Pac-Man low voltage

cyberbmxer

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Hi everyone. Getting 4.22v on my TTL chips. I was at 1.8v so I'm improving and I at least get video output now. Lots of fried parts that had to be replaced. And now I'm stuck.
Test rig is using switcher and jamma adapter.
Here's what I did.:
First I repaired the edge connector it was burnt badly.
Cap kit on entire board 10Ku caps were not in spec
Replaced both r52 and r53 (r53 was out of spec)
Replaced voltage regulator IR1
Replaced dmv44 with Tip31c it was shorted

I get 5.02v at c3 and c4
Tip31c e=4.22v b=5.02v c=5.02v
D8=5.13v
D9=5.13v
D6=5.11v
D3=11.56v banded side
D4=11.56v banded side
C29=11.59v
C51=5.84v
C10=4.99v
C5=9.33v
C6=4.99v
C7=5.08v
C8=5.08v
D1=.01v
D9=4.23v
Voltage regulator
Pin1=4.23v
Pin2=5.0v
Pin3=9.33v
Pin4=.01v

Not sure why the emitter is only 4.22v ? I have another I could replace it? C5 is curious to me to thought that would be 12v?

Anyone have any ideas? I have been out of video games for 23 years. Trying to refresh my skills.
D3 bad?

Thanks, Don
 

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Hey - assuming you're still working on this: doesn't C3 connect to pin 1 of the voltage regulator?

Which of those measurements is correct, 4.23v or 5.02?

Regulator.png
 
@jeema Yes. I am still working on this. Struggling. Big time. from pin 1 to neg. on c3 is 4.25v
I replaced the ram and got this:

so, ignore the monitor colors it needs to be degaussed.

There is a droop in the characters and mazes and the sound is off.

My test bench is a pac to jamma adapter running off a switcher, no monitor. I have to run down stairs to try board in cabinet.
 

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I'm going to swap out the 96-1 with EPROMs. May be easier to troubleshoot. I don't have pacman character roms so ms pac will do.
 
Based on what you're saying, you're seeing a drop of 5.08v - 4.25v = 0.83v across R53. I see a drop of 0.15v on my board.

Assuming R53 hasn't drifted way high, seems like something must be drawing too much current.

Swapping out the 96-in-1 sounds like a plan. If the 5v rail is still low after that, I would check if anything on the board is getting hot that isn't supposed to be.
 
Yes, I'm trying to figure out where that drop is. I don't have a 100% working board to check against. But, I did measure the ttl chips in the cabinet and they are getting 4.92 volts so all the chips should be firing correctly.
 
If you are feeding a switcher in, it can't be +5V, if you are not bypassing the voltage regulator. The pass transistor will have approximately a 0.7V drop across it. That is normal. So if you want it to be able to provide 5V on the emitter, the collector must be at least 5.7V and really it should be a bit more. Everything is actually working as it should, given the input voltage. There is a reason the schematics list the output of D7/D8 at +7.5VDC. If you are using a 5V switching supply, it has to be feed directly to the VCC rail -- + side of C3, pass transitor emitter, etc -- or bridge R53.
 
Well it's normal that the transistor will have a 0.7v drop from base to emitter just from being 'on'. But from collector to emitter I am only seeing about a 0.15v drop on my board. I am also using DC by the way (plugged into a supergun).
 
The D44VM4 has a Vce(sat) of 0.4V to 0.6V in the current ranges it would operate at on a PacMan board. You are right about the 0.7V Base to Emitter drop. Interestingly enough, OP lists the output of the voltage regulator as 5V. With a base voltage of 5V, the emitter voltage would be around 4.2V. The voltage regulator should be driving the base voltage up, trying to get it's sense input to 5V. The base voltage should be 5.7-5.8V, given the other voltages given. The regulator would be limited by R52 to providing about 115mA of base current, which is good. Otherwise it would burn up the transistor. The resulting emitter voltage should be in the 4.8-4.9V range depending on load.

1739503738722.png

A very simplified representation of the power supply. The voltage regulator is represented by the three diodes -- it has a dropout voltage of 2.0-2.5V, it's trying to drive the base voltage as high as it can -- just shy of 5.7V, in this simulation. Now, OP mentioned replacing a bunch of bad parts. If the board is drawing about 9.1A on the 5V line, then you would get voltages right about what are being reported.

1739504362261.png

It would also be pushing the pass transistor beyond it's continuous ratings and making it quite hot.

With a normal ~7.5V on the collector of the pass transistor, the voltage regulator should be supplying ~5.8V to the base, resulting in a 5V output. That 5.8V won't change based on load, it's the current drawn from the regulator that will change.
1739504864127.png

So, question @cyberbmxer -- how much current is the board drawing on the 5V output of your switcher?
 
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