Lately I have been getting into modding older game consoles (I just finished overclocking a model 1 Sega Genesis with a VA7 board), and so now I have turned my attention to repairing the broken Colecovision. The primary issue is that, upon turning it on, there is only a black screen with a constant tone. No BIOS screen is displayed. From what I have read on this site and others, this is a somewhat common problem without a clear culprit. That being said, I have played around with the Colecovision and have drawn the following conclusions:
1. The problem must not be the power supply, since it properly powers the working Colecovision. Also, the Atari 2600 module still works with the broken Colecovision, so it must be getting enough power. I have also tested this with a second power supply.
2. The problem may be the power switch, but I doubt it. Even when I would get the Atari 2600 module to work, it occasionally took a couple of turn-on attempts and some pressure on the power swtich. However, I have never seen the Colecovision BIOS screen appear, and I would assume that if the issue was just poor switch contacts that I would eventually get the BIOS screen to appear.
3. The sound chip works. This I assume because I still get a constant tone when I turn it on, and the tone isn't always the same. I'm guessing it's just outputting whatever random value it starts with. I've wondered if this implicates the CPU or the BIOS since I would assume one of the first things it would do is set the sound chip to output no sound.
4. The video chip probably works. I found that with a cartridge inserted, I could occasionally get random color bars to appear on screen after jiggling the power switch some or rapidly turning the console on and off. However, it's very rare that I manage to achieve this, and I only remember it happening from when I first attempted to repair the console a few years ago.
At this point, I am unsure about how I should proceed, and so I humbly ask for your advice on this issue. Specifically, I want to know the following:
1. Which component is most likely to fail and cause this to happen?
2. If a chip has failed, I know that most of them are (or were) common and could be replaced, but what about the BIOS chip? I'm sure these can only come from other Colecovisions, and I would prefer to repair mine with destroying another. If I do need to replace the BIOS chip, would it be possible to create a custom chip?
3. Could it be a capacitor? I've never encountered a faulty dielectric capicitor, but I've read that their tops puff out when they leak. How visible is that effect?
4. What can I do to diagnose the problem? If need be, I might be able to borrow an oscilloscope.
5. I've seen mention of a Colecovision service manual which gives detailed information on how to test each of the components. Does a .pdf of it exist anywhere?
Any input on this problem would be much appreciated.
Edited by InfiniteSquareWell, Thu Jun 7, 2012 3:48 PM.