Shawn Posted March 4, 2015 Share Posted March 4, 2015 Hey, So I did the easier AV mod and the transisitor I used was junk I think. I was getting audio from the bottom of r5\r6, removed R3 and piped the pokey from the C10 cap up by the channel select switch. Hooked it up with the RF box still attached to the board. I've done this exact mod with full success in the past but this time I got nothing so I removed the mod. I assume it was a junk transistor. Went back to RF and now I only have video and no audio at all. Anyone got ideas? I'm really hoping the TIA isn't cooked and I don't really think it could be but alas no audio. I forget what the resistor value was at R3, I replaced it with a 47k one for now, perhaps that is wrong? It didn't make a difference compaired to when there was nothing in R3 anyhow, still no sound. Could something in the RF box got fried maybe? I dunno and I'm rambling. Hope someone else has had this issue or just knows the solution. Please help this 7800 sing once again. Shawn Quote Link to comment Share on other sites More sharing options...
alex_79 Posted March 4, 2015 Share Posted March 4, 2015 According to the schematic on AA, R3 is 1k. I think that a wrong value here (or a missing component) might be the cause for the issue, because that part of the circuit modulates the audio so that it can be mixed with the video in the RF modulator.You might also find useful this wikimedia picture of the 7800 board as reference. It's in high resolution so you can actually see the color bands on each resistor. Quote Link to comment Share on other sites More sharing options...
Shawn Posted March 5, 2015 Author Share Posted March 5, 2015 According to the schematic on AA, R3 is 1k. I think that a wrong value here (or a missing component) might be the cause for the issue, because that part of the circuit modulates the audio so that it can be mixed with the video in the RF modulator. You might also find useful this wikimedia picture of the 7800 board as reference. It's in high resolution so you can actually see the color bands on each resistor. sweet thanks. Quote Link to comment Share on other sites More sharing options...
iesposta Posted March 5, 2015 Share Posted March 5, 2015 You mean it was broken. My 7800 transistor mod was pulled FROM junk, so even though it is all junk it works great! A 90's cordless phone handset has at least 2 of those transistors. It is what I call an evil 7800, so I actually removed the Longhorn mod, soldered the IC back, put back the resistors I removed, then modded it with the easy transistor mod. It works, but doesn't play some homebrews and kills CPUWIZ 's Dev Carts. It didn't play Beef Drop or Scramble even before I started modding it. It is a final run serial number starting with X series. I removed Atari's fix that lets it play 2600 Dark Chambers so it will now play Supercharger, Robot Tank, (just not Dark Chambers). You shouldn't have too much trouble getting it back working somehow. I recommend any mod as I have never got a good RF picture from a 7800 (although others have). Quote Link to comment Share on other sites More sharing options...
Shawn Posted March 5, 2015 Author Share Posted March 5, 2015 After replacing the 47k with a 1.1k I have audio back now Silly resistors! Quote Link to comment Share on other sites More sharing options...
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