I am working on a gottlieb sys 1 power board for a friends pinball machine. When the power is turned on two of the diodes (CR3 and CR4)burn up. The input voltages are reading high. pins 1 and 2 are 12.7v instead of 11.5v, pin 3 is ground, pins 4 and 5 are 15.4v instead of 14v, and pins 6 and 7 are 75v instead of 69v. Voltages on pins 1, 2, 4, and 5 were taken from ground and pins 6 and 7 across the pins. I was wandering if these voltage differences would fry the board or are they within tolerance. I know that there is probably something else wrong with the board, but I haven't gotten that far yet. I'm trying to remember everything I learned about 16 years ago. It's amazing what you forget if you don't use it. Any suggestions will be greatly appreciated.

 

You will have to post a schematic before we can tell what you're talking about.

 

It would be very helpful to have a schematic to look at.

 

Ok, This is a schematic that i found online. http://tuukan.fliput.net/power.gif It is very close to the board. the only differences is that C9 and C10 are not on the board an C1 is a 4700uf 35vdc cap instead of the 2900uf 30vdc cap. I'm mapping the actual board myself, but that takes alittle time. I have also found out that lic1 is a LM7912 -12v regulator if that helps.

 

Start by suspecting C4 as shorted. Then check the 7912 to see if it is passing excessive current to some shorted load.

There's a start. Check back if you need to.

 

C4 is not showing as being shorted. However, as I was removeing the mounting bracket, which aslo serves as a heat sink for Q1, Q2, and LIC1, I noticed there was no mica under LIC1 and Q2. It looks as if there was mica there at one time but is gone now. I checked to see if there was a conection between them and found out that pin 3 on LIC1 and pin C on Q2 were conecting throught the mounting bracket. I think this could be at least be a part of the problem.

 

Make sure the tab of the 7912 isn't accidentally grounded; the pinouts are different between the 78xx and 79xx family.
/mike

 

SCR201 is part of a crowbar circuit. Something may be tripping it. This might be a case where loading the 14 VAC lines down with a couple of automotive lamps to prevent the voltage from coming all the way up could be useful. If the caps check, then something is going on with the crowbar - the SCR could be shorted.

 

hi
new to this posting business... just wondering if you managed to fix the power supply? Im having the same issue with cr3 and 4 burning... thanks