Colecovision Black Screen Repair Help

[dafa_123]

2 hours ago, nick3092 said:

A friend of mine actually designed that mod many years ago. At the time I don't think he knew that was the best place to pull the audio from. Although back then, I don't think it mattered because it mainly affects the SGM, and it didn't exist back then.

 

I believe the preferred spot now days is to pull audio from the leg of C88 closest to the modulator. As I understand, pulling it from the chip directly causes low volume on anything from the expansion port (module 1 or SGM - but since module 1 doesn't mix with the Coleco sound the way SGM does, it doesn't matter as much for that). Not sure if pulling it from the RF box pin header/connector causes the same issue.

I will try it. I will get the missing components tomorrow and try to built it this week end. I'll keep you posted on my progress.

 

Thanks.

 

Daniel

[ChildOfCv]

3 hours ago, nick3092 said:

I believe the preferred spot now days is to pull audio from the leg of C88 closest to the modulator. As I understand, pulling it from the chip directly causes low volume on anything from the expansion port (module 1 or SGM - but since module 1 doesn't mix with the Coleco sound the way SGM does, it doesn't matter as much for that). Not sure if pulling it from the RF box pin header/connector causes the same issue.

You're correct.  Pin 2 of the RF box should give you exactly what the sound modulator is seeing, but it would also mess with/weaken the RF audio output and may also collect artifacts from the audio modulation circuit.  C88 is on the other side of a capacitor from the RF box, so better protected from that.

 

It may still be better to have an audio buffer between the AV out and C88 though, but nobody seems to bother.

[dafa_123]

10 hours ago, ChildOfCv said:

You're correct.  Pin 2 of the RF box should give you exactly what the sound modulator is seeing, but it would also mess with/weaken the RF audio output and may also collect artifacts from the audio modulation circuit.  C88 is on the other side of a capacitor from the RF box, so better protected from that.

 

It may still be better to have an audio buffer between the AV out and C88 though, but nobody seems to bother.

I may need your guidance once more.

 

I followed the schematic but replaced C3 with a 470uF as you mentioned and put a 10uF at C2. Unfortunately, the store did not have any ceramic 10uF so I got the electrolytic one for now. I don't get an image when connecting to the TV. I took some oscilloscope reading on the input at pin 6 of the RF box and the RCA output. The input signal seems different when the AV Mod is connected. I don't know if it's normal.

 

 

[ChildOfCv]

Looks like you just have a 12V oscillator.  Did you build the circuit yourself, or do you have a local source for one of the pre-built ones?  We may need to look at the wiring.  Also, the datasheet for the 318 says that C2 should be 0.1uF, which should be easier to find as a ceramic.  So I don't know where they came up with the idea that 10 would be an improvement.

 

Make sure the electrolytics have the correct polarity in your circuit.  The + on the 470 needs to face pin 6, the minus on the output side.

[dafa_123]

3 minutes ago, ChildOfCv said:

Looks like you just have a 12V oscillator.  Did you build the circuit yourself, or do you have a local source for one of the pre-built ones?  We may need to look at the wiring.  Also, the datasheet for the 318 says that C2 should be 0.1uF, which should be easier to find as a ceramic.  So I don't know where they came up with the idea that 10 would be an improvement.

 

Make sure the electrolytics have the correct polarity in your circuit.  The + on the 470 needs to face pin 6, the minus on the output side.

Purchased the parts and build it myself. C3 - pin is facing the 75 ohm resistor and the + side is on pin 6. For C2, I have some ceramic 85nF on hand, would that work or should I go back to the store and get 0.1uF?

 

The IC I got from the store is a LM318P

 

I checked the schematic, and everything appears Ok.

[ChildOfCv]

85 might work.  The purpose of C2 is to stabilize the power input, so it should be connected as close to the power pin as possible.  It should not fundamentally change the nature of the circuit, and if you're just trying to spit something out it's optional.

 

But I don't see how you could be feeding back into the input signal unless your output differential is too large.  The only way I can see that happening is if R2 isn't connected but C1 is.  That would turn the op-amp into an integrator, but as it slams each rail it reverses.  Normally an integrator doesn't become an oscillator, but the datasheet mentions that you need some extra components to prevent oscillation with this chip in that configuration.  So it's plausible.

[ChildOfCv]

31 minutes ago, dafa_123 said:

should I go back to the store and get 0.1uF?

In fact, for building the minimum circuit that will "work", leave out C1 and C2.  The circuit will do its basic job without either.  C1 is intended to improve the response so that it outputs a more accurate signal.  C2 is intended to smooth power and clean up issues related to temporary higher power requirements.  But the most basic circuit has only R1, R2, C3, and (R3/R4).  And power and ground, of course.

 

Also, if you soldered this all together rather than beginning with a breadboard, make sure you don't have any solder bridges.

Edited January 18, 2020 by ChildOfCv

[dafa_123]

3 minutes ago, ChildOfCv said:

In fact, for building the minimum circuit that will "work", leave out C1 and C2.  The circuit will do its basic job without either.  C1 is intended to improve the response so that it outputs a more accurate signal.  C2 is intended to smooth power and clean up issues related to temporary higher power requirements.  But the most basic circuit has only R1, R2, C3, and (R3/R4).  And power and ground, of course.

 

Also, if you soldered this all together rather than beginning with a breadboard, make sure you don't have any solder bridges.

Replaced C2 with the 85nF and it works. Super clear and sharp image.

 

For the Audio, should I connect the RCA directly to pin 2 of the RF box or should I put a capacitor or some other component between pin 2 and the RCA connector?

[ChildOfCv]

3 minutes ago, dafa_123 said:

For the Audio, should I connect the RCA directly to pin 2 of the RF box or should I put a capacitor or some other component between pin 2 and the RCA connector?

You should probably actually connect it to the inner side of C88 (relative to the RF box) instead.  As mentioned, C100 (under the RF box) separates the C88 point from pin 2, and pin 2 exposes you to the oscillator that modulates sound for the RF.  C100 also acts as an attenuator, so your sound may not be as strong there.  If you do use pin 2 though, then yeah use a capacitor, like maybe that 10uF that you pulled from your other circuit.  Again, + side towards pin 2

[dafa_123]

2 hours ago, ChildOfCv said:

You should probably actually connect it to the inner side of C88 (relative to the RF box) instead.  As mentioned, C100 (under the RF box) separates the C88 point from pin 2, and pin 2 exposes you to the oscillator that modulates sound for the RF.  C100 also acts as an attenuator, so your sound may not be as strong there.  If you do use pin 2 though, then yeah use a capacitor, like maybe that 10uF that you pulled from your other circuit.  Again, + side towards pin 2

I used pin 2 of the RF box with a 10uF capacitor and it works great. Image and sound are crystal clear. Completed reassembly and cleaned the two very dirty controller that came with the console and voilà! Just like new, well... almost

 

Once again, thanks for helping revive this dead console and saving it from the scrap yard.