Kenwood KA-9100 - Is it Worth Repairing???

hey everybody well while I'm waiting to get some more material for the video I'm currently working on I thought we would stop that for a little bit and work on this one uh kind of getting the urge to work on another piece of audio gear here so I thought this would uh satisfy my urge to do so this is a Kenwood model ka9100 and it was purchased Again by my good friend Dave who is really the driving force behind this channel because you know he's kind of we have kind of a symbiotic relationship I love working on these types of audio gear and he loves collecting them and and he loves the story lines behind him and everything so do I really and so you know he always seems to find some really cool stuff this is one of them uh I don't know anything about this thing right now and I I if I recall I think the story was that this does not work and it will need to be troubleshot and repaired but I'm not sure we're going to find out uh cosmetically it doesn't look too bad there's a little tiny bit of surface corrosion you can see right here and here and on the bottom cover there's a little bit like this as well and there's a little boo-boo right here and I do not think that happened in shipping I think that that was there from before so nice looking receiver it's strange but it appears as though this is uh right around 100 watts per Channel maybe or I'm not even sure what the we'll have to look up the specs of a ka9100 I do know it weighs an absolute ton it has dual power supplies it has a separate Transformer and power supply for each channel so one transformer for the left one for the right and big huge heat sinks I love to see those you know for all that class A B goodness and uh should be pretty exciting to to find out how this sounds now a good friend of mine growing up that that I grew up with he had uh the smaller model of one of these his was rated I think at 40 watts per Channel if I recall correctly but it looked very similar to this one it's from the same era he also had the matching tuner that went along with it and had a really nice set of Kenwood speakers with the 12 inch woofers they were three-way floor speakers really nice sounding system so it's pretty I'm pretty jazzed about hearing this thing uh fire up and kind of hearing that sound once again so let's uh get started and just see where we are see what what it does or doesn't do okay I have my buzzy isolation Transformer power supply turned on and I really do need to remove that Transformer and relocate it across the the room and put it on the floor or something so we don't hear the buzziness uh yeah that's that and another million things to do so I have removed the pipe cleaner and I have the current limiting bulb set up over here move some of this stuff out of the way I have so many projects sitting in the queue right now you just wouldn't believe so all right okay let us see what happens well that's a good thing the bulb flashed and then went totally dim and it charged up but I do not hear I don't hear the protect relay coming out so or come I should say coming in yeah now I don't know if this even has a protect relay as per se I don't know I again I've done not even looked at the schematics or anything on this but one thing I can tell you is the speaker uh the power meters don't usually when you turn these on sometimes you'll see the power meters will just pop a little bit you can see we're not getting that and if I turn it off wait about 10 seconds or so for the capacitors to bleed down that should be good right there and I turn it on you can see the the light will Flash and then go dim and the longer I wait the more it'll discharge and the brighter that'll flash when it comes on so it tells me the power supply at least one of them seems to be alive okay let's have a look at this schematic I have the bigger Pages too but this we can see what we need here so that chip which is a ta-100wa it appears that pin 7 of the board or pin five of the Chip is going to be the output so if we have any kind of crazy DC offset it would be there either there or on the other side right power amp board so we'll have to look at that and it appears that pin eight looks like pin 8 is the output of the other side so we have to look at that and I think I'm just going to try to go for the chip because I think I can get down in there and look at it of course there's their test points there so I think we can just look at the test Point test point seven that would be right there and test point eight would probably be on the other side which is right there so yeah so I should be able to look there and see if we have DC offset I think that's going to be our quickest way out if I see crazy DC on there then uh well we're pretty much wasting our time on everything else huh let's turn that on turn this on it would help if it was plugged in okay ground is ground all around let's see pin eight nothing there okay so how about over here pin seven nothing there so obviously there's there's no offset and again two separate power supplies you can see the two Transformers but they do share a common protect circuit and you can see this is this is it right here if I get it in the light and there's the little LED on the front which I don't know if light on means protect I'm assuming it does and it if you notice it never went out it stayed on the whole time so this thing's staying in protect for some reason uh let's look at some other things let's check some of the voltages that b plus and B minus so for instance we can look at 6 and 12. so let's see what 6 and 12 look like all right so here is pin six is this six yeah 55 volts and pin 12 I'm not getting good contact minus 55 so that's good 6 and 12. so we have power but we don't and we don't have any DC offset right so that would kind of in you know I have 58 millivolts so that's kind of telling me that this thing's happy over here and let's look at this side okay so on this side the B plus is going to be B plus and B minus is going to be 13 and 8. so pin H is going to be the Plus or not would I say 13 and 8. Goofy 12 and 6. that's what we want 12 and 6. so let's go here and let's go can't be right am I getting ahead of myself here let's see oh I'm sorry 5 and 11. 5 and 11. so there's five yeah no voltage pin 11. no voltage do you see that five and eleven there's nothing so maybe we have just something dumb like a blown fuse which why didn't I check that first it wouldn't have mattered because they all appear to be intact well no maybe this one's open all right let's shut it off let's unplug it and I'm gonna wait for it to bleed down a little bit and then we're gonna pop these fuses and check them real quick okay and you didn't see what I was talking about did you so here's two of them here there's the other two down there you can see them okay so let's check those so F2 F3 and F4 are all good but F1 which is this one right down in here is Kaka look at that now I'm not just going to go and replace the fuse we're going to try to find out why said fuse blew and looking up here so there's F1 and F2 and that's on one side and then let's see what we have here I don't see the fuses going to the other power supply here but I'm sure it's in there someplace and I'm seeing four other few okay so I have two fuses here and four fuses here so five six three and four so three four five and six are here one and two are there do I even have the correct schematic ka9100 should be it does not look exactly the same but I might be looking at it incorrectly so let me check around a little bit here and make sure that uh I'm not crazy because you know quite often I am well that's interesting after a little bit of research here a little bit of reading first of all five and six are not used on this amplifier this must be for the 240 volt model uh none of these fuses are for the power supply so well they are but so three and four are the 120 volt Mains fuses for the two power Transformers one and F2 come off of the 30 volt windings of those Transformers and feeds into this and this bridge rectifier is what controls your low voltage power supply this is really interesting and everything is like mixed up on the boards now one power supply for one channel goes over to the diode board and you can see two filter capacitors and the bridge rectifier and these leads go on up and they go to this Transformer up here the other channel the wires from that come down and go here and they go into the diode slash relay board and there's your capacitors and your Bridge rectifier for the other channel and on this board they also have the speaker protect circuit but they do not have the protect relay they show that over here on the power supply board so part of your protect circuit is here but the relays for the protect circuit are here and your low voltage power supply that's regulated Supply is here so you have several boards with the circuits kind of scattered all over it and if F1 is the one that's blown which is this one here that's coming off of the AC 30 volts from this top Transformer up here that means that more than likely either you're positive or negative low voltage supply has a problem so I think what we're going to need to do is we're going to need to pull this low voltage board to look at it a little more closely and see if there's anything wrong here I really don't want to put a fuse in here unless we know for a fact that nothing is dead shorted I don't want to cause any additional damage and what we might do is we may disconnect these couple of wires that go out to the outside world because these are your these are your power supplies this is B plus and B minus you see them pins 23 and 25 and 24 appears to be your ground or your common point so what we're going to do is we're going to pull pull those two wires off I think might be a good start we're going to just wring out these transistors if they're good we'll put a fuse in it see if we can power it up with you know without these being connected and see if we get voltage there I'm just trying to go slow and and not cause any more damage the last thing I want to do is damage these chips okay the wires are disconnected hopefully nothing's shorting out I'm going to turn this on and I don't know what our fuse did but I did put a new fuse in there and let's go from here to here I have nothing so it tells me I just blew my fuse so I probably won't make that mistake again yep nothing so it just probably cremated that fuse again so we have a problem on this board because our power supply is disconnected there so well cost me a fuse which I hate because fuses are expensive now but now we know I probably could have put something in line with that like a little lamp or something but that's all right so let's uh let's start troubleshooting there's not much to this board so we're going to find the problem one way or the other oh and by the way measuring either of these wires to ground statically I'm just it's way up it's kind of reading either a capacitor or you know High ohms very very high ohms and reading from either one of these pads to ground again very very high ohms so I don't think there's a short there it's going to be somewhere prior to that and most likely one of the transistors or one of the capacitors is shot okay let's just stop right here and back up for a minute kind of in the process of making a video I've kind of got sidetracked from the fact that I'm also troubleshooting this problem so what did what did I do wrong here okay so the first thing we did was we measured if you recall I measured the B plus voltage for this amplifier and this amplifier for the right and left channels and we had plus and minus you know B plus and B minus on this side and no B plus and B minus in the other so we looked at the fuses on this regulated power supply board and without actually getting a schematic first found that fuse F1 was popped we then printed out a schematic and I looked and found that it was over here which is your AC 30 volts so this is feeding your regulated power supply well that's all well and good we started tracing chasing the problem on the regulated power supply but the problem with that is if you notice this board really doesn't use the regulated power supply it is just using the plus and minus 55 volts so what we did was we got sidetracked here right that's what I did and so even though this is another problem that's that I've identified still an entire power supply is out so either this side or this side so I think before I continue troubleshooting the regulated Supply I really need to take a look and see if both of these Transformers are working properly and why there is no power you know the B plus is missing from one of these channels it does concern me that we have all that you know corrosion there or whatever the paint modeling on those Transformers did did they overheat and did one of them actually fail because if there's a bad power transformer in this thing that's kind of game over right now until we can find another Transformer so I think that really needs to be the course of action and we'll come back to this regulated low voltage Supply afterwards okay let's look at the schematic so the input of the two Transformers here you have the red wires if you notice they're tied together let's get that out of the way so red wires are tied together here and here and yes I'm holding the schematic up and it's shaking a little bit so have to live with it white wires are separate though they're not tied together between the two Transformers you have one of them going through one fuse and the other one going through the other fuse so what I did was I popped these two fuses out so now they're no longer tied together and that allows us to check across each coil you know each Transformer's primary individually so I'm connected to the red wire which are the two that are tied common together which is right there see the red and the other lead here here's our two white wires there's one here and then the one above it and I'm set to ohms hopefully you can see everything so if I go to first one I have about two and a half ohms I would say that's okay for the primary uh oh and for this Transformer we have open circuit we have a bad Transformer bad power transformer so right there this amplifier is game over so yeah I put about half an hour into troubleshooting this and kind of wasted enough about 10 or 15 minutes getting sidetracked on the low voltage Supply you know the regulated Supply but and again the fuses strangely enough are not blown uh it looks like the transformer shorted and overheated now let's see if this is the one that has the most paint worn off of it or melted off of it actually when you look at it this one here looks far worse you see it this is the one that's testing good be the top wire yeah that's strange but this one's actually still testing good this one tests open so unless we can find ourselves another power transformer for this this thing is going to become a parts unit because without the Transformer the rest of the amp is pretty useless okay the bad news just keeps coming so what we have is I've disconnected the power wires from the amplifier module that's on the side that has the bad Transformer um [Applause] so that would be this module up here there's two modules of course right left channel that our left channel is the one that's got the problems and there's only one two three four five six connections on the entire board you have two grounds you know for your input and output you have your input signal and you have your output signal and then B plus B minus so really all I need to do is disconnect B plus B minus and and connect those to an external power supply and I should be able to power this whole module up and see if we get any kind of DC offset or anything at the output if if it comes up then what we can do what we would be able to do is put a signal in here to see if it passes the signal here's where the ugliness starts I've done that here is the plus b plus and B minus and I have a split power supply that you can see over here and it only goes up to 30 volts instead of 55 but you can see it right here and I have it in differential mode so it'll do a plus and minus uh power supply and that goes in here and then we have the ground lead and then we have our meter connected to this yellow wire which would be the speaker terminal it goes down to the output of that uh power pack module now watch what happens as I turn up the voltage yeah that's at the speaker terminals or would be so you can see the further up I go the more it does this and eventually it starts to draw current and uh well that tells me that most likely the Darlington power pack module is faulty now there's a very slim possibility that one of the small signal transistors is shorted but I would say if that's the case it's probably damaged the chip as well so that ta-100wa power pack module is also bad now chicken or the egg you know did that power module take out the Transformer how could that have happened I mean the primary of the Transformer is open and I've verified that no doubt if the primary of the Transformer is open that means it would have had to have drawn enough excessive current to take out that winding but there's a fuse right in line with that and the fuse in there was good and it was the correct value which is only like a one amp fuse it's not a it's not a very large fuse actually it's a 2.5 amp fuse so you would you would think that the fuse would have blown before the Transformer fried the only thing I could figure is at some point in time somebody replaced it with an incorrect fuse value now something that I found that was kind of strange if you look way inside there I don't know if I can zoom you in close enough but you see down there there's the fuses I have the ends of them pulled out but look at the fuse terminals how terribly tarnished they are and again that could be a sign of heat or it could just be that the type of metal they used was crap um either way I do have those new little Clips I can replace and uh this is turning into an absolute Parts amplifier typically what you would do with it that being said I'm gonna continue testing it a little bit there are workarounds for this I actually found online a toroidal Transformer that not only has plot the the two instead of 43 volt it has 45 volt winding so there's a two volt difference that might be a problem if the chips can't handle it but I think they can and I also found a company called Pyramid Audio and I have used their kits before they specialize in building uh replacement discrete component modules to replace the STK you know chips by uh by Sanyo and the sanken chips and these Trio modules and they have one specifically designed for the ta100wa and so I can order a pair of those and I can order a pair of those toroidal Transformers by the way they are much much heavier Duty than these and being toroid Transformers they're pretty much better in every way but the really crazy thing is not only did it have the the 245 volt windings for the power amp it had an actual it had two 15 volt windings for the low voltage Supply that we could wire in series and get our 30 volts so it's a drop in replacement I will have to make a different mounting bracket for the Transformers but if I decide to do this and I go ahead and do it we can actually rebuild this thing and actually make it a very special amplifier make it way better than it was before um so I'm going to think about it because what's going to end up happening is if I do this I'm going to end up spending more money on this than this amplifier is worth maybe someone who really would know understand and appreciate what this is if it works the way I'm thinking might pay enough to for me to get my money back out of it but uh I don't know it might end up being just a special project that sits on Tony's shelf or maybe sits on my stereo rack I don't know but uh I think I'm going to wrap this video up for right now and uh I'm gonna think about it a little bit and I may go ahead and order those two Transformers and two of those modules which is probably going to be about a 300 plus investment by the time I buy those four items because of course whatever I do to the one channel I want to do to the other channel because I want them to match you know I I could definitely just rebuild the one channel but I'm not going to do that if I'm going to do it I'm going to go all in of course the thing's going to need recap you know all the capacitors and you can see all the things stuffed up in there but that's all small potatoes we can fix that essentially if we have a good power supply and we have a good power amplifier module everything else is fixable there's nothing else in here we can't make perfect so I'm going to wrap this one up I'm gonna get this video posted up there and uh let me know in the comments what you think I'm sure some of you are going to say Tony you're crazy and well you're right I am uh some of you might say Gee let's do it I want to see it I want to yes yeah I bet you do you're not paying for it oh I'm just kidding with you guys but uh let me know your thoughts I think this is a really cool amplifier and I do think it will work very well and reading on the audio Karma website there's quite a few uh threads involving this amplifier these modules different things like that and there's mixed bags of comments you see some people don't like them because of the unreliability of those chips but other people think this is a great and great sounding amplifier when it's working and very well built built like a tank I mean it's so everything's just absolutely over built as far as you know structural Integrity is concerned so I don't know I I think uh I don't know I'm almost convinced I'm gonna do it but let me know what you think until then peace joy happiness and good health to all of you and uh well you're either going to see the continuation of this project or you'll see another project go up on the bench or hopefully I'll have the materials I need to finish my uh computery type video all right take care bye-bye

Channel: xraytonyb

Views: 15,306

Keywords: electronics, vintage electronics

Id: AoMB5U3_nm4

Length: 32min 12sec (1932 seconds)

Published: Thu Jun 22 2023