5200 black screen - need help

[jum]

Hi

 

I have a 2-port 5200 running from 220V via a step-down xformer. Both VR1 and VR2 voltage regulators (LM340-5) and their heat sinks had been ripped off the m/board, so I replaced them with 7805 voltage regulators.

 

I've checked the voltages on the replaced VR's, and they seem to be working OK.

 

However, I just get a blank black screen when I power up with a cart inserted, and a rolling block screen with no cart. The screen shows red and the audio buzzes for about a quarter of a second when I hit the power switch, but then goes black.

 

I have the video cable from the 5200 plugged directly into the RF in of my portable LCD TV.

 

I need help with troubleshooting what's wrong.

 

Regards,

 

Jum

 

PS: The "channel select" switch on my 5200 is missing - does anyone know how to wire this up correctly?