DR #53 - Yamaha M2 - Minor Service and Testing

hey everyone this is Nick with Duality repair and this is a Yamaha M2 stereo power amplifier this has at least two maybe three or even more issues that I'm aware of number one although it feels to be working physically I can't power this unit off with the power switch so in order to turn this off you have to physically unplug this which is obviously not ideal issue number two although I believe the circuit itself is working you can hear the relay engage and disengage there's no light behind the speaker set B switch and then although I haven't confirmed this the owner tells me that there's a loud scratching or popping noise when he turns this on now that may or may not be related to the power switch we're going to take a look at it either way I do have the schematics so let's go inside I now have access to the power switch and yes there was quite a bit of disassembly to get to this point I think I'm done showing disassembly and reassembly steps unless there's some very specific tricks that I can show and in this case there weren't any all you really need are the two tools shown to get to this point a standard fil up screwdriver and a wrench so let's zoom in on the switch the switch appears to be intact at least externally but I'm not getting that definitive actuation feel that I should get when I'm pressing it in and so I think I'm going to have to go inside to open this up I see two tabs on this side two tabs on this side oh wow well as I was messing around I was able to effectively fix the switch it's actuating properly now you can definitely hear it so now I have a decision of course this should probably work as is but how long until it fails again no idea so I think I'm still going to open it up I want to take a look inside and see if there's anything that's just right on the margin and if there is we'll get that corrected the switch is basically separated into two halves this is the first half we have a spring this clip holds the spring onto this piece this is the piece you're actuating then we have this assembly which contains the body itself it's got a leaf spring on the very bottom and then a more Standard Spring above that all of these pieces here appear to be intact and there appears to be plenty of Grease so I'm going to reassemble this I've reassembled this half of the switch and I've actuated it dozens of times without failure so it's possible that this half of the switch was causing the issue but I haven't been able to verify that and everything appears to be functioning properly so I'm going to move on from this half take a look at the second half here's the electrical half of the switch and everything inside here also appears to be intact and I see plenty of grease inside here as well so I'm going to reassemble the entire switch and actuate it plenty of times if everything looks good there I'll move on it's reassembled and still working physically let's make sure it's working electrically yeah let's move on here's the front panel which houses the lights for the power switch speaker set a switch and speaker set B switch these are lamps not LEDs so it's very likely that the filament opened up which caused this to fail so I'll pull this speaker said B lamp out we'll take a look it's probably difficult to tell but yes speaker said B's lamp filament is open so there's another decision to make these are 12vt lamps I can replace this with another 12volt lamp or I could use LEDs I think I'm going to order some LEDs and some current limiting resistors and I'll see what those look like if they look okay I'll keep those in otherwise I'll think about replacing this with the lamp I decided to go with LED and I'm not just going to replace the bad lamp I'm going to replace all three so that they're uniform I'm testing one out here this is a warm white LED it's dropping about 3.1 volts so I have a 470 Ohm resistor in series my power supply is outputting 12 volts and we're drawing exactly 20 milliamps so that's exactly where we want to be assuming these work I will put the part numbers in the description here's just a quick shot of the lamp board for reference I'm all done installing the LEDs and I think they turned out pretty nice there are a few constraints number one this channel back here has to mostly remain empty due to the mounting plate that will fill that space number two these four tabs that are on each button also have to have some clearance above them because those are going to actually move in and out as you can see on that first button there so that's why I have the LEDs at an angle one potential trick or issue that you can run into is although the old lamps have colorcoded wires I would not pay attention to these because the polarity did not matter so they probably could have put these in any orientation on the board but of course with LEDs polarity does matter so you can see on this first led the black wire is closest to us same with the second but on the third the red is actually closest to us so you do have to pay attention to the polarity if you're installing LEDs all right I'm going to get this mounted and installed and we'll test everything is back together now and I'm ready to test I did double checked the schematic and I realized I made a mistake I made the assumption that the common trace on the board was for the ground pin for each LED but it's actually the reverse the common Trace is for the positive 12 volts so I had to go back and reverse all of those wires but we're ready to test now so I'll power it on and you can see it obviously poers on now so that's working fine let's make sure that both speaker selection switches are working or at least the LEDs are working speaker a is working fine speaker B also working fine and they look great so I think we'll hook this up to the dummy loads and we'll test this out during the testing I'm going to be using the m2's preamplifier the Yamaha C6 shown here as far as I know there's nothing wrong with this unit it was just sent with the M2 there are a few things I want to look at during this testing number one I want to look at this peak output level meter I just want to make sure that both channels meters are going to rise and fall basically in sync with each other so I have a 1 khz sine wave at the input I'll turn up the volume now and I'm just going into two 8 ohm 500 watt dummy resistors so you're not going to hear anything I'm really liking the way that they rise together and we're hitting overload which is clipping so I think the meter looks fine let's go up to the scope take a look at how clean the waveforms are and see just how much power this can produce all right I'll start out at 5 volts per division on the scope turn up the volume left channel is on top right channel is on the bottom and those are some pretty clean looking signals let's make sure they're balanced they are they look perfect let's crank it up a bit 10 volts per division still looking really clean and balanced so let's see how much power this can actually output we'll go to my Scopes Max 20 volts per division we might have to put these in the middle or we're going to run out of space on the screen and I'm not going to hold it there long but we'll drive it all the way to clipping right there I'll back it off a little bit right there and we'll do some math on the scope we were seeing somewhere between 120 and 125 volts Peak to Peak that's the equivalent of somewhere between 230 and 240 watts into an 8 ohm load 240 watts is exactly what the manual specifies this should be able to Output per channel so I would say this is performing exactly as it should I'm very happy with the way this turned out thanks for watching