Requesting help with Coleco Video/Audio Problem

[Sam Bushman]

Hey guys, so a few weeks ago I purchased an "as-is" ColecoVision off eBay. The graphics were all garbled on the display, so I assumed it was either a power or video RAM problem. Being a very novice electronics tinkerer, I figured I could give a go at repairing the console. After replacing the power switch, verifying the power supply was providing the appropriate voltages, replacing the video RAM with the 5v RAM mod, and repairing some broken traces on the RAM bus I got functioning video! However, I am not getting any audio out of the Coleco. I next wired an RCA cord to Pins 7 and 8 on the audio chip and confirmed it is outputting audio. Suspecting a broken trace somewhere between the audio chip and the RF modulator (due to my experience with the memory bus) I used my multimeter to follow the signal from Pin 7 (audio out) along the various hops on the board. Confirming no break between pin 7 and the last point before entering the RF modulator, I removed the metal box from around the modulator, separated the daughterboard from the modulator, and reconnected the board with wires so I may explore inside it. I have found no broken trace or component between pin 7 and C88. The only symptoms I now have are the staticy audio (and no game audio as far as I know), and the video signal occassionaly gives out. I'm stumped as to what the possible issues could be. I've made a YouTube video of the issue in hopes of getting some interesting leads from those on this forum. That video may be found here:

 

https://www.youtube.com/watch?v=ME-nVOafMpA

 

Thank you for any time and advice provided for this.

Edited November 13, 2015 by Sam Bushman

[SiLic0ne t0aD]

You did an AV mod on this? Grab your sound from C88 if so

[Sam Bushman]

I have done no AV mod. The only modifcations made were to the video RAM and the power switch. I still assume there is an issue with the RF Modulator if audio is not coming out of it, and I'd like to fix it. The video going in and out is also troubling (and all the composite video mods I've seen pull the signal from the modulator).

[nanochess]

There is a pair of potentiometers that could be adjusted slightly. Never tried it though.

[Sam Bushman]

So I'm a dummy. When I disassembled the RF Modulator, I failed to solder the grounding tabs of the top board of the modulator to ground (previously being serviced by the RF modulator metal casing. Once I got the top board grounded, the video became stable. I still am not getting any sound. Does anyone know where I should be looking on the RF Modulator for the sound signal. I assume the signal from Pin 7 should be getting routed to the modulator (and eventually out the RF port) somewhere. My initial following of the leads just piped through a resistor in the RF Modulator area and to C88. Any thoughts on that would be helpful. Thanks for all the feedback so far.

[Sam Bushman]

Looking at the schematics posted on Console5's Tech Wiki, I *think* I've interpreted it correctly and found the pin on the Video Modulation chip (LM1889) I should expect the signal from Pin 7 of the sound chip to get to (in this case, pin 15 of the video modulation chip). I found there was infinite resistance going from the pin on the sounds chip to pin 2 of the RF Modulator's daughterboard. I soldered a wire connecting C88 to C100 to match the schematic (despite not seeing any obvious traces on the motherboard). After Making this connection, I had a connection between the sound chip and the video modulation chip on the appropriate pins. I still wasn't getting audio on the final RF signal though For funzies, I soldered an RCA cord to Pin 2 (Audio signal) and Pin 1 (Ground) of the RF Modulator daughterboard. I again got sound (just like when I soldered the wire directly to the sound chip). Therefore I can conclude the audio is now making it's way through the ColecoVision's circuitry. I am now suspecting the video modulation chip is the problem. At this point, I think I can confidently perform a composite video mod without other potential problems preventing it from working.

[Jinks]

Looking at the schematics posted on Console5's Tech Wiki, I *think* I've interpreted it correctly and found the pin on the Video Modulation chip (LM1889) I should expect the signal from Pin 7 of the sound chip to get to (in this case, pin 15 of the video modulation chip). I found there was infinite resistance going from the pin on the sounds chip to pin 2 of the RF Modulator's daughterboard. I soldered a wire connecting C88 to C100 to match the schematic (despite not seeing any obvious traces on the motherboard). After Making this connection, I had a connection between the sound chip and the video modulation chip on the appropriate pins. I still wasn't getting audio on the final RF signal though For funzies, I soldered an RCA cord to Pin 2 (Audio signal) and Pin 1 (Ground) of the RF Modulator daughterboard. I again got sound (just like when I soldered the wire directly to the sound chip). Therefore I can conclude the audio is now making it's way through the ColecoVision's circuitry. I am now suspecting the video modulation chip is the problem. At this point, I think I can confidently perform a composite video mod without other potential problems preventing it from working.

This is my problem as well. Everything is good but the rf does not work. So doing a av mod will fix it as there is a chip that is probably screwed?

[Kamshaft]

Good job. I installed the +5 volts mod from Console5 too, no delay BIOS, and the AV mod from Ebay (http://www.ebay.com/itm/ColecoVision-A-V-Composite-Video-Mod-Kit-NTSC-PAL-DIY-/181061180246?var=&hash=item2a28165b56:m:m_9WzGb9DsMt7YHk-d5g4GQ)... and mess up one of my Coleco's sadly (after the ram install which was the last step). 100% my fault.

 

I think that I either killed my VDP, or one of the RAM modules is bad.

 

I'll eventually get a pro to look at it.

Edited November 14, 2015 by Kamshaft

[NIAD]

and the AV mod from Ebay (http://www.ebay.com/itm/ColecoVision-A-V-Composite-Video-Mod-Kit-NTSC-PAL-DIY-/181061180246?var=&hash=item2a28165b56:m:m_9WzGb9DsMt7YHk-d5g4GQ)...

I'm not knocking this A/V mod bread-board, but honestly I don't know why anyone would want to go with this over Yurkie's professional quality A/V mod board with cable that he sells for $29 and provides great instructions and I'm sure, excellent support.

Edited November 14, 2015 by NIAD

[SiLic0ne t0aD]

I recommend the Yurkie AV composite mod too, it'll solve all your problems. Sound is pulled from C88 and the other 3 connections are made on the top of the RF module. Very easy to do and looks clean from the outside.

[Jinks]

I recommend the Yurkie AV composite mod too, it'll solve all your problems. Sound is pulled from C88 and the other 3 connections are made on the top of the RF module. Very easy to do and looks clean from the outside.

I just ordered a Yurkie composite mod. Hoping it all works great!

[Sam Bushman]

Well, I tried building a simple composite mod based on one of the variants of the famous BenHeck mod floating around the internet. After installing it, I lost all video picture. After removing the mod, the lack of video remained. I'm *hoping* I've just damaged the video modulation chip and can just replace it. Without that chip working, no composite mod will work. Furthermore, I don't plan to perform any mod that will disable the use of the Expansion Module #1. My next step now is to order another LM1889 IC chip and install it. Hopefully this will get me back to where I was before my screw-up with the homemade Composite mod.

 

What's really hurting me right now is my own lack of knowledge about what to be looking at or testing to identify the source of my woes. The best I can do at this point is half-educated guesswork :/

[Jinks]

I'm not knocking this A/V mod bread-board, but honestly I don't know why anyone would want to go with this over Yurkie's professional quality A/V mod board with cable that he sells for $29 and provides great instructions and I'm sure, excellent support.

Do you know where to find instructions?? I have this kit but not sure where to connect it?

Edit. after 20 min of forum searching found the pdf

[Jinks]

Can the composite mod be installed without the rf daughterboard??