Aristrocrate IGT lost 24 volt ac, entirely.

[bob in phx]

This is a continuation of my work on an Aristrocrat IGT 1981 slot machine. I previously repaired acid damage, figured out and modified the play credit buttons. and had teh old girl working great!!! But now a new disaster.

I closed the main door and it appears that the 24 volt ac wires that feed the festoon lights on the right side of the reel glass shorted to ground. Now, on power up I get no reel motor reset, and no 24 volt lights. but the good part is that I do have credit displays and the service credits button, adds credits!!! The 24 volt dc counters still work too!!! so I dont think I blew the cpu!!! But no coin lock out opening, no hopper movement when the collect credits is pushed. All the 24vac systems are dead......

I have meter checked all the fuses and they appear to be good. I tore open the power supply and it looks like the 24vac just passes through the power supply, stopping long enough to get a fuse placed in line. I have checked all the other fuses too... Alltest fine and all are the proper amp ratings.

I was thinkng that there must be a master 24vac relay or triac/scr that turns on the 24vac for use everywhere,,, but I cant find it. It would have to be big to control the reel motor etc.. I just hope that I have not fubar'ed the transformer!!!

any ideas????????

[Op-Bell]

One end of the 24VAC  is connected to 12VDC unregulated, which is about +18V. So if you shorted the 24VAC to the frame and the 24V fuse appears to be ok, you're going to have to check the power supply board for damage. Here's a diagram to help you. Note that the "ground" symbol shown connected to pin 8 is NOT the same as 0V (pins 4, 5, 7). It's a but confusing IMHO but I think it's intended to show that it's the return line for everything 24VAC.

Looking at the diagram, that little assembly TR3/TR3A probably didn't survive the short. What that is is a pseudo-SCR. TR5 turns on after the 5V comes up and triggers it, then it latches itself on and pulls the 24VAC common line up to unreg 12V (18V). The reason for this is the triac triggers in the IO board are referenced to true ground, 0V, and I think the triacs won't turn on unless there's a sufficient voltage differential, so this is a sort of power-on safety device to stop anything 24V turning on at random before the CPU wakes up. Hence, if it blew out, nothing is going to turn on ever!

[bob in phx]

Interesting that the power board that I have does not have a tr3a. There is a single tip42b in tr3. I tested against a new one, both in and out of the circuit and it looks bad. I will not have time this week to put all the parts together and see if this change out fixed the issue. I really appreciate the power supply schematic!!!!!!!!

[Op-Bell]

Well, the fact that the diagram shows TR3 and TR3a, R1 and R1a, D1 and D1a suggests that these 'a' parts are a later addition. I misread the diagram - they're not a pseudo-SCR but a current limiting circuit, probably added to protect TR3 from exactly what happened to you.

[bob in phx]

without that schematic, I would never had found the issue. The tip42b tested on the diode setting as .456 and .732 as measured from the center leg, with the negative meter probe. I would have guessed that it was fine. A new tip42b tested .456 and zero.... again, I would have never found it... I got some time tonight and put it all back together and.... IT WORKS!!!!!!!!!!!!!!!!

Ill button it all up on saturday and put another notch on my belt as yet another slot has been raised from the dead!!!!!!!!!!!!!

bob in phx.

[knagl]

Excellent!   

K+ to you and Op-Bell!