Pacman PCB no sync signal

[vsl]

I need little help with pacman recovery. I bought the board from ebay as not working.
All the Eproms were gone. I replaced them with healthy ones that I programmed with the correct code.
When I turned on the board ... there is a signal on the red blue and green, but no synchronization.
I de soldered 2S 2R 3S 3R and 5M, but all 5 chips is OK.
I would appreciate any advice as to where the problem might be?

 

[ArcadePCB]

The copmposite sync output is 5N, pin 11 (manual page 32, left side). First check the continuity of the trace from there to the output. Then check the inputs and the output of 5N. Next check 3N, 3P. 2S is aready checked.
Good luck!

 

[vsl]

I checked with the oscilloscope and where I crossed out there is no signal.

 

[qjuk]

As previously mentioned by ArcadePCB, I’d certainly be checking 3N and then 3P, especially the output on pin 9 on the flip flop at 3N.

 

[Lurch666]

Have you also checked the pins on 2R and 2S that are held high (3,4,5 on 2S and 5,6 on 2R) and the ones held low (6 on 2S and 3,4 on 2R)?

I've had damaged tracks that left those pins floating thus causing the outputs to fail.

 

[vsl]

As previously mentioned by ArcadePCB, I’d certainly be checking 3N and then 3P, especially the output on pin 9 on the flip flop at 3N.

 

[NivagSwerdna]

161s and other things that flip flop often fail.
Just replace 2R

 

[NivagSwerdna]

Another obvious thing to do if you have socketed the counters is to swap 2R and 2S and see if you get the same behaviour.

 

[vsl]

Have you also checked the pins on 2R and 2S that are held high (3,4,5 on 2S and 5,6 on 2R) and the ones held low (6 on 2S and 3,4 on 2R)?

I've had damaged tracks that left those pins floating thus causing the outputs to fail.

2S: 3,4,5 AND 2R 5,6 :


2S 6- LOW
2R 3,4 LOW

161s and other things that flip flop often fail.
Just replace 2R

2R is replaced but problem is still aviable

 

[Lurch666]

If 2S: 3,4,5 AND 2R 5,6 have a signal then it's either differences between an LS161 and LS163 or you have signals where you should have logic high only.

 

[ArcadePCB]

The signals at 2R pins 12 and 11 can't be correct. The lower valued stages (pins 14 and 13) are toggling, but the higher ones don't? Because there are no signals on the higher two stages, the carry output will never become active and so 2S won't be clocked. It's plausible why there are no signals on the outputs of 2S. Are you sure your replacement 2R is working fine? If yes, check if the trace at pin 12 is stuck at high or low. For testing, you can remove pin 12 from the socket and then check the signal on that now not connected pin 12. If pin 12 is toggeling now, you have to look for shorts along the trace that leads to pin 12.
If not, what's the signal at 2R pin 7?

 

[vsl]

After investigation i find shortcut between 9 and 10 pin on 2R under socket. I fixed this.
I have the following problem and I don't know what to believe.
Until now, I have always checked the chips from the 74LS series with my Xgecu T48 programmer and i trusting it,
but I decided to check same ic with the chinese IC tester:

which shows me different results like the healthy ic according to the programmer this thing gives them damaged!
Give me some advice which of the two should I trust ....or should I buy something else that works properly?

 

[ArcadePCB]

I've got the same chinese IC tester you have. I think, its a good tool for a quick check. It also checks roughly switching levels so it can decide if it's a 74HCxx or a 74LSxx device. But for more complex ICs like the 16x counters, it will not even recognize in automatic mode which type it is. You have to select manually the type and then it tests the counters OK.
For being really sure a IC works perfectly or not, you will have to test it manually (check switching levels, output levels with different loads etc.), depending on the chip your testing or just replace it in the circuit with an known good part. Test circuits can be found in the data sheet of the IC you want to test.

 

[vsl]

Thank you
This pacman pcb is not my luck.
I found differences between the schematic which is everywhere on the Internet, and the board which puzzled me:
by schematic : 5M pin 13 --- 2R pin 1--- 2S pin 1--- R 1k--- GND
but on pcb is: 3R pin1,9---2R pin 1--- 2S pin 1--- 2S--pin 3,4,5----3S pin1,5------- R 1k--- GND
by schematic : 5M pin 13 is connect to 2R pin 1--- 2S pin 1--- R 1k--- GND
but on pcb 5M is not connect to R pin 1--- 2S pin 1--- R 1k--- GND
i use this schematic:


where is the true ?
Does anyone have any more reliable schematics of this original Midway pcb?

 

[vsl]

Yes... have a picture :

After removing 4 sockets 2S 2R 3S 3R i discover a "small" shortcut .

 

[ArcadePCB]

I've tried to figure out your measurements. Where is this 1k resistor you're mentioning? Is it R30 (next to 9C in the schematic)? What about GND? This slash with a P next to it isn't a ground symbol, it's +5V. The ground symbol is the three horizontal parallel lines, e.g. at 7L / pin 11. I can't see some differences you mentioning between the schematic and your measurements. All points in the schematic marked with this slash P symbol are connected.
If I've misunderstood what you're meaning, please let me know.

 

[vsl]

Hi, and thanks to everyone who gave me advice on getting the ms-pacman board up and running. While troubleshooting I had put the roms on the regular pacman. After investigation I found 6 shortened paths in ram section 4k-4r . At the moment I have returned newly overwritten roms to ms pacman but there is some problem with the graphics that is making it difficult for me. Any advice if possible?

 

[vsl]

After much investigation i couldn't i find the graphics problem
Even i changed the roms to stock pacman but the graphics still have the same problem.
I guess its somewhere around 5e, i checked all the connections but i couldn't find anything.
I am asking for advice to help me


i checked every chip in this area and everything is work:

 

[ArcadePCB]

It seems to me something goes wrong in addressing the graphics ROM. (And I would highly recommed to cover your EPROM windows.) So I would suggest the problem at a counter responsible for the video signal - e.g. check, if any counters have been replaced - maybe the wrong type was used (74(LS)160..74(LS)163 are almost similar, but you can't interchange them because e.g. one type has an asynchronous reset, the other one has a synchrounous reset, etc.) or the problem may be located on the VRAM adresser board, (replacement board for 5S) etc.
But first, I'd look for the area 2R/2S/3R/3S/8C/5M/2P if all the chips are the correct type. Also have a look that there are LS type chips, not HC. But you can't be sure - I had already chips marked 74LS.. the tester tests 74HC.. This may work, but it may not, too (because of the different levels defined for High and Low). A 74LS.. takes 2.4V input as a High, a 74HCxx may take it as a Low.
Good luck!

 

[qjuk]

You’ve mentioned three different incidents about finding “shortcut and shorted paths”. I assume this is all related when you fitted sockets? If it was me, I’d be checking all your socketed work again to make sure there’s not more shorted pins and/or broken tracks using the schematics and a multimeter checking the continuity between every pin you have worked on. It’s a painstaking task but to me it sounds like that’s where your problem is.