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Finally got my XEGS to start, though lacks colour


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#51 ChildOfCv OFFLINE  

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Posted Sat Mar 16, 2019 11:56 AM

The labeling might be different, but that does look like the schematic.  No worries there.  The extra components you mapped are because +5 is connected to everything.  But now that we have all components labeled according to the XEGS, could you take a resistance reading from R72 on the crystal side and ground?  If it's close to zero, you have a short to fix.  If it's closer to 470, then Q6 is dead.  If it's significantly higher than that, then I guess it might be okay, but with the voltage readings you took, it wouldn't make sense.

 

If it turns out you have a short, check C53.  Also look for small objects in the area that don't belong and might be fusing connections.



#52 Macross_VF1 OFFLINE  

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Posted Sat Mar 16, 2019 2:42 PM

Resistance to ground between R72 and Y2 is 37 ohm. Would that qualify as close to zero? I have checked C53 just to be on the safe side and I got 11 nF capacitance on it. I've also checked and double checked for debris and other stuff that don't belong but I just can't find anything.



#53 ChildOfCv OFFLINE  

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Posted Sat Mar 16, 2019 7:12 PM

Yeah, 37 ohm is terrible.  It does clear Q6 of suspicion, but I see no immediate path to ground other than through C53.  Now, C53 should be 100nF, not 11--at least according to the 130XE schematic.  Try getting a resistance reading across just that capacitor to see if the short is there...but actually you'll have to remove it from the circuit to test it, since it's nearly the same test point as the R72-crystal junction.  Well, assuming it's supposed to be using the same basic component values, maybe replace that capacitor no matter what.


Edited by ChildOfCv, Sat Mar 16, 2019 7:28 PM.


#54 Macross_VF1 OFFLINE  

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Posted Sun Mar 17, 2019 4:08 AM

I've checked it ut of circuit but with almost no difference, 36 ohm compared to 37 previously. Replace I suppose? However I don't know what to replace it with. It's marked with A5E 104 ZTX 738.


Edited by Macross_VF1, Sun Mar 17, 2019 4:08 AM.


#55 ChildOfCv OFFLINE  

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Posted Sun Mar 17, 2019 12:12 PM

With capacitors, look for a string of 3 digits.  Two will be significant values, and one will be the power of 10, just like a resistor.  So in this case, 100000pF, which also means 100nF or .1uF.

 

Well okay, 738 also technically fits that description, but the numbers are out of the typical decade values for capacitors, and the exponent is way too high for a ceramic capacitor.

 

As much as I hate to say it, the next logical failure point is the crystal.  Could you disconnect it and see if the short goes away?  And after disconnecting, see if there's conduction between a leg and the case.



#56 Macross_VF1 OFFLINE  

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Posted Sun Mar 17, 2019 3:16 PM

With capacitors, look for a string of 3 digits.  Two will be significant values, and one will be the power of 10, just like a resistor.  So in this case, 100000pF, which also means 100nF or .1uF.

 

Well okay, 738 also technically fits that description, but the numbers are out of the typical decade values for capacitors, and the exponent is way too high for a ceramic capacitor.

 

As much as I hate to say it, the next logical failure point is the crystal.  Could you disconnect it and see if the short goes away?  And after disconnecting, see if there's conduction between a leg and the case.

 

 

I've disconnected the crystal and taken another resistance reading between R72 and where Y2 would connect. At first I got 4.6k ohm but then I remembered that I had not resoldered C53 and with that returned to its rightful place I got 37 ohm again.

Conduction between a leg and the case? Do you mean the metal shield? If so, that's impossible, all this time I've tested the machine without the shield.



 



#57 ChildOfCv OFFLINE  

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Posted Sun Mar 17, 2019 4:25 PM

No, I meant the packaging of the crystal.  Those packages always looked like metal cans, though looking into it some more, perhaps they're surrounded by glass or resin and painted silver.  Anyway, if the crystal isn't shorted, you're okay there.

 

Anyway, the 4.7K ohm reading is much nicer.

 

Well, let's get rid of some pronouns to make sure we're talking about the same thing.

I've checked it ut of circuit but with almost no difference, 36 ohm compared to 37 previously. Replace I suppose? However I don't know what to replace it with. It's marked with A5E 104 ZTX 738.

By "it" did you mean that you pulled the capacitor and measured the resistance of the capacitor at 36 ohms?

 

Or did you mean that you tested the junction at R72/Y2 and still got 36 ohms even after pulling the capacitor out of the circuit?

 

If it's the former, then definitely replace that capacitor!  Burn it with fire, destroy it beyond recognition, eradicate its existence, etc.  Capacitors should NEVER have a DC resistance reading across them, especially not one in the 2-digit ohms.

 

If it's the latter, then there's something weird with the traces in order to get your situation, and it's apparently somewhere near C53's connections.


Edited by ChildOfCv, Sun Mar 17, 2019 4:27 PM.


#58 DrVenkman OFFLINE  

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Posted Sun Mar 17, 2019 4:33 PM

This is a long (and growing) thread and I may have missed it along the way, but have you tried replacing the 4050? I've had one go bad and leave me with black and white video before. They're rather fragile chips all things considered, but modern equivalents are still being made and they're cheap.



#59 Macross_VF1 OFFLINE  

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Posted Sun Mar 17, 2019 4:42 PM

No, I meant the packaging of the crystal.  Those packages always looked like metal cans, though looking into it some more, perhaps they're surrounded by glass or resin and painted silver.  Anyway, if the crystal isn't shorted, you're okay there.

 

Anyway, the 4.7K ohm reading is much nicer.

 

Well, let's get rid of some pronouns to make sure we're talking about the same thing.

By "it" did you mean that you pulled the capacitor and measured the resistance of the capacitor at 36 ohms?

 

Or did you mean that you tested the junction at R72/Y2 and still got 36 ohms even after pulling the capacitor out of the circuit?

 

If it's the former, then definitely replace that capacitor!  Burn it with fire, destroy it beyond recognition, eradicate its existence, etc.  Capacitors should NEVER have a DC resistance reading across them, especially not one in the 2-digit ohms.

 

If it's the latter, then there's something weird with the traces in order to get your situation, and it's apparently somewhere near C53's connections.

 

My particular crystal is mostly covered in some kind of shrinkwrap so it severely limits the possibility of a leg touching the case. Still, I checked just to be sure and the legs do not appear to touch the case.

Anyway, yeah I got 36 ohm when testing the actual capacitor when it is pulled from the circuit. When testing R72/Y2 junction with C53 pulled I get 4.6-4.7k ohm.


Edited by Macross_VF1, Sun Mar 17, 2019 4:43 PM.


#60 Macross_VF1 OFFLINE  

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Posted Sun Mar 17, 2019 4:42 PM

This is a long (and growing) thread and I may have missed it along the way, but have you tried replacing the 4050? I've had one go bad and leave me with black and white video before. They're rather fragile chips all things considered, but modern equivalents are still being made and they're cheap.

No need, it's already been tested and found to be working though it's a good suggestion none the less.


Edited by Macross_VF1, Sun Mar 17, 2019 4:44 PM.


#61 ChildOfCv OFFLINE  

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Posted Sun Mar 17, 2019 4:50 PM

 

My particular crystal is mostly covered in some kind of shrinkwrap so it severely limits the possibility of a leg touching the case. Still, I checked just to be sure and the legs do not appear to touch the case.

Anyway, yeah I got 36 ohm when testing the actual capacitor when it is pulled from the circuit. When testing R72/Y2 junction with C53 pulled I get 4.6-4.7k ohm.

 

Okay, then we've found the source of the short.  Sorry for exposing the crystal to unnecessary heat.  I should have communicated better what to test and report on.



#62 Macross_VF1 OFFLINE  

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Posted Mon Mar 18, 2019 6:24 AM

No problems mate. 

Replacement parts have been ordered and will arrive in a few days. I'll post again when they have been installed and I (hopefully) have the good news.



#63 Macross_VF1 OFFLINE  

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Posted Tue Mar 19, 2019 3:42 PM

Ok, here's the good news. With C53 replaced I get a rather different set of readings at Q6:

Q6 C 1.6V
Q6 B 0.5V
Q6 E 0V

I've also replaced 74LS74 and now I finally get colour!

Just for shits and giggles, here's all the repair timeline.

  • Bought it several years ago and it didn't function at all.
  • Started tinkering some months ago with the capacitors and finally got it to power on, though only to a black screen.
  • Replaced all the electrolytic capacitors. Still black screen.
  • Replaced all the transistors. Still black screen.
  • Started removing the major chips except Pokey to test them in a 600XL. All major chips worked fine.
  • Removed Freddie to test in a 130XE. Finally found something faulty.
  • Borrowed the working Freddie from the 130XE to the XEGS. Got a picture but lacked colour.
  • Ordered a new Freddie while asking you guys for help.
  • One resistor connected to the colour trimpot found to be of the wrong value. Replaced though still no colour.
  • 4050 chip tested in 130XE, worked as intended.
  • Colour issue finally traced to faulty 74LS74.
  • Ceramic capacitor C53 closely connected to transistor Q6 found to be faulty.
  • 74LS74 and C53 replaced and the machine works as intended!

Here's some pictures to celebrate:
 

Mj5efPq.jpg

 

RjaxeXe.jpg

Elm7Wal.jpg
 


Edited by Macross_VF1, Tue Mar 19, 2019 3:42 PM.


#64 Mclaneinc OFFLINE  

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Posted Wed Mar 20, 2019 8:36 AM

Woo Hoooooo, that's great news.......Glad to hear it!!

 

Paul...



#65 Macross_VF1 OFFLINE  

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Posted Wed Mar 20, 2019 1:35 PM

I forgot to add that I've also replaced the RAM chips and the OS chip as well. I kinda wonder what could cause this much damage. I suppose it is likely the original PSU might have died and damaged the system with it, or it might have been connected to a PSU with the wrong specs.

Anyway, thanks everyone who helped bring it back to life. I couldn't have done it without you. Extra kudos to ChildOfCV who just didn't quit even after the colour problem was found.



#66 Nezgar ONLINE  

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Posted Wed Mar 20, 2019 4:03 PM

Congrats! Threads like this amaze me the technical knowledge contributed throughout the thread. Not only did you solve your problem, but we all got to follow along to learn from, and the information is information now here for reference in the future as well. Gives me confidence if I ever encounter a similar issue too.

 

:thumbsup: :thumbsup: :thumbsup: 






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