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Banane

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Hey guys (and ladies ?),

this is Christian from Germany. I must say I spend more time on german Arcade forums, unfortunately we have just two types of them:

A) the ones with all the newbies who ask the same studpid MAME/RaspberryPI/HowToInstallAComputer-Questions for over and over again
B) the ones with all the fu***d up old techies from the 80ies who do all this arcade repair stuff for 40 Years now and laugh at you if you ask "too obvious, simple" questions or automatically put you to group A ;)

So then, I´m somewhere in between and I´m here to ask for some help at the relaxed and nice guys.
I must say I don´t have much electronical experience, it´s just the basics,but I am able to read and compare 1 and 1. So with a little kickstart I think I could solve the following problem.

At the moment I'm reconditioning an old Ms. Pac-Man Cab from the US. With the Cab itself and the monitor I'm almost finished (full recap of the chassis, tube swap with swap of the yoke etc.), but now I´m kind of stuck at the PCB.

What I did already to got it running:
  • swapped the 6 2114-RAM-ICs to new ones
  • tested and reflashed the EPROMS
  • built a new ribbon cable for the cpu board
  • repaired the board connector with a new fingerboard
  • changed the opposite edge connector to a new one (cause I thought I had some voltage problems)
  • swapped severals caps and IC in the AC-to-DC-conversion circuit section ( cause I thought I had some AC voltage problems). I measured everything, I can say for sure now that I got 5VAC running
Nevertheless I cannot get the PCB running within the CAB, with AC voltage, so I did a DC conversion (bridged the four diodes and the 10W resistor with crocodile claps) to continue testing on my work bench.

While running the PCB on a regular arcade DC power supply I found the (hopefully final) problem: The board is super voltage senstive.
On a DC PSU I can play with the 5V voltage, I found out that PCB "starts" only at 4,93VDC, the Z80 then runs at 4,6VDC or so.
When I touch any GND point with one leg of my multimeter (well, not all the time but often) the CPU resets.

Something in the whole circuit is causing this voltage sensitivity.

What can I do to narrow this down ? I have an oscilloscope, a logic probe (just the probe, no comparer or so), a soldering and de-soldering station and an EPROM burner to test IC.
To be honest the osci and the logic probe I never really used so far, as I´m a medium-beginner fool, but I understood the basic techiques from youtube videos.

Whats´s the best way to start looking for the voltage sensitivity, for e.g. to test TTL ICs in circuit for voltage problems (not logic problems !) as I'm not interested to desolder 20 ICs to test them externally in my EPROM burner :).

I´m thankful for any hint. ^^

Bests
Banane / Banana
 
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replace the capacitor in the reset circuit and post a board foto showing the area with the power circuits
 
replace the capacitor in the reset circuit
What do you mean by capacitor in the reset circuit? I dont see one around around 7L, 9C etc.
IMG_20200620_130125.jpg


I have it now running with with crocodile clamps on DC/Jamma to measure with the oscilloscope

The continous high on Z80 reset Pin 26 / NOR out Pin13 of 7L, a 74LS02 looks kind of dirty:

IMG_20200620_123814.jpg

Even without the IC there is some kind of sinus waveform:

IMG_20200620_125150.jpg

Could this be the problem?
 
that looks like ripple (ac noise) to me. if you measure 5v anywhere on the board do you have a similar image on the o-scope as your last 2 pictures of the reset line?

if you do that is ac getting into your circuit as the ac filter caps wouldnt be doing their job.


https://quarterarcade.com/pac-man-/-ms-pac-man-repair-log

Board would constantly try to boot (1/23/12) – Looked like the game was booting up, but it would never quite get there. Reset line was going from high to low corresponding to the resetting at booting. Started pressing on a few sockets and it seem to get further on booting, so I replaced a Z-80 daughterboard socket and rom socket, but no real change. Finally noticed that if I left the game on for a couple of minutes it would get closer to booting and sometimes boot up fully and work. Tested voltage on the two large caps and it was 7.0 VDC. I replaced these caps and it the board booted fine.
Lesson learned: always test power supply voltage first.
 
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i need to see the board, to convert galaxians/pacman to dc needs more than jumping some diodes.
 
I've never owned an original Pac Man/Ms Pac Man, but I was always told it didn't like voltages even as high as 5.0v.
Seems most adjust the 5v line down to about 4.8v to get it booting, again I'm told this is normal?
 
original pacman / galaxians boards dont have ajustments, they take AC from a transformer and have a linear regulator and some big caps & diodes near the edge connector - that's why i want to see what's been done.

old B&W atari games are the same - it's before seperate psu's became common.
 
that looks like ripple (ac noise) to me. if you measure 5v anywhere on the board do you have a similar image on the o-scope as your last 2 pictures of the reset line?

if you do that is ac getting into your circuit as the ac filter caps wouldnt be doing their job.


https://quarterarcade.com/pac-man-/-ms-pac-man-repair-log

Board would constantly try to boot (1/23/12) – Looked like the game was booting up, but it would never quite get there. Reset line was going from high to low corresponding to the resetting at booting. Started pressing on a few sockets and it seem to get further on booting, so I replaced a Z-80 daughterboard socket and rom socket, but no real change. Finally noticed that if I left the game on for a couple of minutes it would get closer to booting and sometimes boot up fully and work. Tested voltage on the two large caps and it was 7.0 VDC. I replaced these caps and it the board booted fine.
Lesson learned: always test power supply voltage first.
sad part is when i have ran the pac-man boards i rarely have used the ac section. this was my assumption of the ac section. which the issue likely is the ripple or noise on the 5vdc line that is from converting the 7vac from to 5vdc. meaning the noise is likely the 2 filter caps have gone bad.

when i get these boards i usually run +5dc volts down pin 18. this is actually the pin for running a comparitor or coin multiplier board. however it can be used to power the logic on the pac-man boards. typically havent had any issues doing this.
 
you will burn the pad, you cant run enough current through 1 pad and your probably backfeeding bad caps and the voltage regulator
 
Been working well for 15 years. That board is only 2 amp is why it works
 
haha, thank both of you very much for the reply and the discussion.

I just found out that the reset line is shorted to GND.

The noise I also see when there is no AC power at all ( see http://www.ukvac.com/forum/tut-how-to-dc-mod-a-pacman-pcb_topic343979.html).

first of all I need to find this stupid connection. Then I´ll come back to this (if it´s not working then either).

do you have any hint to do this? I don´t have a multimeter which can measure such low resistances unfortunately.
 
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reset lines usually consist of a capacitor, a diode and a resistor.
check them for shorts or other problems - and make sure the pins on the parts arent flattened against the board
 
So, unfortunately it took a while but I found out that the shortage was a bad soldier joint near the power plug. Well, more dirt, I just cleaned it. Allright, back to the actual problem.

On the Sync circuit board just by probing all the pins I noticed one of the U0 / 74LS139 was HIGH with too less voltage (working condition) but toggeling with the right voltage (not working).

I replaced the 74LS139 and had the same problem as before. Well, I´m an idiot, I´m still learning to measure correctly. The U0 / 74LS139 is not the problem, already the enable PIN of it is not set correctly.

This pin goes straight to the /IORQ (I/O request) of the Z80 CPU. There is no I/O request at the beginning.
I also notice that in working modus INT is LOW at the beginnung just before the program starts there is one HIGH interrupt. In the non working modus it´s toggeling all the time.

okay... hmm. What does that mean ?
 
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Tested with another z80?

Check the rams, are the A- and D- lines moving?
If yes the processor is starting if no, you still have a reset or clock problem.

On old boards those ceramic disk caps <in the clock circuit and on the clock lines> can 'drift out of spec' and cause funny issues.
 
Swapped the Z80, the same. A- and D- lines are moving (in both states).
INT and HALT on the CPU are toggeling. Once I decrease the voltage to 4,84V on the PSU the HALT goes steady HIGH, INT goes LOW in test Mode, HIGH at the end of the Routine and then starts to move again.

What Caps do you mean? I think this was also STJs first answer :).

Page 31, https://www.arcade-museum.com/manuals-videogames/P/Pacman-Mspacman-Troubleshooting-Guide-Part2.pdf
this 100pF C4? (Edit: damn, there is also one "unmarked cap" near by, CP25 ... what the heck is an unmarked cap ?)
 
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What speed cpu. Has to be 4mhz or faster. Cant be the 2 mhz version
 
I put in a nearly new Sharp Z80A, LH0080A. It´s the same as with the old CPU.
I also changed all electrolyte and disk caps on the board now - just to be sure.
 
the biggest problem i have with those boards is the pins and sockets linking the modules.
i usually replace the cpu socket with a quality one, fit the cpu in it and program a set of unencrypted roms to go directly on the board.
 
thanks, before I continue I also replace all the six sockets and the ribbon cable :thumbup:
Ah yes, and the ROMS just to be sure ...

I´ll come back when all this is done
 
roms, or eproms?

eproms last forever, just the data degrades.
i usually erase and then reprogram the eproms on boards i repair.
 
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