Super Bagman power supply....bizarre behavior

cwilkson

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Got a strange failure here. I'm troubleshooting the power supply in my Super Bagman. It's a Stern PS-1200 (rev H). Without the game PCB connected to the supply, the +5V output is adjustable from 4.6 to 6.1V. But when I connect the PCB to the supply the +5V goes *UP* to 8V.

!!!

+5V goes up when loaded!

!!!


Anybody know what would cause this? As far as I can tell, the power transistor is good. I put in a new LM317 regulator. I don't have an ESR meter, but the caps seem ok. No opens and no shorts, values are all close. I pulled the fuses for the +12V and -5V supplies so the +5V section is running by itself.

One other thing...when I got the game R8 (22 Ohm) was burned but still measured about 24 Ohms. I left it in. The power trace going from R8 to the transistor was also toasted. I fixed that and the game ran fine until the flyback blew about 6 months later. It has been sitting for a couple of years now. I finally found a flyback replacement, fixed the monitor, and powered the game only to find that the PS was dead. Tried replacing R8 with two 43's in parallel tonight, but no change.

I don't have any spare PNP TO3 transistors....I tried subbing a TIP42C, but got the same behavior. Gonna try to get another transistor tomorrow. I think NTE219 is the cross. I'm stumped.

Oh yeah...I should state that I'd like to keep the game original so putting a switcher is not my first choice.
 
Well, I found the problem. The transistor was leaky! Or somethings. But the crap meter I have (lost my good one) didn't show the problem. Curve tracer to the rescue! Somehow the transistor, once turned on weakly was turning itself on harder. And latching kind of like an SCR. So as soon as I pulled more than about an amp, the thing acted like a switch and shorted out. It was repeatable. Very strange.

Anyway, I picked up a NTE replacement transistor today and installed it. Now when I max out the regulator I get 6.07V on the regulator board and 4.77V on the PCB, farthest corner from the edge connector. The game works, but it acts a little strange. So I'm going to change the regulation resistors a bit to give me about half a volt more range at the top end. Hopefully that fixes it.

So close....

But it's fully playable. I just put up 65,410 on my second game. After a 2 year layoff. Wow. Not bad, but the game is hard as #$%!@ on the second level.
 
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