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INTV System III - Black Screen

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I have an Intellivision INTV System III that doesn't seem to output any video and I'm looking for suggestions of things to try.

 

Here's what I've done so far (part names based on the Console5 Schematic here )

 

  • Replaced power supply capacitors: C1, C2 and C3
  • Replaced main board capacitors: C27, C28, C30 and C40

 

Before replacing the above components, i would get no video output whatsoever. Only static. After replacing the components, I get a clean black screen and static "blip" if I hit reset.

 

Additional things I've tried and tested:

  • Used contact cleaner on cartridge slot
  • Cleaned the contacts of two games (Poker & BlackJack and Space Armada) but get black screen for both
  • Tried inserting cartridges at various depths for possible better contact
  • Tested voltage for all test points based on this diagram. All were correct except for TP-4 which is supposed to read 16V but was actually 25v (no load) and 18V (when connected to main board). TP-6 which is supposed to read 5.65V was 12V (no load) and 5.9V under load from main board. 

 

Are there any ideas on other things to try or is this thing toast?

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You might try testing or replacing the two 2N3906 transistors closest to the CPU. That cured the black screen on my Intellivision (not a System III). Good luck! My repair notes are in this thread:

 

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On 9/26/2021 at 2:37 PM, mthompson said:

You might try testing or replacing the two 2N3906 transistors closest to the CPU. That cured the black screen on my Intellivision (not a System III). Good luck! My repair notes are in this thread:

 

Thanks, mthompson. I just replaced Q1, Q2 and also C26 (bi-polar) for good measure.  Unfortunately, it still has the same black screen output. 

 

Continuity between the ROM cartridge and socket pin/board traces appear to be good. It's probably to the point now where I'll need to locate a known good unit and start swapping out chips.

 

I also have access to an oscilloscope but am no expert with it and am not sure what all can be tested other than the clock signal (which is next on the list).

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When you do check for the clock, make sure to test it at pin 10 on the cartridge slot and not at pin 15 on the color IC. Doing it this way, you know that the clock signal is making it all the way where it needs to. 

 

Another think to do is to try and tweak the variable cap at C2 near the crystal and color IC chip. Best to do this with a way to measure the clock accurately so that you can get it back where it needs to be. But I had one that would only give a black screen and when I tweaked the C2 cap just a tiny bit, it suddenly sprung to life. I'm assuming it was dirty or something so I spritiz'd it with some de-ox-it and alcohol, turned it back and forth about 1/4 turn left to right from where it was initially and then using my Freq counter, I adjusted it back to factory. Seems to have been fine since.

 

I also had one that was doing this and it ended up being the crystal itself so if you can't find the clock at pin 10, then check at pin 15 on the U10 color IC and if that fails, it is likely an issue with the crystal itself although that would be rare but it can happen.

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I was able to do some clock measuring on an oscilloscope and got the following after adjusting C2 (see also attached images):

 

 

Is it normal for the cart slot to have a triangle wave form? 

From the service manual it looks like there's a 5.8K (R4) resistor between the cart pin and clk signal so not sure if that's affecting it. 

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