oogabooga
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I just modified my SRM-323S to work on 117V (it was previously setup for 100V). Thanks go to spritzer, kevin gilmore, and the many others whose posts and diagrams were helpful. Please do NOT treat this as a guide! I may not have fully described all the stops, and I'm NOT ready to take responsibility if you ruin your amp or electrocute yourself. All I'm doing is sharing my experience in case someone finds it interesting.
Removing the bottom panel of the chassis provided some nice testing points. The top two pairs of tested voltages are the AC outputs of the transformer, while the bottom two pairs are the corresponding rectified DC voltages. The photo below shows the test points and the absolute value of the voltages before rewiring: if they didn't match after I rewired the amp, I'd know to something is wrong.
After removing the top panel of the chassis, I could see of the 7 wires coming out of the transformer input, two were cut. There were also two cut wires on the PCB. Looks like my amp originally had all 7 wires in place, then someone cut the 120V wires in two places, leaving four short fragments of wire that needed to be removed. On the transformer side, both cut wires had a dab of glue on them that I was able to easily peel off.
This photo shows the two cut wires (this is after I managed to open the top plastic cover of the transformer... more on that in a moment). The cut wire in the middle goes to "PURPLE" on the PCB, while the one on the far right goes to "BLUE".
That plastic cover was originally closed and coated all over in some sort of thin glue. At first, I wasn't sure how to open it, so I used a flat screwdriver to pry at it and break up the glue seal as much as I could. Eventually, after breaking up glue around the edges, and using a small flathead screwdriver as a 'spudger', I was able to pry the cover off by coming at it from the sides. Since the transformer is screwed into the PCB, I had to be gentle with the force, or hold the transformer so no force gets applied to the board. Once I was able to get it off entirely, I realized that it in the absence of being glued shut, the cover is designed to come off by pushing a flathead screwdriver down a channel in each side (circled in red below) and pushing the tab inwards.
Once off, I could see that none of the windings were cut. For 100V, the green and brown wires are both connected to the incoming AC. Meanwhile, as spritzer has said and KG has drawn up, for 120V, the 'purple' and blue wires are what we want connected to AC. Measuring the resistance between green and yellow (or brown and yellow) gives ~10 Ohms. I measured resistance of ~12 Ohms with between blue and yellow, same for purple and yellow. So, all is good so far!
Completely desoldering the purple and blue wires proved to be impossible for me. No matter how much heat I used, or how much soder-wick, I could not get them to come off. The copper windings form a HUGE heatsink, and I think that was the problem. I also never applied heat for more than five seconds, because I don't know if there's any risk of melting the insulation around the windings. I was able to suck up almost all the solder and 'twist' some of the wire off, and cut the rest. When twisting, it was tricky to not twist and break the copper winding wire, and when cutting, I gripped the free end with pliers so no strands would fall into the amp.
I also desoldered the two wire fragments in the PCB. Then I used two pieces of 22AWG 600V wire (same gauge as the other connections) to connect the middle winding to PURPLE and the right winding to BLUE.I was able to get between 1.5-2 turns of the wire around the post before laying down the solder. Soldering the other end to the appropriate place on the PCB.
With all seven windings connected to the PCB, I could now de/resolder the jumper wires (on the underside of the PCB) from 1,4,6 to 1,3,5. Removing the jumper wires was easy with lots of soder-wick and a small flathead screwdriver to lift each side of the jumper as I heated it. The final product is below - the two new wires are at the yellow arrows.
Testing the resistance across the finished product shows ~12 Ohms!
Connecting the amp to 120V AC and measuring the AC and DC voltages described above gave values within 2V of what I had measured previously! That's good enough for me
A note on the sound "quality":
Before the mod, with the amp connected to a VC-100J stepup transformer, there was no noise/hum when connected to my Concero DAC. The VC-100J connects to power with a two-prong plug: the amp is not grounded.
After the mod, with amp plugged directly into the wall via 3-prong cable, there is an easily heard noise/hum (I can hear it at 3/10). The noise goes down a bit if I connect a wire between the ground post on the back of the amp and one of the RCA grounds.
The VC-100J also has a 120V output: when I plug the amp into that, the VC-100J acts as a 1:1 isolation transformer, and the hum goes away. I have to read more to see if I will ground my RCA cables, shield them, or use the isolation transformer in 1:1 mode.
Removing the bottom panel of the chassis provided some nice testing points. The top two pairs of tested voltages are the AC outputs of the transformer, while the bottom two pairs are the corresponding rectified DC voltages. The photo below shows the test points and the absolute value of the voltages before rewiring: if they didn't match after I rewired the amp, I'd know to something is wrong.
After removing the top panel of the chassis, I could see of the 7 wires coming out of the transformer input, two were cut. There were also two cut wires on the PCB. Looks like my amp originally had all 7 wires in place, then someone cut the 120V wires in two places, leaving four short fragments of wire that needed to be removed. On the transformer side, both cut wires had a dab of glue on them that I was able to easily peel off.
This photo shows the two cut wires (this is after I managed to open the top plastic cover of the transformer... more on that in a moment). The cut wire in the middle goes to "PURPLE" on the PCB, while the one on the far right goes to "BLUE".
That plastic cover was originally closed and coated all over in some sort of thin glue. At first, I wasn't sure how to open it, so I used a flat screwdriver to pry at it and break up the glue seal as much as I could. Eventually, after breaking up glue around the edges, and using a small flathead screwdriver as a 'spudger', I was able to pry the cover off by coming at it from the sides. Since the transformer is screwed into the PCB, I had to be gentle with the force, or hold the transformer so no force gets applied to the board. Once I was able to get it off entirely, I realized that it in the absence of being glued shut, the cover is designed to come off by pushing a flathead screwdriver down a channel in each side (circled in red below) and pushing the tab inwards.
Once off, I could see that none of the windings were cut. For 100V, the green and brown wires are both connected to the incoming AC. Meanwhile, as spritzer has said and KG has drawn up, for 120V, the 'purple' and blue wires are what we want connected to AC. Measuring the resistance between green and yellow (or brown and yellow) gives ~10 Ohms. I measured resistance of ~12 Ohms with between blue and yellow, same for purple and yellow. So, all is good so far!
Completely desoldering the purple and blue wires proved to be impossible for me. No matter how much heat I used, or how much soder-wick, I could not get them to come off. The copper windings form a HUGE heatsink, and I think that was the problem. I also never applied heat for more than five seconds, because I don't know if there's any risk of melting the insulation around the windings. I was able to suck up almost all the solder and 'twist' some of the wire off, and cut the rest. When twisting, it was tricky to not twist and break the copper winding wire, and when cutting, I gripped the free end with pliers so no strands would fall into the amp.
I also desoldered the two wire fragments in the PCB. Then I used two pieces of 22AWG 600V wire (same gauge as the other connections) to connect the middle winding to PURPLE and the right winding to BLUE.I was able to get between 1.5-2 turns of the wire around the post before laying down the solder. Soldering the other end to the appropriate place on the PCB.
With all seven windings connected to the PCB, I could now de/resolder the jumper wires (on the underside of the PCB) from 1,4,6 to 1,3,5. Removing the jumper wires was easy with lots of soder-wick and a small flathead screwdriver to lift each side of the jumper as I heated it. The final product is below - the two new wires are at the yellow arrows.
Testing the resistance across the finished product shows ~12 Ohms!
Connecting the amp to 120V AC and measuring the AC and DC voltages described above gave values within 2V of what I had measured previously! That's good enough for me
A note on the sound "quality":
Before the mod, with the amp connected to a VC-100J stepup transformer, there was no noise/hum when connected to my Concero DAC. The VC-100J connects to power with a two-prong plug: the amp is not grounded.
After the mod, with amp plugged directly into the wall via 3-prong cable, there is an easily heard noise/hum (I can hear it at 3/10). The noise goes down a bit if I connect a wire between the ground post on the back of the amp and one of the RCA grounds.
The VC-100J also has a 120V output: when I plug the amp into that, the VC-100J acts as a 1:1 isolation transformer, and the hum goes away. I have to read more to see if I will ground my RCA cables, shield them, or use the isolation transformer in 1:1 mode.
