dafa_123 Posted September 17, 2018 Share Posted September 17, 2018 Hello, I have a Colecovision that when I put the power on, I get a continual hissing sound. The image is good, but the sound is just a hiss like when you put a TV on a channel with white noise. I checked the PSU and it is good, same with the RF cable. I cannot see anything wrong visually on the board. The sound is also present even if there is no cartridge inserted. At the moment I am suspecting either a bad RF module, memory module or CPU. I have attached a sound file. You can hear a change in pitch when the blue screen appears for # of players and difficulty. Anyway I can easily test to figure out what is wrong? Thanks. Daniel Recording_1.zip Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 First place I'd start is by checking the audio chip, SN75489 as this is what generates sound. Also, there's some good advice here. The troubleshooting steps I'd suggest are as follows: With the ColecoVision turned on, and with a cartridge that is generating sound/music inserted, open the RF box and using a small plastic screwdriver turn the "Audio subcarrier alignment" a very small amount one way and then back again, as suggested in the topic linked above. With the system powered on check for +5V DC on pin 16 of SN75489. I believe the acceptable range is +4.5 to +5.5 V DC so verify that it's within that range. If the voltage is out of range then try another power supply or clean/refurbish your power switch. If the voltage is good, then verify what the audio is like directly from the SN75489. This can be done by soldering an audio jack to the location highlighted with a red circle in the picture below. Don't forget to also solder a ground wire to the audio jack and the CV mainboard ground plane. If your audio is still a problem then it definitely isn't the RF system but the SN75489 that is at fault. Resolder all of the SN75489 pins to eliminate any dry solder joints. Replace SN75489. 1 Quote Link to comment Share on other sites More sharing options...
Hannacek Posted September 17, 2018 Share Posted September 17, 2018 (edited) I had the exact same problem with a friend's ColecoVision. I couldn't fix the sound, so I bought an AV mod kit on ebay. The sound still didn't work, so I followed Ben Heck's ColecoVision mod instructions and got the sound directly off the sound IC chip. I'm not a fan of modding, but it was really the only way to fix the system so it was usable. If you don't want a modded system, you can just sell it and buy a working regular system. Follow these instructions to get the sound off pin 7 of the sound chip. https://www.benheck.com/bens-colecovision-composite-video-mod/ Edited September 17, 2018 by Hannacek 1 Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 Just to clarify that the location I proposed to take audio from within the RF box is essentially the same as taking it directly from Pin 7 of the SN75489 except that in the former case the audio has been conditioned by resistor R18 and capacitor C100. Quote Link to comment Share on other sites More sharing options...
dafa_123 Posted September 17, 2018 Author Share Posted September 17, 2018 Ok, so I fiddled with the both alignment screws with no luck. I get 4.86V on the audio chip. When I connect the audio directly on the solder point mentioned, I get clear audio. It would appear that the RF module is faulty. Is there a way to repair the RF module? I could also replace the RF module or put in a A/V Mod chip. What do you guys think? Thanks. Daniel Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 (edited) I'm not aware of a known fix for this specific kind of problem with the RF module other than the previously mentioned stroking of the audio subcarrier alignment. Looking at the schematics there is very little of consequence between the audio output of the SN75489 and the TV Modulator (LM1889) chip located on the underside of the RF board (assuming yours is an NTSC ColecoVision) and so my suspicion would fall onto the LM1889 being faulty and would suggest it may need replacement. These are readily available online. Note that removing the RF module to, for example replace the LM1889, is not an easy task. You need to desolder the surrounding RF shield from the motherboard as well as desolder the eight solder points that connect the module to the motherboard. Personally I have never done this and so there may be more involved than this. There are a few electrolytic capacitors around the RF circuit which are old and may be failing but as far as I can tell none of these are associated with just the audio side of the circuit and so doubt these would be the culprit. Nevertheless, if you do remove the RF module then I would recommend changing them anyway. These are C68, C87 and one on the RF board itself. A kit to replace all of the electrolytic capacitors, including those mentioned, can be found here. If you don't care about the RF output and are happy with composite or better then the simpler solution may well be to install an a/v mod board. Edited September 17, 2018 by Ikrananka Quote Link to comment Share on other sites More sharing options...
dafa_123 Posted September 17, 2018 Author Share Posted September 17, 2018 I think I will go with the A/V Mod option. It will be simpler. Thank you for all your help, it was really helpful. You guys know your stuff! Daniel Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 I think I will go with the A/V Mod option. It will be simpler. Thank you for all your help, it was really helpful. You guys know your stuff! Daniel You're welcome, and do let us know how you get on with the a/v mod Quote Link to comment Share on other sites More sharing options...
Hannacek Posted September 17, 2018 Share Posted September 17, 2018 Just to clarify that the location I proposed to take audio from within the RF box is essentially the same as taking it directly from Pin 7 of the SN75489 except that in the former case the audio has been conditioned by resistor R18 and capacitor C100. The instructions that came with the mod kit said to take the audio from that spot in the rf box, and I got nothing. Pin 7 worked great. Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 The instructions that came with the mod kit said to take the audio from that spot in the rf box, and I got nothing. Pin 7 worked great. Broken trace perhaps, or a bad solder joint. Quote Link to comment Share on other sites More sharing options...
chart45 Posted September 17, 2018 Share Posted September 17, 2018 The instructions that came with the mod kit said to take the audio from that spot in the rf box, and I got nothing. Pin 7 worked great. you should try at the c88 cause on the pin 7 you will get low volume with the sgm Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 you should try at the c88 cause on the pin 7 you will get low volume with the sgm Is that due to the C100 capacitor? Quote Link to comment Share on other sites More sharing options...
chart45 Posted September 17, 2018 Share Posted September 17, 2018 (edited) Is that due to the C100 capacitor? i never tried on the c100 sice it's after the c88 where is the jonction of the exp port sound and audio out from the sn chip... but i know for sure the sound will be low with the sgm cause my first coleco had this av mod and i had to move the wire to the c88 to have the correct volume with the sgm Edited September 17, 2018 by chart45 Quote Link to comment Share on other sites More sharing options...
Ikrananka Posted September 17, 2018 Share Posted September 17, 2018 (edited) i never try on the c100 simce it after the c88 where is the jonction of the exp port sound and audio out from the sn chip... but i know for sure le sound will be low with the sgm cause my first coleco had this av mod and i had to move the wire to the c88 to have the correct volume with the sgm Interesting, I guess it must be C100 that causes the problem. At C88 the output from SN75489 still goes through a resistor. So, I wonder is it best to take the audio directly from SN75489 or from C88. I've seen different mods take it from all of these locations - surely there must be a "best" location. Edited September 17, 2018 by Ikrananka Quote Link to comment Share on other sites More sharing options...
Hannacek Posted September 18, 2018 Share Posted September 18, 2018 Broken trace perhaps, or a bad solder joint. There wasn't anything I could find. Quote Link to comment Share on other sites More sharing options...
Hannacek Posted September 18, 2018 Share Posted September 18, 2018 you should try at the c88 cause on the pin 7 you will get low volume with the sgm The volume was on the high side. And my friend doesn't care about SGM, and won't be paying what they charge for it. I only got a SGM because it was less than the retail price. Quote Link to comment Share on other sites More sharing options...
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