I need some advice/troubleshooting?

I have owned a Gottleib Hulk machine for a few years & had no problems until 1 day I put a LED bulb in one of the playfield lights to see how it looked (great) I put the regular bulb back in & it blew. I replaced it & it worked fine for awhile (at least 8hours for a party) then the fuse blew. I replaced the fuse & they kept blowing (with none of any of the playfield lights working). I tried taking all the lights out & replacing the bulbs 1 by 1 but the fuse would blow also. I lost power to the game for awhile due to Hurricane Sandy. Then I was able to get a local pinball guy to come & look at it. Of course he shows up & we put all new bulbs in it & it works perfectly, no blown fuses ect...

Here is my problem, when he left & I did a diagnostic on it & it gave me an error code (40- #10 point switch (5), and then at the end of diagostic it gives me another code (70- #4 rollover switch).

The game plays fine, all the lights light up & the 4x rollover works & scores properly.

But...When I start it up I get a backwards C in the scoring display area with a number and it won't work until I add a credit. But then it will show the 50+ credits that are on the machine (somehow added 99 credits when running through the diagnostics). The credit thing has happened before but I don't know how I did it & can never duplicate it when I want to.

Anyway..I can keep adding the credit & working around it but I don't want to cause any other problems & I want to have a machine that works the way it's supposed to (and did).

I would appreciate any insight anyone might have. I'm really a noobie at repairs & if I have to pay the guy to come back I will, but thought I would try to get an answer here & see if it's something simple I can do myself..Thx