Second XEGS Repair

[MyPawnsEatPpl]

I picked up a second XEGS while I continue to repair my original and get parts and things. That one had bad RAM and a bad STELLA chip. I accidentally pulled up some copper traces getting the STELLA chip out that I have to fix. Oops.

 

This one has a red screen. The power supply tested fine.

 

Using my old, not very trusty analog multimeter, I get a strange reading of about 12V in this circled area. I'm still a beginner at learning electronics (I just started a class on it) but that isn't normal, is it? What could be the problem here?

 

 

Everything else seems fine. I believe all of the chips are working after testing with my logic probe.

Edited August 23, 2018 by MyPawnsEatPpl

[+Nezgar]

First indicator is red screen is generally bad OS ROM. I've read this is fairly common for the XEGS. Nice thing is a 27C256 EPROM is a drop-in replacement, if you are able to desolder the original chip and install a socket. I would be happy to send you a programmed chip for cost of the EPROM and postage. PM me if interested.

 

There is another recent thread also suspecting the OS ROM, but his machine 'mostly' works:

http://atariage.com/forums/topic/281967-xegs-issues/?p=4094371

 

If that doesn't fix it, the next target would be either the two RAM chips at the front-left of the board, or the MMU.

[MyPawnsEatPpl]

My logic probe is showing pulsing signals on the OS ROM chip so I assume that means it's good.

 

All of the chips besides the two 8-pin chips have some pins with pulsing signals. Doesn't that indicate that they're working?

 

I think the two 8-pin ones handle keyboard input or something, right?

 

What about this 12V signal on my multimeter? Could that be a bad resistor or something?

Edited August 23, 2018 by MyPawnsEatPpl

[_The Doctor__]

think you circled a capacitor... 12 V?

[1050]

GTIA pin 17 will make 12 volt (IIRC) so if that's it

then nothing to really worry about there. Phase 1 clock

is voltage doubler pumped to provide that signal and

it is required. You sure you are not on AC voltage

setting?

 

Doesn't that indicate that they're working?

You are certainly assuming wrong, logic probe tells us

there is activity but not if that is going in or

coming out or what it should be. If you had a dead ROM

I would hope all the support chips would be trying to

talk to it in an effort to wake it up. Dead doesn't

mean all signals enter the twilight zone and cease to

exist, it means instead they are mostly ignored by the

vacant party of concern.

 

These XEGS ROMs are known to lay right down and die.

You can only prove a positive by swapping with a known

good ROM and that doesn't fix it outright.

 

I don't believe STELLA is a chip found here. Please call

it GTIA if you meant GTIA instead.

 

XL/XE schematics are almost identical as to device number.

C42 is circled but I can't find it in XL/XE nor do I

know if XEGS schematic number match XL/XE scheme as tightly

as does XL to XE. Atarimania does not have XEGS in

SAMS which means all we have is Sobola Schematic, is that

correct?

[MyPawnsEatPpl]

You're right. I meant to say the SALLY chip.

 

It was set to DC voltage when I measured.

Edited August 24, 2018 by MyPawnsEatPpl

[MyPawnsEatPpl]

I don't see C42 in the Sobola schematic.

 

I also have a power switch on this thing that doesn't always turn on. You have to rotate it to just the right position and press it in for it to turn on. Let me know if anyone knows how to fix that.

[_The Doctor__]

having a switch like that can introduce spikes and brown out conditions leading to cooking, things run hot when browned out, out static/spikey power is introduced...

[1050]

I don't see C42 in the Sobola schematic.

And that's part of the problem with Sobola schematics

sometimes, they are not accurate.

 

I also have a power switch on this thing that doesn't always turn on. You have to rotate it to just the right position and press it in for it to turn on. Let me know if anyone knows how to fix that.

Contact cleaner such as DeOxit can be used on them

but you may need to push down on the ball socket to

allow the magic fluid to enter the switch contact

area before it will do any good. Tried to locate a

sample board since it's been years since I've fooled

with mine enough to know more about what I speak here.

But alas, it's playing hide and seek with me and winning

by a large margin at the moment. It looks like you are

on your own until my luck changes. I had a flashing idea

cross my mind about tracing C42 better too, but that

also requires a real XEGS in my hot little hands.

[1050]

Further snoopings show C42 to be the bypass cap for

GTIA on XL/XE where Sobola has his numbered C49 for

GTIA on his schematic and is likely in error there.

 

Yet I've seen a picture of the board that clearly

shows that cap circled is C42. Please confirm 12 volts

there and upper or lower end for that measurement.

[shoestring]

My logic probe is showing pulsing signals on the OS ROM chip so I assume that means it's good.

 

Not necessarily, chips fail in so many different ways and the probe will show activity but it won't tell you what the signals should be.

[MyPawnsEatPpl]

Thanks to 1050 for helping me to diagnose the problem in private messages.

 

We traced back that strange 12V reading on C42 to pins 1 and 2 on the Freddie chip. It was sending out insane voltages and that chip appears to be the problem. I just hope that the chip didn't take anything else out with it.