LUXMAN Preamplifier Repair - The LUXKIT A3300 Preamp

hi everyone and welcome to another episode of Mr Carlson's lab the repair today is a luxman lux kit a3300 stereo preamplifier so the complaint is the headphone jack does not work it went in for a partial recap quite some time ago and then it was discovered that the headphone circuit doesn't work so let's find out what's going on with this thing and fix anything else along the way Deluxe K pramp is in fantastic condition most of the lettering is there minus the volume and yeah basically I would say probably nine out of 10 you know physical condition wise anyways wood grain is looking really nice still you know just the standard dust and stuff on it now to get inside these things is really easy oh by the way this is what the back looks like lots of input jacks on the back and the park connector right here that goes to the external power supply which I'll grab here in a moment and it also has a dropping Transformer because this is a 100 volt device or the power supply is a 100 volt device so it needs a dropping Transformer to take 120 down to 100 getting in these is relatively easy all of the screws are on the bottom so you just remove these you can see they've been removed quite a few times so this does have vacuum tubes in it right so chances are people have been tube rolling that's what it's called basically swapping tubes out and then you just undo the feet here by the way take these out fine threads so anyways all of these here come out and once the last foot is removed it very easily just slides right out of the top so just grab each side and pull straight out and that's it and it allows you to get the wooden case out of the way and we are in very nicely laid out look at that not a nice looking preamp inside did a really nice job with that so let's take a look at the bottom here the camera up just a bit so you can see that just a touch better very nicely done so you can see the partial recapping in here I do see some Originals down in here so that's that so what I'll do is I'll grab a schematic we'll take a look at the schematic check out the headphones circuit on the schematic locate what's going on in here and go from there I managed to locate some schematics for this preamplifier and it looks like they've done a really nice job designing this now mind you there are some errors on this schematic here so if you managed to find this schematic I would look for the newer version which I'll show you here in a moment I'll show you where the errors are at least I I saw two at a quick glance at any rate so this here is the phono input section and then if you were to say run a CD play or something like that you would put it into the auxiliary input bypassing this and it would just go in through these and then out now you'll see some transistors down here for those of you that are vacuum tube purists that want just a vacuum tube preamplifier chain this provides that the only reason that we have these two little transistor amplifiers down here is to provide lower impedance drive for headphones all right so basically what happens is when you stick your headphone jack in here there's a whole bunch of switching that goes on it disconnects the preouts and it feeds the signal into here and then of course you know what this is only one channel here by the way it shows two channels here for the headphone section but it only shows one channel of the actual preamp so this is one tube two triodes in one tube right and then of course you know whenever you have this kind of a dotted line here that means that there's a second half to it so as you can see we have a 12 X7 right and then we have a 12 au7 11 12 au7 so this is 1/2 this is the other half of the 12 au7 and then we have another uh basically another 12 X7 so we have three tubes in this chain of course if there's two in each tube we have six parts right so anyways back to this so there's a lot of switching that goes on in this phone's Jack so this here would be definitely suspect I can't really see it failing on both sides you would get you know headphone output on either one side or the other probably if one of these was you know uh I guess you could say if one of the switches was corroded or something major had happened very rarely would both sides fail but hey it is possible right so we'll check all of this so now supposedly the preamp chain itself does actually work and sound quite nice so that was what that partial recap was for it's just the headphone section that isn't so our Focus really is from this section here right and then out so we're looking at the solid state portion of this preamplifier right here so when the errors is right over in here we can see that uh see we have a switch that'll either go up to this point or go down to this point here so you can see that has a little arrow right there I can zoom in a little closer to this it's a little bit uh not quite in the center of the schematic so so you can see here we have a a switch that'll move up to the top or down to the bottom here so if we were to move this down to the bottom this would be a completely open connection so I imagine this is probably one of the attenuation networks all right so if you were to say click that down this is completely open and nothing would work so that would be a very very good attenuator circuit uh very very good so you can see if we look at this here and then we slide over to the newer schematic you can see they caught the mistake right they connected this point to right here instead of connecting it to right here all right so get this side by side so you can kind of see that all right so you can see they instead of connecting it to here they jumped it over to here where it should actually be so one mistake very quickly seen right there and uh you'll see that we have a 22k resistor here which makes sense because we have a cap right here going across this so you have a nice little filter on this side right here and this side here we have the cap same cap but they forgot to put the resistor between here so a couple of things and you know I haven't really spent a whole lot of time with the schematic so there could be more in this particular one so my suggestion is is uh look for this one here if you're going to be trying to service this you can see that you know they've shown one side here right and then they have the other side down here this is the complete schematic with everything drawn the way it should be all right so definitely a little bit more time put into this so right now our focus is going to be this and that is is this little board right here so I'll zoom into this little board and we'll check it out here's a closer view of this board and you can even see it in the last shot if you look carefully look at these transistors they're blown right in half at that and another one here is completely missing Its Top This resistor here has popped and sprayed out the side it looks like it's actually sputtered the board and then we have another one back here here that's sprayed out its side and then looks like it's you know sprayed all over this resistor over here as well now the supply for this board according to the schematic here you can see this schematic unfortunately this is the best resolution of a schematic that I could actually find this one and the other one they're all like this so but um it's enough to read at any rate so we have an 8vt Supply Here coming off the power connector on the preamp and it runs down here and that's the power for both the preamps here right right in here so if we look on the back of the unit itself here's a power connector and as you can see we have an 8vt Supply that's coming in so that 8vt Supply is for this board right here and let me tell you if anything was to go wrong on this board we wouldn't get this type of an action you're probably saying what this is a very very fast event that caused this to happen so basically uh this board got hit with very high voltage and this immediately went pop and then did this right here and same with this over here and of course it cracked these transistors right in half and and blew the top right off of this one here completely so what that tells me is in the past somehow some supplies got shorted either in here or in the power supply itself this board got fed uh higher voltage than it was you know supposed to be supplied with and this happened normally with an 8vt Supply on this if something was to burn up on this board would have been a much much slower event so you get Browning of the body and then usually what it does is it burns the board underneath so you get this big black thick charm Mark the way this has happened and sprayed out like this basically like sputtering almost is an extremely fast event so basically this thing just went poof like that fast and uh the same with this one over here so some supplies would have had to have been shorted to create it you know to create an issue like this and here's a really good example of why when somebody tells you a headphone circuit or a power amplifier circuit doesn't work you don't plug your headphones into it now mind you there are blocking caps these caps here are designed to keep DC off the headset right so basically designed to pass AC right but to keep DC off of them and you can see these have been replaced so maybe somebody tried to feed this board with a higher voltage in order to get a little bit more drive out of the headphone section here maybe to drive headphones just a little bit louder and uh maybe it took out the the blocking caps here so they replac those only because you can see these are all original everything else on this board is original aside from the headphone blocking caps so just uh a little word of precaution there if somebody does ever bring something in to get repaired or something like that and they you know say something is wrong say you know well one of my speakers isn't working on an amplifier on a power amp or say a headphone isn't working on this never plug a good pair of headphones in uh if you have a pair of sacrificial headphones that's okay but uh you definitely don't want to be taking a chance just because of things like this you know so but uh I imagine there's probably not a whole lot going on on this thing anymore imagining we're probably going to have a lot of dead transistors so we'll see if I'm right so let's try and get this board out of here it's been out before obviously because those caps here have been replaced right so these four screws look like they're attached to the standoffs in here so that's probably all it is and I just need to desolder some wires now before you desolder anything it's always a good idea just to draw a square on a piece of paper draw the little tabs where they're located and then uh give yourself some wires coming in and then draw the color coding for the wires it's also okay to snap a picture with your phone or with a camera but in a lot of cases uh if some wires are on top of one another sometimes things will get confusing at some connections especially if there's many wires so never rely on a camera alone always for a secondary precaution draw the board and just draw lead off wires you know black BLK whht you know and then another you know little tab here with yel for yellow and then you know you can see here we have an orange wire coming in over to this terminal and we have a green wire going to here another white wire and just do that we have our inputs here so you can see that the signal lead is here this is Center conductor and then this would be the outer shield and then again this is the center conductor here and outer Shield so we have green and pink over here and that's the safest way to do things and uh that way if uh you need something to fall back on you have uh two sources to fall back on for for [Music] is [Music] all the transistors on the board have been removed and the two VAR I've left in so we can actually uh test those just right in circuit with the Curve Trace right here so let's test the remaining transistors and see what happens here so I'll wake this back up again so let's test this one here first and see what it's doing so that transistor is leaking from base to emitter and base to collector and it's a dead short from emitter to collector very bad transistor so let's test this one here that shorted from base to collector and base to emit and emitter to collector that's toast as well let's check this one here leaky leaky shorted well it's actually not dead shorted this one here is actually slightly resistive so it's kind of a resistor this one so leaky and a resistor between the emitter and collector so from base to emitter and base to collector it's completely open and from emitter to collector it's dead shorted and this one here from base to collector is shorted base to emitter is shorted emitter to collector is shorted that one is completely toast so all the transistors on this board are completely toast so that further goes to tell you that yes this was most likely a uh some form of uh voltage Bridge somebody bridged something in the power supply so let's test the varistors here here okay that is normal so that ver is looking okay and this one here yeah no problems that's because I've got the leads here reversed either way you can test it so there we go no problems so both fers are fine so no these are actually denoted like diodes on the schematic right here you can see this here so it's those two part parts right there those are still fine but every semiconductor device on this board is toast so yeah this thing uh unfortunately I think is the result of a bunch of pins maybe being bridged in the power suppli or in the power connector I've noticed in the actual on power supply the negative Supply and the 8 volt Supply are right next to each other so you can see right here so the negative Supply are those two pins and then that pin is right next to it so if you were to accidentally test and slip with your test lead uh you would give that oh 80 volts or something like that I think is the voltage on the back of the uh back of the supply this have the voltage written on it this one here does yeah so 75 volts or something like that that looks like what has happened to me with this thing so I think this thing got hit so anyways now what I need to do is do some research and find a newer transistor that I can just substitute all of these old transistors with something that'll work just fine you maybe require just a slight bias adjustment if anything like that and um we'll get all the transistors put in and Power It Up on the bench make sure everything works before we re install it in the unit and of course we're going to test the supply voltage to this just to make sure that there isn't something wrong with the supply itself and it's gone high but I very highly doubt that okay I'll get some transistors Happening Here it looks as though the BC 327 and BC 337 transistors would make very nice replacements for the original faulty components right here now one of the differences with this newer part is the pan oute is a bit different than the 2s part so on the 2s part which are the original transistors here this outer lead is the base lead whereas on this newer BC part the center lead is the Bas lead so I just need to move the legs around which means the transistor will sit a little sideways on the circuit board but I think this is a very nice replacement for these ones here I guess we'll find out when I put them in the board right so hopefully I'm not going to end up eating my words here so I'll test one of these new transistors here and I'll show you what it looks like on the curve Tracer so this here is an npn part so put this down here so from the base to The Collector we should have a nice knee there it is all right and then from base to emitter we should have a nice knee that looks just the same and then from collector to emitter we should have an open connection so nothing should change just like that and that's what a new transistor looks like on the curve Tracer so this is also a very low Distortion signal source as well so this can be used as basically an audio signal generator for testing audio amplifiers as well which you'll see when we test this board right here and Vary the output amplitude by just turning this up and down it's just that easy so this is one of the pieces of test equipment that I've designed and released on patreon this is actually a $ oscilloscope from uh a radio swap meet that I picked up and I just put the little boards I designed in inside this thing so you can almost turn any of these old oscilloscopes into a very nice curv Tracer an extremely useful piece of test equipment for the bench so if you're interested in that check that out that's on my patreon site in my electronics course so let's see if this comes off with just a little bit of lacker thinner not too bad again that's what tells me that this was a very fast event it didn't actually burn a big chunk of the PC board out it was just a very quick little I guess you could say uh event that happened here that caused this if I work away at this a little bit more with another Q-tip I could probably clean that up even more so I'll do the same over here as well and I'll get rid of the capacitors these ones here I'll replace and uh we should be ready for testing all the transistors have been replaced on the board as well as all the electrolytics I've even replaced these two here just because we don't know the history of the board so these things are the only two things that block the DC from getting into your headphones so nice to be safe and I've also upped the voltage of these two capacitors as well to be even safer so that's all done I replaced the two resistors on the board on the bottom side oh you can also see the transistors have a bit of a Twist to them right because the leads the lead out of these particular transistors is just a little bit different than the 2s Series so looks pretty good you know unless you were to really look at it you really wouldn't notice and on the bottom side I cleaned it up to try and make it look just a little bit more Factory looked pretty bad there with the um all that flux on it so there it is there's the board so what I'm going to do is get this hooked up to a power supply feed it a signal and uh let's see if anything comes out of it the headphone amplifier SL driver board is ready to be tested so mimicking the headset I have two 33 ohm resistors attached to the outputs so 133 Ohm resistor for the right side 133 ohm load resistor for the left side the scope probes are attached across each output so I have right and left displayed here on the scope both the inputs of the board are tied together and this signal Source right here is feeding the inputs so I have a nice low Distortion Drive signal coming in here there's an 8vt Supply running to the board from a regulated Supply now keep in mind the supply that uh operates this board here in the factory application is not regulated so it could be anywhere from say 7.5 to 8.5 volts somewhere in that range so right now this is getting a regulated 8vt Supply so what I'll do is I'll turn on that power supply and we have output on both channels here so it I'll do is I'll turn up the drive signal and there's a fair amount of drive there but it's starting to clip and I would say it's clean to about 08 of a volt so let's grab a calculator here and we'll go uh here we'll go8 time8 divided by the resistors here which are 33 ohms equals 19 m is what it's putting out so this should be making at least 50 m so it is a little bit shy on the power so that means I'm going to have to take out that beautiful resistor that I put in here I hunted a long time to match a resistor that looks like the original ones here so anyways this 82 Ohm resistor here is going to have to go so you'll notice most of these designs you'll see a transistor here and there is no resistor in the emitter lead dep positive at all so they've put this in here obviously to limit the drive of this amplifier here maybe with these transistors here it makes power a little easier probably does all right these transistors that I've replaced in here uh handle a little bit more current all right so chances are things are just a little bit different so I'll put another resistor in here say only 10 ohms so just in case something goes wrong in here that 10 ohm resistor will go away technically I could completely eliminate it and I'll take this value here on the collector of the first transistor down half so I'll say uh we'll say 2.2k I'll put in there all right and I think that should give us quite a bit more Drive hopefully it'll get us over the uh 50 m Mark so that's what's going on here so I got a my nice soldering job here I got to pull the resistors out again so it would be relatively loud you know would be okay at 20 Ms but um you know you want a little bit of extra Reserve power for you know base and things like that and the newer headphones aren't that incredibly efficient either so it's always nice to have just a little bit of extra Drive power so anyways I'll get those resistors replaced and we will retest this we have a little Powerhouse of an amplifier now so I'll turn on the power supply and if you recall around 08 of a volt is where where we started to run into Distortion before so that's right around 08 of a volt nice and clean right there right no problems so watch this I have to bump the scale up because we're going to greatly exceed that scale so look at that one volt no problems 1.5 volts still nice and clean 1.8 volts still looking good and it starts to Flat Top about 1.9t five something like that so we could say 1.9 this nice and clean right there if we even wanted to have a little more safe Zone we could take that right down to say 1.85 and now we're definitely clean we know that for sure all right so grab our calculator here and go 1.85 times 1.85 / 33 ohms look at that over a 100 m per Channel remember this board is only rated for 50 Ms right so that is doing very good that's both channels fully loaded independently each Channel 103 M and if I move the trace on the screen here all right you can see I'll move this Trace up see how it basically disguises itself and behind the other Trace so both channels the amplitude they track very well so I'll move this down and see that no problems I go down one scale on this one you see that shows up again and then down one scale on the other one this one here was uh went one too far I go down you can see that they track extremely well so they're hiding under one another so very well balanced no problems spending time matching those transistors definitely paid off in this case before I go about reinstalling the freshly rebuilt little circuit board here I want to make sure that the preamp is going to play nice with that circuit board so for example we really don't know if there's any elevated voltages on any of the leads that attach to that board right that may have caused the problem in the first first place right so we want to make sure that everything is going to be okay in this unit that would be a whole lot of work right down the drain if say there was B+ you know like the vacuum tube B+ on the actual leads that Supply the board or say maybe there's a short and it's feeding B+ into the input or outputs or something right so we need to make sure everything is okay and that all the connections are working first so just installing this thing back in and reconnecting the wires and turning the switch on would be a very large risk all right so first thing I want to do is plug in a headphone jack and once it's plugged in I'll unscrew the barrel and we'll make sure that some connections from the wires on the output of this board go up to the headphone jack first all right so I'll just plug that in right now nice positive connection there unscrew the barrel make sure this is facing me now remember how before I said you need to make a legend right so this is a legend so we have out two and out one so the colors are yellow and green should connect to that Jack right now so this is on ohms and it's on continuity so you can hear the beat so what it'll do is I'll grab the green wire here again no power is applied to this at this point right so there is nothing applied power supply sitting there but it's not on so let's try this right first go that was a 50/50 chance let's try the other connection make sure they're not shorted nope so there we go all right no problems there now if I move to the yellow wire here I should have the other connection there it is and of course again to the other and it should be open yep all right so no problems there now the input leads right here the next test that we're really going to do is we need to power this thing up and we need to see if there is any B+ anywhere and and hopefully there isn't so what I'll do is I'll just grab a jumper clip here find one that's got the correct end on it there we go and I'll connect it to the ground for the board so the ground for the board is the black lead right here so I'll just connect that there and this here this orange lead should be the positive so I'll move this to DC and we should have around 8 volts on there so the orange lead here is the 8vt lead and the black lead is positive these two white leads here are just a pilot lamp so we can just completely get these out of the way if they'll stay out of the way they've been bent in a certain position for so many years I don't think they'll form any other way so I'll just put the weight of the chassis on this and that way those things are out of the way there we go okay so the inputs are open everything's floating here now before I power this up and before I go probing around in here if you're working on one of these things you need to know that if the power supply is on and if it's off for some period of time there are large filter cans in here that will hold an elevated voltage remember vacuum tubes anything that runs vacuum tube equipment aside from anything that's space charge uh has elevated voltages on the plate so you need to keep that in mind so if you're working on any of these things and you're following along you're doing so at your own risk so believe me I'm not going to be doing this with my fingers and holding this on here when I first touch this because if there is B+ on here I do not want to feel that all right so here let's turn on the power supply so plug this thing in and here we go is it on yes it is I see the little orange light on the par while come on there interesting all the vacuum tube filaments are in series in this thing so you pull one tube out and they all go dark kind of like the old Christmas lights so uh you know if you have one bad tube you have to test them all to find out which one goes bad since it is a uh they're all in series uh they could have been very neat way back in the day and used as zener diode or xener diode if you prefer in a bulb and have it at each socket so it would very quickly identify which tube is out but and in the series strings with a higher um uh you know filament voltage from end to end you can even use a neon bulb so if one of the filaments goes open the neon bulb that's associated with the tube that it's close to will glow anyways I'll talk more about that in the future I don't really understand why they didn't do that with the All-American 5 radios way back in the day that would have been so easy to find the dead tube right with film cuz you know whenever a tube fails you know probably about 80% of the time maybe through my experience it's always filaments right okay so I'm going to touch this with the probe away it has a bit of a hook on it look at that 8.4 volts no problems so it's there now let's make sure that there is no voltage on the headphone jack sure there's nothing there that's going to cause any issues nope nothing is there let's try the inputs there are blocking caps at the input of the amp nope nothing there and nothing there so perfect there nothing there nothing on the outputs all right so we know that everything is fine cuz I tested up here first so and they're connected right now so what I'm going to do is I'm going to pull the Jack out and retest just make sure right because there's a lot of switching that goes on in that Jack so I'll test this input here nothing there nothing there nothing here nothing here and we should have our eight volts here we do there we go 8.3 volts we're going to be putting out a little more than 10 and some odd Mills with that board with 8.4 now keep in mind that this is a 100 volt system and it has a dropping Transformer over here so this takes line which is you know roughly 120 125 depending on where you are and how good your service is and this will drop it down to about 100 volts so this power supply needs that basically it's like a buck Transformer the curve Tracer and Signal generator is being used to drive the input of the amp through the auxiliary Jack and the oscilloscope is attached to the headphone jack here so we can look at both channels here so what I'll do is I'll turn this on make sure everything is down for first that is set to a level that will drive an input just fine basically 12:00 so I'll just let this warm up in fact I'll give it a bit of volume here the tubes need to warm up right before it's going to start to pass audio right there it is look at that there's life at the headphone jack that looks really nice too look at how clean that is it's a beautiful looking signal right there two of them they look very well matched and so this is on mono left so this will be mono right so there's nothing plugged into the right so if I move this to the right side I should have an input now I do look at that no problems there and if I move the balance let's see what happens perfect one side goes down now the other side should go down no problem very nice it's interesting to notice how the amplitude increases on the one channel when you move the balance control it's kind of neat so there we go and if we go to right plus left we move the balance again same idea stereo normal so we should only be on the channel that's plugged in as you can see that goes down that goes up and of course if I go to the other side it'll increase because it's actually trying to reduce the amplitude of this other channel but it's not there because it's in Stereo mode so I'll plug this in now the blue move this back up to the upper Jack if I can get this out of here boy those are very tight RCA jacks which is very good no complaints there and there it is now the trigger isn't on that channel so it's not going to lock on that I got to move the trigger to B there it is so now want to lock and again moving this and this no problems and if you saw a little bounce on the trace over there just a moment ago that's the heating system turning on big Power draw on the heat system turns on in here let's see that seems to be doing it everything seems to be good so now all I have to do really at this point is I just go through all the rest of the Jacks so plug it into the phono section and do all that kind of stuff and check out the amplitude with that and of course if there's any issues we'll come back and look at that but it's basically going to be an exact repeat of what you see here right and we can look at both channels there right at the same time which is nice so and you can see the movement here my scope lead again here just clipped onto these Jacks they have solder on them so they're uh not really cleaned off so I just have it clipped kind of a loose connection there so no problems everything is looking really good I'm very impressed with this so I'll move this lead around you can see this here there we go scope leads anyways looks good you know what we should do just for a very quick check is take a look inside the power supply and make sure that we don't have any upset capacitors or anything inside that I've seen everything in here all looks good let's just take a quick check and make sure everything is good inside the supply maybe we'll also have a clue as to what uh popped that board originally power supplies unplugged and discharged through the amplifier so let's get inside the power supply boy these screws have a lot of time on them so I have new screws I'll replace those screws on the power supply as well those look pretty bad so that tells you right there are people have been in the power supply before and uh you know to have that kind of maring on the screws it looks honestly like people have been in this power supply a lot before let turn this around and remove what these screws are just as bad wow these are ugly looking screws I don't know if you can actually see on camera how ugly these things are they've been just stripped out look at that so not good at all Hey look it's in real time I could Buzz through this and Skip me unscrewing all the screws but why okay here we go I'm going to lift this straight up now I'm not going to pull this off to the side or anything because I don't know how the capacitors are situated inside here and I don't want to short them if they through dialectric absorption if they've uh tried to charge back up again look at that those fancy old diodes okay look at that now this has been unplugged for quite some time and through the magic of the camera it looks like I'm touching these but I'm not I'm actually a long ways away so fancy old diodes in here lots of dust I'm not putting my finger near these because I have no idea if they're trying to charge back up again a lot of these older capacitors are very good so contrary to popular belief so and of course me being the worst capacitor critic there actually is I don't know if there's anybody worse about critiquing capacitors than myself so okay let's see is this thing charging back up let's see look at that it's at 7even 7.6 volts and is going up not down you see what I mean these are great caps if that was leaky it wouldn't be doing that that's called dialectric absorption the Magic in capacitors so this one here is also going up as well so yeah well we know that it's working well cuz the signal is nice and clean in the amp fire so no problems if there's anything wrong with these caps that would be bad news so we would have to end up replacing little look at this look that can you see that that's not a good thing looks like I don't know if that's me did I do that that here let's make that into an unhappy face there that's unhappy so uh yeah somebody's tighten this under here looks like it's a ground the blacks are usually the ground well it's a center tap here so that isn't very good what's on the other side what's on the what is going on here that's a lead to the primary of the Transformer so somebody uh needs a better pair of glasses that is not a good thing that could make the chassis of this hot that's tightened right down on one of the primary leads this the primary right here running to the neon indicator bulb in there if we can see that zero and 100 if you can see that in there kind of hard with no light but AC right whoa so what we wanted to do is open that up obviously if it had shorted you know we would have big issues here you can see that why was that tighten over there it looks like it had gone well maybe not very close to going through that's pretty dangerous so probably have to I'll slide a piece of heat shrink over that before I put this all back together let's undo the other one see if that one's look at that well it's a good thing we opened this up everything's working okay but it just goes to show you hey you know you never know very close it is a ground so but still wow that could be very very bad now that wouldn't have caused this issue I'm sure because that's just the grounds so many people would probably think that oh that's the issue right there no that wouldn't be the issue wishful thinking though that would be the uh issue here's a bit of trivia for you why are there curls in the leads of these diodes every diode has a curl on the lead why have they done that you want to pause the video for a second and think about it I'll tell you here in just a moment nice looking little diodes thermal relief that's all they didn't want to heat the diodes up so they put a few coils in there before the joint here so when you actually soter this joint it ends up you know giving you thermal relief into the actual diodes a lot of these older parts were very sensitive the heat they didn't want to damage them and this one here has a lot of lead length over to this capacitor so there's no issues over there and we have another C or um another diode here almost looks like the diode setup that you would find in a television in the horizontal section doesn't it for those of you in the know let's see here so what may have happened in here somebody may have been testing this at some time because I do see like a mixture of like fingerprints and stuff inside this like it looks relatively dirty like you know I don't see really too much around here but it did look like there was you know some stuff going on on the bottom down here by our very unhappy face because of the squished wires um you know it kind of looks like somebody may have been in here not too long ago go I don't know because you'd think dust would settle on it mind you the fingerprint marks in there are kind of Dusty as it is so see if I wipe it again yeah you see so you know new dust is probably settled on that so not so incredibly long ago somebody was in here poking around I would imagine and of course that would indicate all the uh the stripped screws now if we look at the power supply schematic which I do have here somewhere had to do a little bit of digging I thought this was right beside me here so if we look at the schematic we can see the filament supplies right here on pin four and five now remember all those 12vt filaments are in series all right so when this is loaded it's around 75 volts and then right besides pin 4 and five which are connected together so we have a nice good connection there we have pin six which is the supply for that little board if you were to slip and touch pin five to pin six bad things would happen to that little board that board would go poof right now and that's probably what happened in the actual preamp itself if you look at the backside here if you look at the back you can see the wires right there are shielded right into the power connector and they run right up to this little amp board right here there's no brakes in it so there really is no way to bring any of the B+ or filament Supply or anything up to the that board unless it's done here by slipping with a probe I think that's probably what happened you know fresh fingerprints in here and everything just kind of look like that so everything looks really good I'm pretty happy with this now a lot of the times I get questions saying Paul you didn't do this and Paul you didn't do that well there's a lot of things that are done off of camera as well because to film it is just tedium right to do any type of videoing with that it's just tedium for example sweeping the amplifier with my sr780 to make sure that the actual headphone amplifier has no oscillations anywhere within a certain frequency range I do that all off of camera I matched transistors for this off of camera uh installing a bypass capacitor was done off of camera and the amplifier all of these things are done off of camera so a lot of the times I just I get these questions probably didn't do that yes it's just a lot of showing a lot of this stuff would just make this video last for hours and hours and hours if I was to show every single step so that is the rundown on this so now what I'll do is I'll end up putting the case back on top of this right I'll put some heat shrink on this and you know finish this all up and I put the case back on the amplifier and at this point it is good to go if I was to restore this well this is a repair uh restore would have been uh a lot of parts replaced at this point but again this is just a repair in this situation I hope you enjoyed the repair today if you did enjoy this repair you can let me know by giving me a big thumbs up and hang around there'll be more videos like this coming in the near future we'll be taking a look at vacuum tube and solid state electronic devices alike if you'd like to take your electronics knowledge to the next level and learn Electronics in a very different and effective way and gain access to many of my personal electronic inventions and Designs you're definitely going to want to check out my ongoing Electronics course on patreon I'll put the link just below the video's description under the show more Tab and I'll pin the link at the top of the comment section so if you click on the link it'll take you right there all right until next time take care bye for now

Channel: Mr Carlson's Lab

Views: 92,220

Keywords: Luxman preamp repair, Luxman A3300 preamp repair, Luxkit A3300 preamp repair, fix a preamp, fix a preamplifier, repair a preamplifier, repair a preamp, test a preamp, test a preamplifier, repair videos, restoration videos, electronics videos, tube preamplifier repair, tube preamp repair, Luxman repair, Luxman restoration, electronic restorations, electronic repair videos, audio equipment repair, electrical engineering videos, learn electronics, teach electronics

Id: 8RJmEfeoL7E

Length: 52min 14sec (3134 seconds)

Published: Sun Jan 21 2024