Stern Scramble

[jonhughes]

Picked up a Stern Scramble board a while back and this was next in line from the repair pile. Board was running but all the graphics were well... 'Scrambled'.

Here's what I was getting.

Very mixed up display. After reading a few repair threads I narrowed the fault down to a group of 3 chip types. 74LS138 being one of them. Here they are in the schematics. The logic probe was showing that 3C pins 13 & 14 were stuck high but pin 13 on 3D was also stuck high.

I had a spare 138 chip so piggy backed the chip on 3C and got a strange response. Removed it and tried it on 3D and boom... the garbage disappeared.

Removed the bad chip and inserted a new socket and replaced the 138.

An odd thing happened next. The board would just hang on the boot cross hatch pattern and go no further. Cut a long story short and after a bit of a panic discovered my Supergun Vogatek 2006 has crapped out. I'm now using a MK III which is all I've got left. There are ghosts in my test rig. Still a nice result in the end.

 

[lesoleil70]

Nice one.

 

[RGP]

Nice work John

What does AB mean in the addressing?

Normally A0-A15 is the primary address bus but then I see them become other things like AB or SAD etc

 

[jonhughes]

Sorry, beyond my knowledge.

I'm guessing it's just address bus - here's an extract from Galaga which pretty much does that.jonhughes2015-01-01 22:31:05

 

[ben76]

Nice work

- Ben

 

[guddler]

AB0-15 just means the address lines are buffered. Well, thats the normal convention anyway.

So in all likelihood they go through a 74LS244 (data lines would use the bi-directional 245). Before the 244 they would be unbuffered, therefore A0-15, after the 244 they'd become AB0-15. Obviously not all 16 address bits would be buffered since you don't buffer the upper address selection bits.

 

[Stevros]

What does "stuck high" mean?

Nice work btw.

 

[guddler]

It means the signal is being forced high and cannot go low. Usually because the IC in question has an internal logic gate shorted to +5v. But not always, it may be a chip that the output pin is connected to that has the problem, or a track on the board, or, or, or...

Also need to be careful that you don't jump to the conclusion that your outputs are stuck in any state when (as was mentioned in the other thread earlier) it may simply be that the chip is working perfectly but the code is never reaching a state that would make the pin toggle. They don't always. Some pins are meant to stay high or low and maybe only very occasionally toggle.

 

[Stevros]

It means the signal is being forced high and cannot go low. Usually because the IC in question has an internal logic gate shorted to +5v. But not always, it may be a chip that the output pin is connected to that has the problem, or a track on the board, or, or, or...

Also need to be careful that you don't jump to the conclusion that your outputs are stuck in any state when (as was mentioned in the other thread earlier) it may simply be that the chip is working perfectly but the code is never reaching a state that would make the pin toggle. They don't always. Some pins are meant to stay high or low and maybe only very occasionally toggle.

Thank you

 

[ben76]

What does "stuck high" mean?

Nice work btw.

basically using a logic probe you can sort of get 3 responses. High Low and pulse (you can get combinations too) if an input/output is stuck then its not working right. A simple example would be on a control.... press a button and the signal should go low (ground) if it doesn't (stays high) then you are on the right lines of fixing it usually. Again it could be a lot of things just trying to give a simple answer

- Ben

 

[ben76]

Arse ninjad lol god it takes ages to type on my phone lol

- Ben

 

[Stevros]

Thanks Ben, about time I learnt myself considering the cost of such repairs.

 

[guddler]

You gave a good practical example of a case where it really would be stuck though - so that's good

 

[jonhughes]

Thanks Ben, about time I learnt myself considering the cost of such repairs.

I started tentative repairing about 10 years ago with no clue at all about electronics, other than having an A level in Physics. I do have a Master of Science degree in Structural Engineering and my day job involves significant problem solving, continuously. I just see this another problem solving exercise.

My soldering skills are far better than my logic trouble shooting skills but I'm learning all the time.

You just need to be dedicated and have a fair amount of spare time. I believe whatever you're prepared to put in to learning the more you will get out of it.

Good luck.

 

[RGP]

thanks all, the stuck input is a good example, have fixed a ton of board input issues after getting a tutorial off Luke about 2 years ago.

Is there a list of these conventions for naming signals somewhere?

With not coming from electronics its useful to be able to design something in such a way that others can also understand it. I know you could include a legend etc but if theres an agreed set of names for things isn't it better to use them than reinvent the wheel.

 

[guddler]

Not aware that there is / was a standard at the time we're talking about, no. Couldn't say if there is or isn't now.

 

[virtvic]

This interests me.

 

[Anthony Graham]

A good video explaining some basic logic stuff. I think this is going to turn into a bit of a series, I hope atleast anyway! Some of the pics from the book are a bit hard to read but you get the idea.

http://https://www.youtube.com/watch?v=7l039y8dX5A

Ant

 

[jonhughes]

This interests me.

Me too! I've just discovered where the buffers will be in the schematics without knowing which chip types are the buffers. Said I was learning all the time.

 

[philmurr]

Nicely documented fix.

Scramble is a fantastic game and well worth keeping it running