Robotron cpu, +12vdc issues

sfluter

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I finally pulled a couple of spare boards out to see if I can get them up and running and ran into issues straight away with the first one. First off, it is missing the 1J2 header and the 5114 ram at 1C, so I am assuming the memory protect interlock and bookkeeping are disabled. Not sure if this has an effect on the problems with this board, so I will continue.

The screen only displays vertical lines, red most visible. It shows 1-3-1 error, and strangest to me is that the +12vdc on all of the 4116's shows as -1.2vdc. This is also true at the header. Basically as soon as I power up it takes out my +12vdc reg. If I unhook 1J1 and turn the power back on I get +12vdc back where it belongs.

I have gone through as many posts as I can to get a starting point, but can't seem to find any on this topic. Am I correct in assuming the only place on the cpu that uses +12vdc is for the 4116's?
 
Sounds like that board needs some serious work. If that is a Rev D MPU board, check the coils by the power input. If not you may have to trace the 12v from the header. You may need to reflow the solder on the power input header.

ken
 
Its a Rev. B., and I did reflow all the solder before I hooked it all up.

Where would be the first place to check? With the board powered on I get the same -1.2 everywhere +12 is supposed to be, header included. This is the same on pin 8 of every 4116 on the board.

I guess what I am asking is what on the CPU can take out +12 vdc?

The power supply is linear and rebuilt and works fine with 2 other Rev. B boards. All other boards are known good.
 
I swapped out all the 4116's and I have my +12 back. I still have a 1-3-1 error, but at least I am back to the rug pattern. My +5 is still a bit low (+4.79 at pin 1). Not fixed, but at a point where there is plenty of material posted on the subject. Time to start reading!
 
Those 4116's can be a pain! When they go out, they like to go out with a bang as well! I was repairing my MPU board and I was getting a 1-3-1 error and all of a sudden I got that "Burnt electronics" smell... I power down everything and the memory chip at 1-3-1 was the problem. I went to pull it and the damn thing burned my fingers - you could've cooked on it! Anyhow, after replacing the memory chip, all was well and it fired right up.

So, if all your solder joints are good on the MPU and the Power Supply, you need to figure out where your +5 is going low. I assume that the +4.79 you measured is off the memory chip. Did you also measure it at the MPU header and at the Power Supply headers to see the output there?
 
The thing to check is at the header where the power first comes in and at the RAM. It may be a little lower at the RAM. That is normal. There are a lot of chips sucking on that +5V rail.

Also watch the rug test, you can get a lot of clues where to look while it is running. If it throws the 1-3-1 on the first pass, that usually means addressing issues. If it is painted in wide stripes (normal looks like 1 line at a time), that usually means that it is one of the buffer chips. If some of the pixels twinkle as the rug pattern is running it may indicate that one of the 7474s is bad or it may mean that the ROM cable needs to be recrimped. Vertical lines through the display usually indicates one or more of the 74153's is bad.

Because of the interconnected nature of all the chips, the above are really only rough guidelines. It takes some loving attention with a logic probe and a logic comparator to narrow down the suspects. And sometimes it is a totally unrelated component that ends up being the culprit.

It also helps to have a working board to compare to.

ken
 
I replaced IC2 on the power board and it resulted in an increase of .10 vdc, so I am now getting 4.89 vdc at the 4116's, which is a drop from the 4.93vdc I am getting at the header. +5vdc at the power supply board is 4.96. Not sure where else I can get a little more on this line.

Nice info on the different rug patterns! I guess all the different versions could make for an interesting gallery. Mine is immediately solid across the screen and stays that way for 10 seconds, then I get a game sound, the screen flashes a solid blue, goes black and then the rug pattern fills in from left to right. This will cycle endlessly, the whole time flashing 1-3-1.

I am ready to start probing the main board using the Video/Recap Troubleshooting guide for Stargate/Robotron, but would like to ask a couple of questions since this is the first time I have used a logic probe. I guess the big one is most flowcharts actually take you to a finishing point. This one takes you to "see recap, circuit schematic". At no point does it say component is bad, yank it. I have gotten as far as testing 1E-37 (high, brief low on reset), same result for 7G and 5H. I basically stopped here and started reading a LOT on different circuits and truth tables. I will attempt to work through the flowchart. The probe won't tell me anything unless I know what I am looking for.

Currently pouring over repair logs. Thanks to all those who take the time to make them.
 
The recap doesn't give any definitive "if a and b then replace chip z" resolutions, but it does help pinpoint bad chips.

Your timeline sounds odd. Normally you get the rug test within a second or two.

If you don't have a paper copy of the schematics, you can get a PDF version from arcarc.xmission.com. I recommend this anyway. I will print a copy of the schematic and parts layout pages and mark them up with highlighters as I trace through the components.

ken
 
That's a good suggestion. My drawing set is going to be dog-eared by the time I get through this.

First starting point according to the flowchart is in the watchdog circuit. Pin 13 on 5H is stuck high when I push reset.
 
When I wrote that the screen was solid I meant that it was a solid rug pattern. And I am pretty sure that it is resetting every 10 seconds or so. If I keep the probe on pin 37 it flashes low and then goes back to high every time the screen goes blue and then back into the rug pattern.
 
That is the watchdog circuit kicking in. Once the hardware starts the CPU must write a specific bit pattern to a specific memory location within a specific period of time. If it does not do this, the watchdog circuit trips the reset line on the CPU and you are back to power up state.

Normally the CPU "pets" the watchdog on a regular basis during the RAM test and when the RAM test is complete, if the RAM test was successful, then the game software kicks in. It is up to the game software to "pet" the watchdog. If there is some issue that prevents the game software from starting, addressing issues, bad ROM(s) or whatever, the watchdog doesn't get the "biscuit" it is expecting and so it pulses the reset line.

ken
 
Thanks for your input Ken, its very helpful. I think I am going to step away from it right now and concentrate on learning how to read the schematics a little better before I try to do any more. I also need a better test setup. In the meantime I threw a working board back into it and enjoyed kicking its ass rather than the other way around!
 
Finally making some progress on this. I tracked down a bad chip in the watchdog circuit (pin 6 on 4G was stuck high) and got to 1-3-1, but it was still resetting every 10 seconds. The rug pattern was in a vertical bar arrangement and I found a post describing this a vertical blind pattern. I tested the chips in the MPU Address Buffer section and found a bad chip at 2D (pin 7 pulsing, but no high) when I replaced it the board fired up, but now I am stuck on the bookkeeping screen and I have a series of thin vertical garbage across the screen.

I am hoping the bookkeeping issue will be cleared up with a new CMOS chip, but not sure where to look to clear up the vertical garbage. I can post a pic of the screen if that helps.

Hope everyone had a good Christmas!
 
Okay, I have hit a roadblock. I was probing on a good board in the Video RAM control circuit and have found a difference on pin 12 of 6E. On the good board it is pulsing, but on the bad board it is stuck high. When I trace it back it leads to notRACNT on the ROM board interface. I googled the crap out of RACNT but couldn't find any mention of it. Worse, there is no mention of it on the ROM schematics other than it goes back to the MPU.
 
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