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Last month, while taking in some of the more obscure non-working effects pedals from our rental inventory, this Boss GT-5 Effects Pedal came along, it having no output. I had no documentation on it, so when I got to this project, I downloaded the Operator’s manual and also searched for a Service Manual. I found one available for $17 to download, and added that website to my notes. I powered it up, noting the Display was working, along with the enunciator LED’s lit. I connected input and monitored the output on my audio analyzer, finding no output, as claimed. I looked in the manual for the Factory Reset code, and applied that, but while that yielded more results as far as the pedals changing functions, I still had no output.
So, I opened it up, finding a large PCB full of surface mount parts throughout on the top PCB. I powered it back up again, after having removed the power xfmr from the bottom panel, and had is set safely to not cause any hazard, I checked for supply voltages. Looking at the layout, having found +/- 25VDC coming out, there had to be some IC regulators here someplace. I finally found what looked like them, and checked for voltages on their terminals. I got nothing…not even the +/- 25VDC. Powered off, checked for continuity, and didn’t find any. I unplugged what I needed from the large PCB, removed the mtg hardware so I could lift the PCB up and check the bottom side to try and follow the supply routing from the power supply at the opposite end. Found the traces that confirmed the regulators, but, no continuity. Not even getting connection before relying on the via’s to make it to the tiny SMD regulators, I added discrete wires over to restore connections, and still had to add small jumpers to get those to the inputs of these regulators.
Powered back up, but still nothing coming out. So, looks like a pair of dead IC Voltage Regulators. I got approval to buy the service manual, downloaded that, and with it now on the computer screen, I looked to see what the regulators were. Panasonic AN78L15M and AN79L15M. I was able to download the AN79L15M, which had the pin-out of the package, while that of the AN78L15M didn’t have that info. And, I found these had long since become obsolete. All I was able to find on line thru distribution were SM versions of 7815’s and 7915’s of various current ratings. I added a few of each to my current parts order, they not being expensive. But, after having ordered them, and looking at the layout, I got the calipers out, then compared the dimensions to that of the Panasonic parts, and found these non-Panasonic parts were both too wide as well as their pinout is not the same as the 7815 & 7915.
After searching on line a bit, I checked ebay, and did find some, which ended up costing me a little under $20…10 pcs of the AN78L15M and 5 pcs of an AN79L12M. -12V regulator….no -15V regulator. I’ll deal with that later.
The parts came in last week, and Monday, I opened the unit up again, got my antistatic tweezers out, put my smallest chisel tip into my soldering iron, and with 15mil solder, tinned the solder pads of the two IC patterns, and carefully soldered them into place. Then, thinking I could put a low voltage zener in series with the GND terminal on the -12V regulator, I could raise that to what I needed….depending on what I had on hand. I found some 1N746A 3.3V 1/2W zeners, body size of a 1N914 signal diode. I looked at the layout & data sheet, finding the input terminal was conveniently on the left side, and there was adequate GROUND foil next to the part, so I was able to cut the terminal away from GND, leaving enough foil to tack-solder the 3.3V zener in place. Scraped off the solder mask at both circuit locations, verified I had isolated the GND pin from Ground, and then bent & tack-soldered the zener into place. Cleaned it up, and checked the results.
NOW I have +15V & -15.2V coming out of the regulators, and appropriate voltage on a few of the SM Op Amps I looked at. Connected test signal to the input, and plugged my analyzer/scope and headphone amp into the output jack, and had a working pedal.
I don’t know how much call we have for a GT-5 Pedal, but……that’s not my problem. Still get paid for the task and the surgery was successful. I was able to get thru this without having to deal with more SM parts problems, which is usually where I draw the line these days.
Roland_Gt-5_Boss_Service_Manual.pdf
AN78L15M_15V_100mA.pdf
AN79L15M_-15V_100mA.pdf
So, I opened it up, finding a large PCB full of surface mount parts throughout on the top PCB. I powered it back up again, after having removed the power xfmr from the bottom panel, and had is set safely to not cause any hazard, I checked for supply voltages. Looking at the layout, having found +/- 25VDC coming out, there had to be some IC regulators here someplace. I finally found what looked like them, and checked for voltages on their terminals. I got nothing…not even the +/- 25VDC. Powered off, checked for continuity, and didn’t find any. I unplugged what I needed from the large PCB, removed the mtg hardware so I could lift the PCB up and check the bottom side to try and follow the supply routing from the power supply at the opposite end. Found the traces that confirmed the regulators, but, no continuity. Not even getting connection before relying on the via’s to make it to the tiny SMD regulators, I added discrete wires over to restore connections, and still had to add small jumpers to get those to the inputs of these regulators.
Powered back up, but still nothing coming out. So, looks like a pair of dead IC Voltage Regulators. I got approval to buy the service manual, downloaded that, and with it now on the computer screen, I looked to see what the regulators were. Panasonic AN78L15M and AN79L15M. I was able to download the AN79L15M, which had the pin-out of the package, while that of the AN78L15M didn’t have that info. And, I found these had long since become obsolete. All I was able to find on line thru distribution were SM versions of 7815’s and 7915’s of various current ratings. I added a few of each to my current parts order, they not being expensive. But, after having ordered them, and looking at the layout, I got the calipers out, then compared the dimensions to that of the Panasonic parts, and found these non-Panasonic parts were both too wide as well as their pinout is not the same as the 7815 & 7915.
After searching on line a bit, I checked ebay, and did find some, which ended up costing me a little under $20…10 pcs of the AN78L15M and 5 pcs of an AN79L12M. -12V regulator….no -15V regulator. I’ll deal with that later.
The parts came in last week, and Monday, I opened the unit up again, got my antistatic tweezers out, put my smallest chisel tip into my soldering iron, and with 15mil solder, tinned the solder pads of the two IC patterns, and carefully soldered them into place. Then, thinking I could put a low voltage zener in series with the GND terminal on the -12V regulator, I could raise that to what I needed….depending on what I had on hand. I found some 1N746A 3.3V 1/2W zeners, body size of a 1N914 signal diode. I looked at the layout & data sheet, finding the input terminal was conveniently on the left side, and there was adequate GROUND foil next to the part, so I was able to cut the terminal away from GND, leaving enough foil to tack-solder the 3.3V zener in place. Scraped off the solder mask at both circuit locations, verified I had isolated the GND pin from Ground, and then bent & tack-soldered the zener into place. Cleaned it up, and checked the results.
NOW I have +15V & -15.2V coming out of the regulators, and appropriate voltage on a few of the SM Op Amps I looked at. Connected test signal to the input, and plugged my analyzer/scope and headphone amp into the output jack, and had a working pedal.
I don’t know how much call we have for a GT-5 Pedal, but……that’s not my problem. Still get paid for the task and the surgery was successful. I was able to get thru this without having to deal with more SM parts problems, which is usually where I draw the line these days.
Roland_Gt-5_Boss_Service_Manual.pdf
AN78L15M_15V_100mA.pdf
AN79L15M_-15V_100mA.pdf