(Topic ID: 86709)

MPU LED locked on stern 200 board

By Tridentphoto

9 years ago


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  • Viper Stern Electronics, 1981

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#1 9 years ago

So I'm trying all the stuff listed in the mpu repair guide, and I've replaced some transistors and diodes. It's still locked on.
I am reading 2.5V at pin 40 of the U9, not 5V, and when I do the short R1 & R3 to ground, my VR1 diode starts to smoke. I'm not sure where to go next. Any thoughts?

#2 9 years ago
Quoted from Tridentphoto:

So I'm trying all the stuff listed in the mpu repair guide, and I've replaced some transistors and diodes. It's still locked on.
I am reading 2.5V at pin 40 of the U9, not 5V, and when I do the short R1 & R3 to ground, my VR1 diode starts to smoke. I'm not sure where to go next. Any thoughts?

You should change the title of this tread to describe what game and board you are having problems with.

#3 9 years ago

How do you change a thread title? I've often wondered that when people post *fixed* after the fact.

#4 9 years ago

It's a stern 200 MPU. I have an ultimate MPU to replace it, but the board is in such great shape overall, I'd like to fix it.
It's off theviper I have

#5 9 years ago

If you have 2.5v at P40 of the MPU there is a reset issue.

Check TP2 for 12v.

Check Q1. Q5, the zener diode, the two 4148 diodes.

#6 9 years ago

There's 12v, I'm using a test rig, so all that's on it is ground, 5V and 12v.
I pulled all the chips too except u9,6,&11
I replaced all 3 transistors, cr5, Vr1, & c3

#7 9 years ago
Quoted from Tridentphoto:

How do you change a thread title? I've often wondered that when people post *fixed* after the fact.

edit the title of the first post

#8 9 years ago

Just looked at the schematic. Make sure your transistors and blocking diodes are installed in the right direction. I dont see anything that would make VR1 smoke besides a shorted or backward Q1. Maybe a shorted or backwards CR5 would combine two voltages... but if you did that i think you would have high voltage on U9 P40............ Not sure what an open R2 would end up doing... hrmm...rambling

Maybe take a picture. The circuit isnt too complicated. Someone 2nd look might spot something.

#9 9 years ago
Quoted from Tridentphoto:

How do you change a thread title? I've often wondered that when people post *fixed* after the fact.

Edit the original post, then you edit the thread title.

#10 9 years ago

So I got the 5v on pin 40 on u9.
Shorting the left side of resistors R1&R3 to ground doesn't bring pin 40 to 0 though, it only drops to 1v

#11 9 years ago

image-442.jpgimage-442.jpg

#12 9 years ago

So the vr1 diode is rated 8.2v, but there's 11.4 on the one side, is that why when I short the resistors it gets so hot?
Basically is coming straight off the 12V TP2

#13 9 years ago

Sigh, this is frustrating like crazy. I'm getting only 2.3V at pin 40 on U9 now, and then the q5 I replaced is showing 5V on two of the three legs and the 2.3v on the other.
I feel like I'm chasing my tail here because I discovered that one of the PIAs I thought was good wasn't, so then I made sure the chips were good, and there was still problems with the reset circuit I guess, because the led wont flicker or flash.

#14 9 years ago
Quoted from Tridentphoto:

Sigh, this is frustrating like crazy. I'm getting only 2.3V at pin 40 on U9 now, and then the q5 I replaced is showing 5V on two of the three legs and the 2.3v on the other.I feel like I'm chasing my tail here because I discovered that one of the PIAs I thought was good wasn't, so then I made sure the chips were good, and there was still problems with the reset circuit I guess, because the led wont flicker or flash.

Your pic looks fine as long as all the components are the right part number.

A bad PIA in theory could hold the reset low. Maybe a bad IC connected to the reset line is shorted... that would explain VR1 getting hot and low voltage.

Pull all the ICs connected to the reset, which i think i u9, u10, u11. Then check the reset line and see if it is high. Collector of Q5 should be 5v.

#15 9 years ago

I'll give that a whirl. I took a break from it for the past few days, but I'm ready to look again.

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