I'm trying to grow beyond the paint-by-numbers EE troubleshooting and understand how to dig into this problem. I've been fiddling with it for weeks and decided to ask for help. Please be gentle.
Short Description:
Trying to troubleshoot a ColecoVision console that suddenly lost chroma output (so it outputs B&W). It was working normally, then one day went B&W. I started following the advice in the Technical Manual and in the process my probe slipped, bridged two pins and instead of releasing magic smoke, the chroma signal came back. When the console is powered off, it comes back on in B&W, but bridging those pins again consistently restores the chroma signal. It takes a second or two of being bridged to re-activate the chroma signal, and while it's bridged the image is completely distorted.
What I *think* this means is that some IC or other is not getting a logic high and that it might be a capacitor that's either dead or has gone high ESR somewhere in the chain, but I've already replaced all of the electrolytics with newer higher-quality ones trying to fix ths. What I really need is some help understanding which components in the signal path are suspect. This board is really old and fragile and I don't want to start pulling components at random and testing them.
The pins that I bridged were pins 3 & 4 of the J4 connector.
Full circuit diagrams are available here. I'll link what I think are the most relevant ones below.
The J4 junction connects the mainboard's audio/video circuitry to the RF modulator. To pull a clean composite video signal from the console, I'm tapping it from pin 13 (Chroma Subcarrier Output) of the LM1889 chip using this circuit.
This diagram shows the RF box where the LM1889 chip is and the 8 connections in section B-2 are the J4 connector pins. As you can see J4(4) is connected to +12v and J4(3) is connected to the R-Y (color difference) input on the LM1889 chip. When I just take a simple voltage rating J4(4) measures the expected 12v and J4(3) measures around 6v both before and after I've made the temporary short to restore chroma.
This diagram shows the other side of the J4 junction and this is where I really start to get lost - probably because I haven't quite worked out how to read the diagram completely. The circuit from J4(3) attaches to a HEF4066 switch. Based on my limited interpretation of the data sheet the switch works similar to a transistor but the base is called a "control" and rather than a collector/emmitter moving in a single direction it simply connects two circuits allowing the current to flow in either direction.
FWIW the R-Y signal originates from the TMS9928A shown in this diagram.
If anyone could take a look at this and help me make sense of what's happening and what to check and why, I would greatly appreciate it.
Short Description:
Trying to troubleshoot a ColecoVision console that suddenly lost chroma output (so it outputs B&W). It was working normally, then one day went B&W. I started following the advice in the Technical Manual and in the process my probe slipped, bridged two pins and instead of releasing magic smoke, the chroma signal came back. When the console is powered off, it comes back on in B&W, but bridging those pins again consistently restores the chroma signal. It takes a second or two of being bridged to re-activate the chroma signal, and while it's bridged the image is completely distorted.
What I *think* this means is that some IC or other is not getting a logic high and that it might be a capacitor that's either dead or has gone high ESR somewhere in the chain, but I've already replaced all of the electrolytics with newer higher-quality ones trying to fix ths. What I really need is some help understanding which components in the signal path are suspect. This board is really old and fragile and I don't want to start pulling components at random and testing them.
The pins that I bridged were pins 3 & 4 of the J4 connector.
Full circuit diagrams are available here. I'll link what I think are the most relevant ones below.
The J4 junction connects the mainboard's audio/video circuitry to the RF modulator. To pull a clean composite video signal from the console, I'm tapping it from pin 13 (Chroma Subcarrier Output) of the LM1889 chip using this circuit.
This diagram shows the RF box where the LM1889 chip is and the 8 connections in section B-2 are the J4 connector pins. As you can see J4(4) is connected to +12v and J4(3) is connected to the R-Y (color difference) input on the LM1889 chip. When I just take a simple voltage rating J4(4) measures the expected 12v and J4(3) measures around 6v both before and after I've made the temporary short to restore chroma.
This diagram shows the other side of the J4 junction and this is where I really start to get lost - probably because I haven't quite worked out how to read the diagram completely. The circuit from J4(3) attaches to a HEF4066 switch. Based on my limited interpretation of the data sheet the switch works similar to a transistor but the base is called a "control" and rather than a collector/emmitter moving in a single direction it simply connects two circuits allowing the current to flow in either direction.
FWIW the R-Y signal originates from the TMS9928A shown in this diagram.
If anyone could take a look at this and help me make sense of what's happening and what to check and why, I would greatly appreciate it.
