Coleco Composite LM318N mod trouble

[Dennis Stith]

Hi I just successfully built and installed the LM318N circuit in one of my Coleco sets. The problem I'm having is, I put in Smurfs Rescue in Gargamel's Castle and when I get to the first set of dark stages (purple sky with black overhang and floor), the graphics on the top of the screen stretch and glitch across the TV set.

 

I'm wondering what I need to check in order to fix this. At first, I thought it might have been the RAM since I replaced all 8kb on the board with the +5V mod. I unplugged the composite video cable and plugged the RFU in and the graphics display properly. This means that it has to be the LM318N circuitry itself that is causing the issue.

 

Any help would be appreciated. I am starting to wonder if maybe one of the capacitors has the improper value. The one on the schematic said to use >1uF for the +12V to GND but that is pretty vague for me. I installed a 10uF 25V capacitor there and it is much smaller looking than the one in the photo on the diagram.

 

Also the ceramic capacitor I bought it smaller and of a different shape (it is a small shiney blue one) than the one on the photo I have in the PDF of this. Its value is 27uF and I wired it in properly.

 

Any help would be appreciated. I'm just starting to think that maybe one of these two little capacitors might not be correct because it is probably for filtering noise.

[Dennis Stith]

I just tried modifying this circuit to no avail so I put it back to the way it was. The issue still persists. Now I'm starting to wonder if it is the video chip causing this (well it doesn't if I use RF) or what it could be. I have 2 coleco sets. I guess I could add a circuit into the other one and see if the same thing happens.

[Dennis Stith]

I just skimmed through this article on impedence http://www.bluejeanscable.com/articles/impedance.htm and it looks like it has a bit of information to help me research this trouble and move along. I'm going to check the quality of my video cables and also check the resistance with a multimeter of the circuit. Apparantly by being around electronics for so long, somewhere along the way I learned of this magical (or not really so magical) value of 75 Ohms which this circuit has in it. Basically I have seen the term "75 ohm impedence" thrown around a lot, mostly in coax cables used to connect cable TV or VCRs to the tv set through the antenna input port.

 

I'm thinking that either the cable I'm using isn't capable of carrying the 75 ohms across properly (which is pretty likely since I'm using a white RCA cable rather than a yellow one, though they do look the same), something in the circuit (two 150 ohm resistors in parallel for example, maybe something is wrong with the values of one or both of these in my circuit), or there's some signal noise on the composite cables I'm plugging in. That or there's something altogether wrong with the circuit or the way it amplifies the signal is not NTSC safe for the yellow color any more.

 

If I get this circuit fixed, I'll come back and post my results here in case anyone else can benefit from my discovery in the future for their coleco set.

[RedRaider]

Get you a good, high-quality Digital-Coax Cable with RCA ends, they are "TRUE" 75-ohm cables.