Omega race PCB repair #4

omegaman

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I have another set for repair, from Member GoldenAge. This one looks very clean.....but don't let looks deceive you!! :)

I pulled all the chips, all the Ram checked out good, apart from the BBU ram. The Roms and CPU were also fine. I could see by looking inside the sockets quite a few needed replacing. In the end i replaced 6 program Ram sockets, 1 Video Ram socket and 2 Video Rom sockets.
The board was now in a state to test

IMG_0966.jpg


I powered the board up and it reported all Ram were bad :( I checked the program Ram, selector IC at PN2 with the comparator.

IMG_0970.jpg


sure enough it was bad!

Self test then passed the program Ram but failed the video Ram and Rom. I checked the 74245 at S1 (Video buffer) and that was bad as well.

IMG_0968.jpg


I now had the Roms passing but all the video Ram were still showing bad. I checked the 7432 at R1, this is the selector chip for the video Ram. Again it was bad.
 
IMG_0971.jpg


IMG_0972.jpg


I now had all Ram and Rom passing self test.

Now I have the self test running but with no video, all the vector stages are in Halt...the State machine is stopped. I think there is a problem with the 74109 at HJ6 so will take a look at that tonight after work.
 
More progress on this board today. I tracked the problem with the vector generator down to 2 x 74161 in the vector timer. Once I had replaced them the state machine was running but I only had activity on Y axis (Vertical)
I looked at the area post DAC and found no activity on pin 3 of Op amp A8 (TLO81). I pulled the 4016 at A6 and it tested bad.

IMG_0978.jpg


I replaced the op amps at the same time.

I now have activity on both channels.

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This is an image of the test screen, obviously something else is wrong....getting close though.

Man this board has a lot of problems!
 
More bad Chips!

IMG_0981.jpg


All 3 74191 counters in the X section were bad.

Now I get this

IMG_0982.jpg


Another bad 74161 in the timer section was bad at M2

Now the picture looks like this .

IMG_0983.jpg


That's enough for one Eve, more tomorrow.
 
Let me ask you a question-

How are you narrowing down which TTL chips to test out? I mean- is there a functional area on the board that you've determined there is an issue in?

I wouldn't know how to figure something out with all these multi-chip failures.
 
Logic probe, logic analyzer, etc, and a knowledge of what the outputs should look like for a given input.

You should read a TTL primer.
 
One of these days, I'm going to try to figure out what a TTL chip is. LOL.

Could all those chips have been fried, by soembody reversing the voltage going to the board? Maybe plugging the 5 or 12 into the ground?
 
This kind of damage is what happens when a pass transistor in the power supply shorts. You get the full output of the unregulated voltage from the bridge rectifier and the filter cap going straight to the board. ;)

Or, someone wires +12 to the +5v line.

DOH!

That board has a lot of fried chips on it.
 
Like Channelmaniac said, probably a bad power supply. I have the power supply here so I will check it out later. All of the ram were good though, I would have thought they would have died but you will never know for sure.
One thing is sure, it's the worst one yet! :)

How do I find the bad chips? Again like CM said. For example, the X and Y channels are pretty much identical . If you have 4 missing inputs to a chip then either that chip is dragging them down or the one that is sending those signals has failed. A lot of the time you will see activity with a logic probe so you need a comparator to check the logic is correct (or the timing)
The state machine is the hard part because an area may look dead but really it's just waiting for a go signal. For that area I keep a section of good IC's on some anti static foam...I can quickly run through each chip with the comparator if something looks odd.
 
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