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Ianr757

Colecovision Video Issue: Plays Blind

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I have a Colecovision that starts up, stays black for ten seconds, then goes blue. At the blue screen, I can start the game, and I hear audio. So, it seems the console is running properly, based on its start up sequence, but there are only solid colors on screen, one at a time. All components and traces check out. The game, power supply and RF cable also all work. The DRAM ICs seems to be outputting close to optimal ratings. In general, it's a little low, but not too much. Pretty much every chip is: pin 1: -4.5v, pin 8: 11.78v, pin 9: 5v

 

Could this be a VDP/TMS-9928 issue? For reference, I forgot to mention, the Atari 2600 add on works perfectly through the console. But the expansion module doesn't use the TMS, right? So would that explain it? Thanks.

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7 hours ago, Ianr757 said:

IFor reference, I forgot to mention, the Atari 2600 add on works perfectly through the console. But the expansion module doesn't use the TMS, right? So would that explain it? 

Correct, the 2600 expansion module completely bypasses the TMS VDP.  But that means it also bypasses ALL of the ColecoVision video display circuitry (other than the RF module) so it doesn't narrow your problem down to being just the VDP, it could still be the VRAM for example.  Your -5V looks quite low at -4.5V and this could be causing the problem.  Do you have a different power supply you can try?  Try cleaning the power switch, there is a method to overhaul it but spraying some contact cleaner into it could help in the short term.  With the power disconnected, physically move the switch back and forth quite a few times and then test the system again and see what the voltage at the VRAM is.

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11 hours ago, Ikrananka said:

Correct, the 2600 expansion module completely bypasses the TMS VDP.  But that means it also bypasses ALL of the ColecoVision video display circuitry (other than the RF module) so it doesn't narrow your problem down to being just the VDP, it could still be the VRAM for example.  Your -5V looks quite low at -4.5V and this could be causing the problem.  Do you have a different power supply you can try?  Try cleaning the power switch, there is a method to overhaul it but spraying some contact cleaner into it could help in the short term.  With the power disconnected, physically move the switch back and forth quite a few times and then test the system again and see what the voltage at the VRAM is.

Well, the power supply works just fine with my other CV. I can try a spare, but I doubt that's the issue. I will try cleaning out the switch. Thanks for the help. By the way, I swapped in a new VDP, and perhaps unsurprisingly, it didn't help. But at least that's out of the way.

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Cleaned the switch with deoxit, but nothing. I've thought that the dram could be the culprit and maybe doing the 5v mod would work. Is this the next step? Thanks.

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Probably the signal to turn on the screen on the TMS9918A chip isn't responding, and it is receiving data into it background color register. Probably check if the pin's connected.

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1 hour ago, Kiwi said:

Probably the signal to turn on the screen on the TMS9918A chip isn't responding, and it is receiving data into it background color register. Probably check if the pin's connected.

Do you know which pin that would correspond to on the TMS chip? I'm assuming you mean that I should check this pin and its continuity from its trace to other destinations? Thanks.

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It's not a pin, but a command received on the data lines.

 

However, there is a pin which decides whether the received data is to VRAM or a command.  That's pin 13.  See if that has continuity with U2 pin 8, for instance.  That's address line 0, which means that even address writes to VRAM and odd address writes a command.  If you definitely get continuity from the pin itself (not just the solder joint on the board), then since the rest of the system works, this pin must also be getting the proper signals.

 

If that's good, check continuity of VDP 2 -> (all VRAM) 15; VDP 1 -> (all VRAM) 4; VDP 11 -> (all VRAM) 3

 

And if that's good, and since you've tried another VDP chip with the same result, the most likely other explanation is that the memory got fried by its last owner, maybe by an over-voltage from a bad power supply.  If you have a jumper wire, try very briefly connecting +5V to any of VDP pins 25-32 to see if it causes stuff to appear.

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4 hours ago, ChildOfCv said:

It's not a pin, but a command received on the data lines.

 

However, there is a pin which decides whether the received data is to VRAM or a command.  That's pin 13.  See if that has continuity with U2 pin 8, for instance.  That's address line 0, which means that even address writes to VRAM and odd address writes a command.  If you definitely get continuity from the pin itself (not just the solder joint on the board), then since the rest of the system works, this pin must also be getting the proper signals.

 

If that's good, check continuity of VDP 2 -> (all VRAM) 15; VDP 1 -> (all VRAM) 4; VDP 11 -> (all VRAM) 3

 

And if that's good, and since you've tried another VDP chip with the same result, the most likely other explanation is that the memory got fried by its last owner, maybe by an over-voltage from a bad power supply.  If you have a jumper wire, try very briefly connecting +5V to any of VDP pins 25-32 to see if it causes stuff to appear.

Thanks for the help. I have conducted the continuity tests, and there is continuity between all the points you have listed. I do not have a typical jumper wire; could I get by using a normal soldering wire, temporarily soldering one end to the 5v leg of a dram ic and then touch the other end to the VDP?

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I still wouldn't dismiss the power switch.  If you're still just getting -4.5V at the VRAM that could be part of the problem.  While spraying contact cleaner into the switch can sometimes help, the design of the switch is such that it can prevent most of the contact cleaner from getting to the contacts.

 

What voltage do you get at the power switch on the -5V rail?  Both on the input and output of the switch.

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7 minutes ago, Ikrananka said:

I still wouldn't dismiss the power switch.  If you're still just getting -4.5V at the VRAM that could be part of the problem.  While spraying contact cleaner into the switch can sometimes help, the design of the switch is such that it can prevent most of the contact cleaner from getting to the contacts.

 

What voltage do you get at the power switch on the -5V rail?  Both on the input and output of the switch.

Sorry, I am not super familiar with the terminology. By rail, do you mean the leads from the switch that are soldered into the motherboard? Thanks.

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3 minutes ago, Ianr757 said:

Sorry, I am not super familiar with the terminology. By rail, do you mean the leads from the switch that are soldered into the motherboard? Thanks.

Yes, just measure the voltages at those solder points.

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1 minute ago, Ikrananka said:

Yes, just measure the voltages at those solder points.

Okay, thank you. I have a parts Intellivision, would its switch be a suitable temporary replacement to rule out the switch if the rails aren't suspect?

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1 minute ago, Ianr757 said:

Okay, thank you. I have a parts Intellivision, would its switch be a suitable temporary replacement to rule out the switch if the rails aren't suspect?

No, the switches are not compatible.  The CV switch can be dismantled and cleaned and it's not too difficult if you can solder/desolder and are good with your hands.  But, if the voltages around the switch are good then this won't be necessary to solve your problem.  Here's a guide so you can see what's involved.

power_switch.pdf

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1 hour ago, Ikrananka said:

I still wouldn't dismiss the power switch.  If you're still just getting -4.5V at the VRAM that could be part of the problem.  While spraying contact cleaner into the switch can sometimes help, the design of the switch is such that it can prevent most of the contact cleaner from getting to the contacts.

 

What voltage do you get at the power switch on the -5V rail?  Both on the input and output of the switch.

The -5V doesn't go through the switch though.  It's always on.  Only +12V and +5V are switched.

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4 hours ago, Ianr757 said:

Thanks for the help. I have conducted the continuity tests, and there is continuity between all the points you have listed. I do not have a typical jumper wire; could I get by using a normal soldering wire, temporarily soldering one end to the 5v leg of a dram ic and then touch the other end to the VDP?

That would work.  Really, if you have good steady hands, you can just hold the +5V end on the power.  One of those huge coils near the power switch is +5V, so that's an easy target.

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1 hour ago, ChildOfCv said:

The -5V doesn't go through the switch though.  It's always on.  Only +12V and +5V are switched.

Well that's embarrassing.  I should have put my brain in gear before opening my mouth.

 

@Ianr757 Sorry for my mistake.  I'll leave you in the incredibly capable hands of ChildOfCv.  If anyone can help solve your problem, he can.

 

@ChildOfCv Isn't -4.5V right on the edge of the minimum voltage spec for 4116 DRAM of that era?  Might that not be contributing to the problem?  With these chips being so old, and that they might now be more susceptible to out of spec supplies, might some of them not be flaking out (misbehaving) with such a low voltage?

 

 

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1 hour ago, Ikrananka said:

@ChildOfCv Isn't -4.5V right on the edge of the minimum voltage spec for 4116 DRAM of that era?  Might that not be contributing to the problem?  With these chips being so old, and that they might now be more susceptible to out of spec supplies, might some of them not be flaking out (misbehaving) with such a low voltage?

Yeah that seems odd. -4.5 is on the very edge of guaranteed good behavior for the RAM chip.  -5V would be better.

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@ChildOfCv I tried the 5v to the VDP pins, and some colors showed up on screen, but nothing appropriate to what should normally be there. Thoughts? Thanks.

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That seems to indicate that it's the memory chips that are dead.

 

One thing to check:  Unplug power, then measure resistance from the -5V pin to ground.  If there's a shorted capacitor, it could be the reason the -5V rail is so marginal.

 

But the memory upgrade kit sold by console5.com would likely fix your issue either way.

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10 minutes ago, ChildOfCv said:

That seems to indicate that it's the memory chips that are dead.

 

One thing to check:  Unplug power, then measure resistance from the -5V pin to ground.  If there's a shorted capacitor, it could be the reason the -5V rail is so marginal.

 

But the memory upgrade kit sold by console5.com would likely fix your issue either way.

Thanks for the extra suggestion. I ordered the memory kit and a cap kit, so hopefully I will be safe either way. Once I get the new memory installed, I'll update the thread. 

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All right, I'm back after installing the new memory upgrade. It turns out that the old VDP was bad all along too, so I put in a new one. After both replacements, the console is back from the grave and working. Thanks for all of the help guys.

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