartridge. 

 

The power board itself has been serviced and tested. 

 

Any ideas what could cause this and what to check for?

[Paul Westphal]

I’m sure you are using a new power supply.. You said the power board has been serviced but I’d check the voltage regulator(s) after replacing the power supply. Also - Crappy switch maybe? 
 

On a side note, I was once told that those B-Key keyboards cause issues with Atari 48k ram boards but I’ve never confirmed it.

[_The Doctor__]

the spring interlock switch on the cart door sometimes does not get flexed enough to get completely engaged, sometimes a little adjustment is needed... I'd check that switch and make sure it's clean, lubed, and aligned/adjusted to fully engage each time... while you are in there make sure there are no cold solder joints and the connectors are clean and fully seated.

[Vyvyan B.]

I had this same issue on a 400 and it turned out to be a cold solder joint on one of the pins of motherboard to power supply connector. Door switch could be dirty too.