Diagnosing A Crate GX 30m Guitar Amp With An Oscilloscope

ladies and gentlemen this is Joe's classic video games back with a guitar amp repair video for you this evening uh we usually do arcade game Pinball Machine and jukebox repair videos but lately on the weekends I've been doing this little segment called amateur repair time where we try to just fix other stuff um and see if the you know just mess in around with it we can get it to work so what we're what we're messing with tonight is this old crate amplifier here it doesn't work right so I'm going to see if I can fix it we'll take it apart and see if we can figure out what the issue is and then uh uh that'll be that so if you're into arcade games pinball machines and jukeboxes and that's the only reason you like our Channel that's understandable uh we will be back Tuesday with a pinball machine video so we'll see you Tuesday uh but if not hang on so this I mean if you'd rather uh if you if if you just enjoy the content hang around a little bit this crate amplifier I picked up I actually bought it online and had it mailed to me and I bought it from a thrift store so it wasn't guaranteed or anything like that it actually works but the chorus doesn't work which is the only reason that I bought it really I've got this cool old school Peavey down there with nice Reverb on it uh this crate amp I like the ones that are made in the USA or made in Mexico or something interesting you know the ones that are a little older from the 80s and the 90s this crate amp is also made in the USA and it's got even nicer Reverb on it I think but the chorus doesn't work at all so we're going to attempt to take it apart and uh see what we can get going so let me let me turn it on and I I'll just show you what it's doing I've got a guitar plugged into it just so you can hear what it's doing and what the chorus is doing which is really strange all right [Music] so see if we can get some sound out of it so it's got a nice Reverb so we'll turn that [Music] up so the Reverb on it is great okay and it's we're on the clean Channel okay so you can turn it on this overdrive Channel [Music] okay and it's got this shape control here too that's really interesting I don't know much about okay so let's go back to the clean Channel we'll turn the Reverb down and I actually have the chorus on right now if I turn that [Music] off so I'll hit a note and then turn it on it's kind of there right but watch what happens when I turn up the uh the rate or the [Music] depth all right so let me turn up the rate so I don't hear any difference in the rate that it's doing anything and the depth doesn't really seem to do anything either so this is on [Music] zero this is on six you know it's just it's not doing anything and it starts clipping so if we turn it off so that's not that's not working properly I'm not completely sure how deep that chorus is supposed to sound but it did sound like we've heard something resembling chorus there for a minute but it's certainly not working right and we shouldn't be getting all that clipping I don't believe uh so this is a crate gx30m and just so I'm clear none of these are worth anything so this is I don't I hate whenever people are making videos where they're bragging about oh look I got this was worth $3,000 this this amp might be worth $100 if it was in nice shape you know so it's it's not worth a bunch of money but this is the type of stuff I like working on stuff that's kind of been discarded we're going to put some Grill cloth on it too but stuff that's kind of been discarded like uh oh that old crate amp you know it's got problems it's not working right uh let me get something nicer so I like I like finding stuff like that um that's decent quality in my opinion but kind of lower end stuff that's my favorite you know so I've got this this guitar we're playing here is just an old K from the '90s that's just the Cheapo copy of like a a Gibson right but that's my kind of stuff I might rather have it than the Gibson but um so that's what it's doing let me show you the schematic there is actually a schematic of this bad boy and uh we'll see if we can figure out what might be one of the problems and it's a crate GX 30m all right folks so here is the schematic made [Music] by slm Electronics is that it so this is from them somehow they must have gave away the schematics back in the day that's pretty cool on the arcade games and the pinball machines they do that well they used to that you don't get schematics with the new arcade games you still do with the pinball machines though I believe okay so this is the amp now I don't know much about guitar amps but I've been looking this over and it's we may be able to figure out what's wrong with it uh just by thinking about it so the Reverb works works the EQ works the overdrive works and the clean Channel works the only thing that doesn't work is the chorus so that means all we have to do is eliminate all of the stuff that we know works so here is the speaker or maybe there's the speaker whatever the speaker is in this area right and so all of this comes from up here and it's coming from this area they call the power amp and so does that work yeah we know that works because we can hear that everything's Amplified like we're getting it works fine just the chorus doesn't work so all of this works therefore we don't have to mess with any of that okay and then just before you get to that if you look see how this is the speaker blah blah blah blah the input to all of this is this one line sign here which they have labeled as tp7 test point 7 and that is a NE 5522 is that correct I think it's 5522 obviously I've got this on the computer too where I can read it a little better than this print out but it's an op amp and if you notice on the opamp it says positive 15 Vol C and then negative 15v C now the reason they're saying C is because down here in the power supply section there are actually four different 15 volts 15v a b c and d and then negative a negative B negative C and negative D and all of those are derived from a positive 20 and a Nega - 20 so what they're showing you there is that that particular op amp runs off of the C uh rail so plus 15 volt CGA 15 volt C if you look over here it's off of the 20 volt all right so we know all of this is working so let's start over here at the other end of the schem it says foot switch you can plug up a switch that literally just as they say you use your foot on and with that switch you can move between the chorus being on or off out or in and the channel switching out or in okay now this is hard to see but you can see it on the computer that is an A so this signal here is a and this signal here is B so I'll show you why that matters here in a minute the guitar plugs in right here and uh one part of the Jack goes to ground directly to ground and the other part is this signal and so we get here and on these two diodes we have 15 volt a and5 a we go into an opamp 15 volt a 15 volt a uh blah blah blah blah blah it runs through a bunch of stuff capacitors here is our EQ section so you've got mid low and high these little arrows on the jagged line are showing you a potentiometer right here's another one level okay so this is just how they equalize that signal and then here you have it says a well that's telling you that that's the switch that's mirrored up here with the foot switch and so a is the channel switch right this will make more sense as we go along yeah okay we get up to this preamp plus 15 volt a minus 15v a right and so the signal you can see it's flowing from the Jack through here we'll ignore this for now through here right and then it all kind of comes together here here so you've got um it goes through the op amp it also goes through a resistor it also goes through a cap and all of those those three lines where test point 6 is goes through a little resistor go over here into our opamp and then into our final amplification stage so we know that works right so what this is showing you is how the clean signal is created so when it's clean that's what you've got going on and it's turned off if this switch isn't in the right direction right so you have this signal that ends up right here if the switch goes in I guess that shorts it the ground where it makes both of the inputs to this ground and so none of that signal gets through so if that were to happen the signal would still be here I suppose but it comes in and it also connects this way it goes through this one capacitor here a 10 microfarad 16vt and it comes up here to H right goes through these two resistors now this also says a so what's happening is is if you hit that switch it'll switch between either the clean Channel or the overdrive Channel okay so let's say that we've got overdrive on well then this is connected and this is grounded so your signal from the Jack will come in of this way and go through the overdrive section now remember our clean signal all worked fine and our overdrive signal all worked fine so let's say that the signal Comes This Way it gets over here to this op amp and it's also running running on the a rail right it goes there's some dodes here and resistors and capacitors here is the gain uh knob for overdrive right here is yet a third leg of the switch that must be a heck of a switch so it turns this on we go through another stage of the the op amp there's another test point and it goes on and on and on here's the shape control and here's the level control so you've got gain shape and level and the signal goes through and blah blah blah and it's processing it making it sound however it does now keep in mind folks this is amateur repair time so I'm just giving you the Layman's explanation someone who really knew what they were doing could explain this in better detail to you but this is basically how it's working we're trying to figure out what's going on with our chorus all right we're going to get there we're going to figure it out and so that's signal that has now been routed through the overdrive so that it can go through a different set of op amps different diodes involved these switches uh turned on right um different set of controls that signal is now coming out right here and it circles back around through that resistor and Bam it ends up right where the clean signal would have ended up now the clean signal would have been right here our overdrive signal is right there but if you're on clean these uh this and this are not connected right so you're you're not getting a signal into this area because it's cut off right here so the overdrive would have nothing on it if the overdrive is on then this isn't connected so right here you've got either the overdrive or you've got the clean signal so all of this seems to be working fine okay so then here we have the Reverb the overdriven signal or the clean signal will be right here and then come out right here at test point 6 and then of course it also is going through the resistor and through the cap right so our signal will be present right here so while it's running this way it also can run this way right it will be at the same time so it goes through a cap and into this off amp and they've got it arranged a little differently and then it goes over to a speaker connection um a it's like an RCA jack an audio connector into the Reverb tank so there's literally a tank on the side of the cabinet um with springs and things in it I suppose and they shoot the signal down through there and it makes weird noises and that's how you get your Reverb sound and so it comes back out this side runs through some more processing stuff caps and resistors you know it's all caps and resistors people I mean caps and resistors and then it goes into this opamp which would be a transistor caps resistors and transistors it runs through this and look at it it's running off of the B rail b b right and then it gets over here to the Reverb knob now if you have that on zero it's going to pass none of that weird signal that it just created by running through the Springs the this is the signal right the Reverb signal will just stop right there and you'll never hear it but if you turn the knob you add it you turn it up a little bit and it mixes in bam right here where both of our signals have just showed up either the overdrive or the clean so that Reverb is working as well so we know that the B power Rail and the a power rail are working fine because all of that stuff works just fine okay so what about the chorus that's the only thing we got left well when the clean signal or the overdriven signal gets through this op amp to right here it'll run through the Reverb but then you got this line so it comes this way and through this resistor enters into our chorus section right now notice there's no switch to turn that off at least right here right so your signal from the guitar is going to be right here runs through these resistors and caps and they're using this I guess to just shape tone or something or amplify it or lower it and then it runs into this I always get this wrong MN 3007 chip so that chip is a it's a it's what do they call it a BBD chip it's a Bucket Brigade device and the purpose of that I've looked this up I don't know this off the top man I looked this up the purpose of that is to provide a delay to the input so you run a signal in and then it's going to delay that signal before it sends it out okay now connected to the MN 3007 chip is this MN 3101 chip and you can see that it has three connections to that chip cp1 which is the clock one cp2 which is the Clock Two which is the inverse of clock one they're just I believe that's correct they're it's like the upside down version and then vgg which is a voltage that this chip creates to control this chip okay now if we keep looking now keeping notice that I don't know many details about any of this stuff all I'm doing is just looking for logical things right I'm just trying to logically figure out what in the world's going on here now this chip if you look has this stuff all connected to it and only that stuff see how all all of this doesn't touch anything else all of this just goes to here so there is a path here right so it starts over here with ground and up here with 15 volts or is that negative 15 it's positive 15 all right so we have a look we've got them on these opamps too but look we've got a D here plus 15 D minus 15 D so this is our D rail so You' got two more opamps I'm calling them opamps I suppose that's what they are NE 55 does that say 32 is that what they are ne5532 I guess they all are they're probably all the same this one says rate that's the rate pot on the chorus and then this one says depth the depth pot on the chorus now if you watched whenever we turned up the chorus I got it to a point where it was going duck duck duck duck duck duck and you were getting like a popping out of the speaker but you weren't getting the the chorus effect I thought I heard it there for a second but it it like it went away um but when you turn I think it was the depth one when you turn one of them eventually you do get like this weird popping cracking noise in the speaker so that tells me this stuff might be working so it could be that this is working and we've got a problem here or something but this is the output which is where that sound would come from like if this is doing crazy stuff you wouldn't hear it there' be no way to hear it it has to come out of the output right so the way it looks like it's supposed to work is these shape the signal or whatever or shape something because there's no signal over here they just shape a voltage I guess or a clock and they send it over here into this chip so this if you look has three inputs ox1 ox2 and ox3 and so you see one 2 and three are all connected to the same input but to get to two it has to run through a resistor and a diode and to get to three it has to run through a cap so you got these three that are feeding this chip telling it what to do and you have a ground wire ground connection and then you have this voltage which is -3 so the -3 is created right here they take the -20 run it through a resistor and then use a xener diode um to make it happen I've never understood how these work but they do work so it says this is a 15 volt so I would think that would be 15 volts but maybe the cap changes it or something I don't know um so this chip needs -3 and ground to run and then it needs this stuff to tell these three inputs a certain voltage or pulse or whatever and then it uses that to create the voltage for this chip and the two clock signals for this chip okay so this chip takes the input that remember is coming from right over here our clean signal or our overdriven signal that's running through the Reverb and it runs in and then it puts it it outputs it by delaying it all right so then you get it delays it and then it runs through these resistors right and then bam it runs through this which is a switch this is the B switch again that if you hook it up on the foot switch is the chorus switch right so that is the switch that turns the chorus on and off and then where does it go after that it just comes right over here where the Reverb comes out where the clean and the overdriven signal comes out so if this switch is on you're going to get and this switch is off you're going to get a clean sound right there you're also going to get a reverberated sound right there and you're going to get a chorus sound right there so you can have clean Reverb and chorus on at the same time or overdrive Reverb and chorus on at the same time and Reverb can be turned off by just turning the knob down and chorus can be turned off by hitting the switch it's pretty cool how this works how they have these two chips that like talk to each other okay so there's that um so what is our problem well I'm thinking it may be this D well if you think about that if that was if that was bad if a and C were working but D was out then these wouldn't work right and it would screw all this up this one's ran off of the C rail as well so we need to go in and test some voltages and just figure it out so the first thing I'm going to do is test these now but look over here too this is interesting they have a chart it's a little hard to read on this printed version the overdriven channel uh switch one and overdrive input signal 50 molts Peak to Peak at 1 kiltz shape gain and level at 10 eqs at five okay so when you go to these test points it's showing you what the ac voltage should be and what the waveform should be so if we can't figure it out with the voltage we might be able to use a scope on it I'm not good with the scope but I can kind of use it and then if it's the clean Channel blah blah blah blah blah it's showing you what the waveform should look like so where that might be useful on this one is to see if we have these clock signals if this is screwed up we won't have these clock signals this chip won't be getting its clock signal and so it won't know how to act so it would get the input but it might might not give an output now we could have too a situation where like this resistors burn up or something and the signal is never getting to it but to be honest with you that's not that common because there's not a lot going through this you know we know our Signal's good here cuz you can hear it this is where the the the clean Channel and the overdrive Channel meet right so if that signal is good why would it burn up this resistor unless this chip shorted or something but I mean they're not going to do that these caps are non-polarized so they're going to be little ceramic Caps or something or little glass Caps or something usually they're not going to short but maybe so I think it's about time to take the thing apart and see what we're looking at all right so I took it apart not hard there's four screws on the top that hold the chassis in and then there's four screws that hold the speaker board in it's solid state and so I think I'm fine with running it without the speaker plugged in so we've got it turned on and the first thing I'm going to check is the -3 volts so this R71 has -20 on one side and 13 on the other side so we're reading - 21.1 and 14.7 so that voltage is there it's a little high but it's still there so but that sucks I was hoping it would be missing I need to figure out where I can test for the + 15 and the minus5 um D plus 15D and minus 15 d it's not quite as well laid out or as well marked as it is on the schematics so this is the overdrive section there's the Jack coming in this is the clean section with the EQ and everything this is the power amp here um the red one there is the Reverb then that switch and these two uh potentiometers this is all the chorus this is the part that isn't working okay so I measured the voltage and the the uh all of the op amps are running on like 18 volts and the the voltage is there and none of the voltage is missing so it's supposed to be 15 volts and Nega 15 but it's like 18 and negative 18 and the reason for that is probably because because the line voltage coming from the wall is higher than it is supposed to be on the back of the amp it says 115 volts but my voltage here is like 126 or something like that so they're all running High which is just a problem you run into with any kind of vintage electronics since it's not regulated it just uh the voltage is just higher because it's coming out of the wall like that and it's just running through a Transformer so if if we're looking at this section of the of the um U chorus that we've got those two IC chips that are running on 13 volts is what they designed it for okay so the the 13 volt section is running at a little over 15 so one of the chips uh there is a clock generator so this is what's manipulated with the pot and all of that and it it sends out a clock signal and an inverse of that clock signal over to that other chip right so it says absolute maximum ratings Supply voltage 18 volts negative 18 volts so it can handle 18 ours is at-15 it's on the schematic it's spec at -13 so this is the 3000 seven which is just a chip it's a delay chip that's that's what they're using to make this chorus effect and the voltage for it that they have spec at 13-3 volts uh its absolute maximum ratings is it can handle 18 volts so since our -3 has drifted and now it's -5 it looks like we're still within reason so I don't think that's the problem so this section basically creates a clock signal with this chip it uses Ox one ox2 and ox3 as inputs on this chip to create a clock signal and then the reverse of the clock sign the inverse of the clock signal that is used to shape the speed of the chorus that this chip creates these schematics are really nice it shows you right here test point9 is that clock line so they want you to check that clock line so 3101 if we check pin 4 now I never do this but I actually had to get my scope out one of my Scopes out you know I don't like Scopes because the the whole thing is I like being able to fix it with like a screwdriver I don't like having to use extra tools and stuff and it takes forever to even learn how a scope works and blah blah blah they're really good instruments and really good tools but I like fixing things as simply as possible but in this instance there's no simple way to fix it without using a scope so if I check out pin 4 let's see what we get here so this is test point9 that's a pretty Square wave here I'll do it again if you look down at the bottom see it says Channel One 5 volts per Division and that is jumping three divisions so that's 15 volts which is what everything's pretty much running on now if you look over there on the right it says Channel One Peak to Peak 17 volts so all of this is running on like 17 volts um but that is a clock signal see it and if you look this is the overdrive Channel there's our clock signal and it does look right and then this is if it's clean there's our waveform and it it looks correct ac voltage it's saying 13 is that what it's saying maybe I can zoom can I zoom yeah there we go it's saying 13 volts Peak to Peak but we're getting 17 Peak to Peak because of what we talked about a minute ago DC voltage ne5 volts DC okay so the number 10 waveform looks correct I mean the number nine waveform looks correct we are getting a clock signal there so all that means though is that this chip is working we kind of knew that because whenever you turn this we were getting that changing all right um H yeah maybe I could turn the knobs and see if that changes anything boy you'll love this oh I can't do it I don't have three hands let me get the tripod all right that's the best I can get with my tripod check this out you're going to love this I'm going to check it again and we're going to adjust the the two knobs one is depth and one is rate depth and rate rate let's see which one's which if I can not get electrocuted I would like not getting okay I see depth and rate okay so I'm going to probe that pin again and then we're going to turn the knob so you can watch what happens so see how now you can actually see the wave form on there because I I CH I had I've been messing with these knobs so this is the depth one this is all the way down as you turn it up it goes out and then at some point it comes back in okay so that's the depth one and see how it's doing that on its own I'm not turning the knob right now I have it cranked up pretty good and that's what it does so if I turn it farther it goes farther out so this is the time between triggers basically right so the the depth is down lower now so it's just it's not going out as far now because the depth is down so that's actually a good name for it right okay so that's at about four all right now watch what happens when I turn the rate up look at that so that's what we were seeing earlier we were seeing that because we had the rate and the depth turned way up so that's the clock signal going into the other chip I got to say that looks to me like it's working fine that I mean seems right doesn't it so let's see if the other clock line is appears to be the inverse of that I could hook up both probes but we'll just try to eyeball it so let's see if this looks like anything similar yep it does okay so I think that Chip's working and I think all that stuff before that chip is working but I've got a test point 10 in that area too I'll check it just to make sure it looks right according to this waveform but looks good to me I think I think the one chip the clock chip is working is drawing a jagged line man so now Peak to Peak is 120 molt and it's telling me that yeah it should be a jagged line okay so I have the speaker unplugged and the pen that goes into that chorus chip is pin three so I have the Probe on pin three if you look it says 600 molts that's just where it's at whenever you're not doing anything I have the guitar plugged into the amp I've got the speaker unplugged I've got it on overdrive and I've got it turned up a little bit so if I just pluck a string pretty good see how the voltage goes up and see how the line ripples right so now we're we're just sitting here see how the voltage goes up so that's the input to that chip to the chorus chip so here's the output to the chorus chip so see the 600 now I'm going to pluck it again nothing nothing so I'm going to turn up the chorus both of the the um the rate and the depth way up and see if it changes anything nothing [Music] so I think I chips bad um I get nothing out of it and we're getting the clock signal into it what do you think all right so looking at the data sheet this -3 on mine I just checked it it's -4.6 that's not crazy at all if you look look at the spec sheet it says that it's supposed to be running at-5 anyway so you know it's it's right in Spec they may have even written -3 knowing I don't know whatever okay and then there is also this line this vgg so that line needs to be one volt less a one volt More Than This One um so if this is NE negative 13 it would want that to be -12 and it just it's part of what it uses to make this delay work it's the it's a BBD which is a Bucket Brigade device so it's basically used to make a delay of a certain time depending on the clock signals so anyway so this should be -3 this should be -12 on mine this is -4.6 this is -3.6 which again if you look at the schematics I mean the data sheet that's exactly what it wants so this chip is getting everything it wants it's getting two nice clock signals it's getting its voltage in the ground it's getting the vgg signal and it's getting an input but you're getting no output so it's got to be this damn chip um I mean it I'm going to say 60 70% chance it's that chip reading online people are acting like they don't go bad that often though but dealing with like uh um old arcade games and stuff arcade game pcvs chips that do math fail a lot now uh counters go bad all the time and this sure sounds like a counter to me it's similar right uh this chip here actually creates that voltage it's just part of it this is a really cool chip this is a cool setup I've never seen this but it's really neat so you've got your ground and you've got your supply voltage it uses that Supply voltage and it makes this an output because it's outputting the two clocks and the the vgg signal that these need that a BBD needs so this is designed specifically to do this and it takes this voltage and it makes this voltage 14 15 of this voltage so whatever its Supply voltage is it will make that output voltage 14 15 because it knows that that other chip chip needs that and they're it's doing that and it's got the clock and the inverse of the clock it's doing that to set up that Bucket Brigade thing happening in the other chip so it's just really cool design whoever came up with this um so yeah so I got to try to order one of those they say there's a whole bunch of fake ones and they say that they're also in all of the little toys that do Reverb and anything that does Reverb might have this in it so like a little microphone that you talk into and it it echoes your voice and stuff it's probably got this chip I don't have anything like that though that I know of so I think I'm just going to have to order one all right folks so here is the Reverb tank I think it has Springs and stuff in it and I think we're looking at one right now that little spot there I think it's a spring maybe not um so we'll have to wait for that part I've also got this little Cheapo Dan Electro amp but it doesn't have Reverb or anything on it I was thinking I was considering taking it apart look you're not supposed to be a part swapper so I'll just wait on the part but here we sit I'd like to get this one going uh so I'll have chorus I don't want to add a bunch of petals in I like it when it's just on the amp I feel more like I'm you know like I'm Legit then and I've got some um speaker Grill cloth coming our speaker Grill has seen better days so I'm going to I got some that's like a one of the fender Styles that's kind of like a red well I better not do that mess up my peave one that's got lines in it that's more like a red um and I think it'll look good and we try to put the logo on it and everything and put it back with just nice new um speaker cloth if we can get this chorus fixed so leave your comments below let me know what you think you saw me go all the way through that circuit kind of I know you couldn't see the the the U scope too good because of the angle of the camera but like I said I'm not very good with the scope um you saw what happened so this is my logic here you know I found before that when I ask people questions online about stuff like this I never get a straight answer my buddy Matt that comes in the store sometimes there's two Matts by the way but one of the Matts he's an engineer he usually kind of knows what I'm talking about if I ask him I didn't ask him about this yet though maybe I'll send him a that video of the clock signals I did see that the peak to Peak on that was a little higher than the 13 but I think they were just saying 13 because that was like the input voltage I don't know but it should be all right with the clock signal being higher Peak to Peak but so we're waiting on the chip when we get it we'll pop it in and then that'll be that so let me know what you think so far down in the video I mean down in the comments and uh we'll see you on the next one hey don't forget to check out my brother Donnie on Tuesdays we we usually on this channel do arcade game Pinball Machine and jukebox videos and on Fridays my brother Donnie does videos on Old Vehicles old buildings and he's got a farm so uh go check him out I'm over there with him on his channel usually and then on Sundays usually we do just you know messing around videos like this that we call amateur repair time hopefully this video proved that I'm an amateur but I think I've got a pretty good grasp of how it's at least trying to work and uh hopefully that chip will get his fixed up we'll find out next week now I do still have my Majestic radio here that I'm working on as well I'm also waiting on parts on it so you'll just have to wait on that too but I haven't abandoned anything we're still working on oh I was talking about how I had uh several Scopes look at this big bad sucker here I didn't have a guts to try to use it we'll do another video on it one of these days but that really needs to be I hate to even plug it in you know that really needs to be gone through before you try to use it on anything um and I was considering using my varc to plug up the amp with the voltage turn down just to make sure that that 13 being 14.6 wasn't a problem but I don't think that's it so all right folks so there you go we will see you on the next video hope you got something out of it it's s so long you know me I don't like making long winded video but I hope you enjoyed we'll see you next week hopefully we'll be able to finish it up I'm waiting on the mailman