DR #17 - Art Audio SLA 1 Amplifier Troubleshooting and Repair - Low Output

hi everyone welcome to duality repair this time I have an art SLA one amplifier to look at this is a really small but powerful unit it's got two channels as you can see channel 1 and channel 2 I have a 500 Hertz sine wave as my input and I have my output hooked up to my 2 8 ohm 50 watt done the resistors as well as the two channels of my oscilloscope and I have both channels set to approximately negative 26 dB so let's go ahead and take a look at the failure mode which is very obvious here we are at the scope I have channel 1 on the top and channel 2 on the bottom and as you can see channel 1 has a much lower output than channel 2 so that is our failure mode before we dive into the unit and remove the cover I'm going to first take a look at the schematic so we can try and identify some key areas to test here's the entire schematic for the amplifier unit it all fits on one page conveniently the bottom section shows the power supply which we're not concerned with the top section is what we're concerned about this shows the amplification section for channel 1 and channel 2 as you can see channel 1 and channel 2 are nearly identical and they're just separated by the black dashed line for clarity each signal will go through a series of operational amplifiers before going through the final amplification stage the 7 transistors and then finally their output so to test and try to identify where our issue is coming in on channel 1 I'm going to test the output of each amplification section of channel 1 and compared to the amplification section of channel 2 since channel 2 is a known good reference it'll give us a good idea of where our issue is coming in for channel 1 for instance I'll start with op amp III C for channel 1 we'll check that output you know compare at op amp you 3 B for channel 2 those look good I'll move on I'll move on to the next operational amplifiers you 3 d & u 3 a and if those are good I move on to op amps u 1 a and u 5 a if those look good I'll know that the issue is coming about in the final amplification stage but the BJT here's the unit with the cover off let's take a quick look around before we start probing for signals we have our input power coming in in the back here it's going to pass through this filter board before coming to the square on the left this board on the left has several functions function one is the power supply as you can tell with the main filter capacitors function 2 is to handle channel ones amplification as you can see with the transistors mounted to the heatsink here and function three is to handle the outputs as you can see with the binding pulse in the back obviously in the middle we have our large toroidal transformer and on the right we have another board with several functions function one is going to be to handle the inputs you can see our XLR inputs over there in the back function 2 is going to handle channel twos amplification circuitry you can see the transistors and the right mounted to the heatsink and it's also got the operational amplifier with the 4 op amps built-in right here which is u3 so that'll be our first test point we have one more board here on the front which handles our volume control and then our power switch on the front and then there is a little small fan back in here you can't see it's got a cover over it no big deal nothing to look at there so that's about it so step one we're gonna probe the signals on you three back here here we are back at the scope let's start by checking channel twos first operational amplifier op amp u 3b and this output is going to be pin seven so there's pin seven for Channel two let's check channel one of the same op amp and this is gonna be pin eight and they look identical so let's rule those first set of op amp circuits out let's move on to the next set this will be the same op amp chip with two different outputs channel twos output will be pin 1 so here's pin one now let's check channel ones which is going to be pin 14 and again they look identical so we can roll out this entire op-amp u3 let's move on to the next set which is going to be u 1 for channel a or channel 1 and u5 for channel 2 let's start with channel twos u 5 pin 1 there's channel 2 and I should know it for reference both channels are set to negative 18 DB for volume so again this is channel 2 op amp u 5 pin 1 let's go to channel ones op amp u 1 pin 1 aha so here's our issue op amp you one for channel 1 is not amplifying correctly so to completely verify this let's check the input to both op amps let's start with you 5 for Channel 2 the input for this should be pin 3 there's pin 3 now let's go ahead and check the input for channel ones op-amp you one that's also going to be pin three ah too quick to suspect the op-amp so the op-amp input for u1 is low so let's go back again again this is channel ones input to op-amp u1 let's go back to channel twos input to on pat op amp u5 so take this as a reference compared to that so channel two just has a larger input signal so the op amp itself may be okay but now we're narrowing it down we've ruled out the first set of operational amplifiers which is that main chip u3 and we know we have an issue somewhere before op amp u1 so there's not much in that circuit that can go wrong so we can start probing and trying to identify what's wrong in that circuit so let's take a quick look at the circuit and what components are in between the good signal at the output of op amp u3 and the bad signal at the input of op amp e1 in series with these op amps we really just have three components we have capacitor C 26 we have capacitors C 16 and resistor R 38 by to guess I would say that one or both of those capacitors has gone bad so I'm just gonna check where I can it's really hard the way these capacitors are mounted on the board there's no space between the bottom of the capacitor and the board to get my probe in there and touch one of the leads so I'm gonna try and find some of these resistors and measure their so let's just start with the one I can see is our 35 so I'm gonna check both sides of that one side should have a signal obviously the other side is going to ground so that won't have much of anything so let's check our 35 all right so the first side I probed is the signal side so this is a good signal this signal looks better than the input to the op-amp again let's go back to that op-amp input for reference that was pin 3 of u1 which is here and back to our 35 just here so we have a good signal at our 35 so that means we ruled out C 26 as being bad so either C 16 is bad or our 38 is bad and again if I would guess it would be C 16 so let's try and find our 38 and I'm gonna measure both sides of that or at least find one side of that so here we go our 38 is there so here's one side here's the other side so I can't tell which one is the input but it doesn't matter they're both the same so we have a good signal at the input of C 16 and we have a bad signal at the output of C 16 which means there's something wrong with C 16 let's take a look at C 16 and that's this cap right here so there's nothing really noticeable about it it's not bulging it doesn't appear to be leaking if you look at on the bottom of the board right here these two on the right nothing noticeable about that as well solder joints look okay so let's just go ahead and remove it and see if we can get anything with the ESR meter all right here's C 16 removed from the unit let's take a closer inspection of it again the top doesn't appear to be bulged in any way bottom shows no signs of leaking or bulging as well let's go ahead and test it with the ESR meter so this is a 50 micro farad excuse me this is a 10 micro farad at 50 volts so we should be between 5 & 6 homes ESR you can see the ESR meter doesn't even register it and just to prove that the ESR meter itself is working go ahead and test a new capacitor that I have this is 10 microfarads at 63 volts so it's a little slightly higher rating and this should again be between 5 & 6 ESR and you can see this one's at 2.6 2.7 ohms ESR so this new one is reading just fine the old one is probably completely open so this thing is dead let's put this new one in and test it out all right I replace the capacitor I reinstalled that left board and I have my same 500 Hertz input I have both channels set to negative 26 dB let's take a look at the outputs and there we go they match identically so that was it one little tiny 10 micro farad 50 volt capacitor that was open that was it let me overlay the signals to make sure that they match identical they look beautiful last thing to do let's hook channel one up to a speaker and make sure the audio sounds good welcome to the final test I have channel 2 still hooked up to the oscilloscope as you can see I have channel 1 now hooked up to a speaker so let's test the output by turning up the volume [Music] sounds fantastic this one's done I appreciate you watching we'll see on the next one you