Before I bring out the scope, I wanted to check if anyone has had experience with an SD-1 32-voice not getting audio output on the main or headphone outputs. I know the main and headphone outs share some of the same components such as a TDA1541A DAC, but it will take me hours to find the faulty component(s) and would greatly appreciate any feedback for a starting point. I already checked all the voltages from the power supply and the TL072 op amps in the main and headphone circuits are getting +12VDC at VCC+ (pin 8). I did a quick diode check comparison using my multimeter on all of the TL072s and also both TDA1541As. They had identical feedback on the same pins between the two components. I already replaced the battery and did a reset as well as reflowed the solder on all the audio jacks. I have additional work to do such as replacing a tact switch and some other minor issues but don’t want to go any further until I can get this main issue repaired. I really appreciate any help in advance!
UPDATE: I JUST FOUND THE MAIN BOARD SCHEMATICS (below) so I will continue researching but still appreciate any feedback!
LATEST UPDATE: I traced it all the way back to a 74HCT34 logic / buffer IC. Not sure if the 74HC04 will work as a replacement for this chip? IC, 74HC04 hex buffer - Syntaur
After spending many hours going over the schematics, checking signals at numerous ICs, and tracing down the FX1/2 and dry bus paths, I narrowed it down to the 74LS174 flip flop IC, which of course is under a riser PCB that I have to remove before I can replace the 74LS174. This IC also distributes the AUX signal, which currently works through the AUX and headphone outputs. I was able to confirm this by switching the output bus to AUX and then going through the other busses which currently don’t work. This is all on the digital schematic. Luckily, this part is easier to find then the one I mentioned before, which obviously wasn’t the culprit. So, I’m ordering a 74LS174 and will report back once I have swapped it out!
Right after ordering the 74LS174, I noticed a very small deformation under high magnification on a trace near FB12, which connects U29 pin 8 to ESP (U21) input on pin 1. The first photo is after I removed the defective area from the trace and exposed some copper to run a jumper wire as see in the second photo. This corrected the main issue with the outputs and I was able to repair all other minor issues with the SD-1. Lesson learned here (once again) is to inspect EVERY trace in the circuit for even the smallest deformation before assuming there is a failed component. Before I removed and cleaned the problem area, it was only the size of a pen dot and a darker green than the trace around it. I confirmed no continuity with my DMM before digging into the trace to repair it. I’m so glad I didn’t have to go through the trouble of replacing any chips! Whew!
awesome dude! I have something very similar with my asx-xpro. I believe the SD-1 was of the same period, so the link to that schematic will help greatly. I have the expander, so I usually pipe it through there. fyi theres a guy in poland that makes a rom board that holds all the factory roms (4 of them) on one single rom card. I think it fits into the SD-1 if its expandable. On the asr-xpro I have to toggle a small dip switch to change banks, but on the MR-76 it works great.
great that you found the issue, small amount of corrosion it seems. I’ll have to go and look around that area to see if I can see something similar. thx for sharing!
