High End McIntosh C29 Preamplifier Repair!

hi everyone and welcome to another episode of mr. Carlson's lab today we're going to go through and repair a macintosh c29 preamplifier while we're in there we'll take a look at the overall quality in the Macintosh build that's what really makes these preamplifiers stand out above the rest so let's get started this is the Macintosh C 29 stereo preamplifier in all of its reflectiveness and shininess I guess you could say this is probably going to be one of the cleanest examples of a C 29 stereo preamplifier out there and I'll tell you why so first of all now whenever I accept something I want to know the history of the device I want to know how many hands it's been in if other people have been inside it and so on and so forth and I'll get into this here in just a little bit and tell you exactly why this is so incredibly important before you take on a repair or restoration of any kind so the story on this Macintosh goes like this so it's on to its second owner now and usually Macintosh owners are pretty meticulous with their stuff and this one here is just a pure example of this so the original owner of this Macintosh had a lot of stereo gear and he would like to cycle through his stuff so he bought this thing used it literally for a handful of hours and put this thing back in the box and it's pretty much sat since then so the second owner the person that's dropped this off now it's tried it out and says it works good it sounds fine it's just that when you move some controls around on the face that one channel go away or get scratchy and it's kind of hard to recover and you know it's it he says it sounds good he says he's not sure if there is anything wrong inside or whatever but you know the controls are an obvious issue and he says just go through the thing and pretty much just make it sound good make it do what it's supposed to do he says I want this thing to remain as original as possible since it has solo hours on it he says so basically just go through and replace what needs to be replaced so basically no mass parts replacements or anything like that so that leaves a little bit of a job because basically I have to go through and check all the capacitors one by one making sure everything is perfect you know and we're gonna have to find out what's going on with the controls on the face of this thing and you know see what's happening so we'll have to feed a test signal into this thing and see where the problems actually lie move switches and knobs around and things like that so as you can see this thing is in pristine condition it came in the original box which is flawless and it came with the original wooden case as well which is flawless as well it's uh it's odd it's almost like this thing has been transported through time it really is in that good condition so what I'm going to do now is I'm going to move the camera around I'm going to remove the top cover and the bottom cover and we're gonna take a look inside we'll take a look at the quality of Macintosh's build and we'll talk a little bit about what they've done inside then we'll test the thing out see where the problems are get rid of the problems and bring this thing back to its former glory once again this is the top side of the Macintosh c29 stereo pre amplifier and as you can see right here we have a label with a bunch of specs and this is also a portion of the top lid and I'll talk about that here in just a moment we have a countersunk area here with two VRS on this side and two VRS on this side VR meaning variable resistor on this side it says main output level so we can adjust our main output level with these two VRS and this side here these two VRS adjust the headphone and line output level and these are very sticky so they kind of pop in and out of place so you definitely tell this could use some cleaning these viewers and these VRS all the VRS and everything in this thing could definitely use some cleaning as you can see the upper side is very clean aside from some dust and stuff like that on the lid now this here is a portion of the lid this whole thing is very easy to get apart so if you need to service something like this in the field very easy to get at everything inside so to get at the upper vrs here and dilates if you need to all you have to do is just remove these two screws right here in this little panel just come right off now there's a trick with this thing here this will not come off if you want to take these two screws out you're gonna have to get on the underside and hold a nut this isn't designed to be removed right here but to get it the remainder of the circuit board all you do is you just take out these screws on the perimeter and everything just comes right off you're pretty much right into there and you can access the main circuit board and in a moment we'll take a look at the bottom side and it's just as easy to get in at the bottom side so this thing is very easy to service there's no catches like a lot of the modern stereo gear they'll have a lid but it like pops under a piece of say plastic fascia or something on the front and you have to manipulate the lid to get it off or pry up on the plastic and push down on the lid and slide it out things like that there are no tricks with this thing at all it's designed to be serviced very very quickly and very very easily so basically you remove these two screws these two these two these two this one here and you pretty much have access to everything that you need to get at on the upper side so what I'll do is I'll remove all of this stuff we'll take a look inside then we'll flip the thing over and take a look at the circuit board on the bottom side we'll get a really good idea of the quality behind mcintosh I now have all the screws removed on the lid and this front panel here now you'll see where the screws have been tightened down on this panel and the rest of the lid you see how much just scoring you might be thinking to yourself oh that lids been off a bunch of times to create that kind of scoring well the truth of the matter is that scoring is absolutely crucial and it needs to be there you'll notice the lid of this preamplifier is painted it's painted on both sides we'll take a look at that here in just a moment in the rails where the screws tighten into here is also painted so in order to make this lid become part of the case so this lid needs to have electrical connection to the case there needs to be that scoring so if there is a heavy paint on this lid and say it was to use plastic screws all the way around here this would just sit on top of the chassis and this would not make electrical connection to the chassis in order for this thing to reject hum this thing needs to be a completely shielded case if for some reason this upper panel this lid here was insulated from the rest of the case so it was not common to the circuitry in here like the rest of the case this thing this lid would be an incredible hum magnet especially for sensitive stages like foam stages and things like that so this scoring that you see here is very very crucial to keep this lid making connection to the rest of the case the screws need to be tight and this needs to have that scoring so when I remove this lid here in just a moment I'll show you the rails here and how everything is painted and how this could very easily be insulated from the rest of the case this is done at the factory so let's remove this little panel right here so this comes off just that easy two screws and this is off and as you can see we can get right to the vrs and here in the switches and everything the panel lights are right in here and we'll take a look at the panel lights here in just a little bit and to get it the rest of the circuitry very easy as well you can see it just comes off like that and as you can see the bottom side is painted as well and as you can see there would be no electrical connection here nothing right this looks like some dust or some surface dirt on here from sitting on there for so long so there is no electrical connection from the case to the lid other than where you tighten the screws into the perimeter here this is even painted as well very very important if you're designing some form of preamp or anything like that the case itself needs to be an entire shield especially if you're getting into phono stage sensitivity as you can see inside this thing there's a little bit of dust but it looks just flawless inside this thing could use a bit of a Dee dusting other than that it's looking super super clean nothing has been touched all the original caps and everything your car in this thing super super clean just they say just dust definitely a very low our unit so what I'm gonna do now is I'll turn the thing over we'll take a look at the bottom side all the screws have been removed on the bottom lid so there's one two three screws here two three screws here that's it comes off just that easy look at that just spotless you can really see the quality everything is loomed and they wrote everything on the side of the board metal conduit right here and you have the wires running through a metal conduit that's strapped to the chassis everything is nice and square brought in connectors on the boards here very easy removal if you need to service something pull the connectors out take the board out and away you go honestly the thing just oozes quality and it really does boards nice and clean did a beautiful job cleaning the board up a little bit of flux residue here and there I can see slightly but nothing major beautiful job you can even see they have signed off on different parts of the board different people is through the inspection stage just you know just really really nice work so what I'm going to do now is I'll move this around and we've taken a look at the bottom side and everything here and we'll take a look at the dial lights we'll start testing this thing and find out where the issues are and address the issues in order to replace the dial lamps or panel lamps inside this Macintosh c29 preamplifier you need to access these orange colored rubber grommets here so there's one under here as well and there's some on the bottom now they're pretty easy to locate because of those grommets but there's a couple things you need to keep in mind if you're gonna try and do this job on your own first of all they are incandescent lights so they have a filament inside them the bulb itself is pretty much stuffed right in the center of that grommet as you can see the wires come right out the backside make a sharp turn and go right back behind that panel again so there isn't a whole lot of wire there to work with the reason that they've done that again is for Hum consideration they're shielding all of the power leads to the lamps between this panel and the front face so if you want to replace the dial lights on a macintosh c29 and do a nice job you got to remove the entire front face and all the knobs and everything to get at the wiring now you can use newer style LEDs if you like but that requires a bit of a circuit modification to do that again this thing is working all the dial lights are working in this unit so the owner of the device wants this left as factory as possible so you see if I lift this wire loom up here you can see that there's another little panel lamp here if I lift it out you can see that it's glowing nice and bright so everything is working just fine in this unit here another thing to keep in mind is since there has been a very hot incandescent bulb glowing in the center of these grommets for a very long time it tends to dry them out and they get really crusty so if you're gonna remove the old incandescent bulb and take it out of the backside here you have to be very careful that you pull straight up or you'll crack the grommet sometimes they'll fall inside the chassis so you'll have to start replacing grommets so if you're going to be putting an LED modification in here or some you know maybe even new incandescent bulbs if you like in here sometimes it may be a good idea to change those grommets so if you want to retain that same look you just have to be very very careful with those grommets again even these grommets even with a low time on them over time the grommets just on their own kind of get crusty so if you haven't hot incandescent bulb in the center of it it just makes it all that much worse so there's dial lamps up here our panel lamps up here and some on the bottom as well all of them are working just fine in order to start troubleshooting this Macintosh c29 stereo preamplifier I need a low distortion signal source to feed to the input of the preamplifier and then I need to watch the output of the preamplifier on this oscilloscope here to look for noisy controls or staticky controls basically just to see if anything's wrong when I move anything on the face here in order to get that low distortion signal source I'm going to use this repurposed piece of test gear here this is a device that's up on patreon and it's designed to help you look for problems in circuitry without schematics this is known as a signature tracer this is also a curve tracer and it's a low distortion signal source as well for testing audio equipment so all the plans and everything and even description videos are all available up on patreon to build this device this was a five dollar oscilloscope that I put these modifications into no kidding this thing was picked up from a ham swap meet for five bucks and it has a bunch of circuit boards and some modifications into it this is an extremely useful device I use this thing all the time for troubleshooting power supplies and all sorts of other pieces of equipment it really is quite handy so check that out if you get the chance so what I'm going to do is I'm going to use this to feed this with signal right now so I'll turn up the CRT brightness excuse me an idea of amplitude here and I'll turn the amplitude up about there is fine what I'm going to do is I'm going to increase the volume control until we see a signal on the screen here and that's about half way up so the volume control is pointing pretty much rate at 12:00 o'clock right now so what I want to do is I want to move the controls around on the face and we're going to take a look at the screen here and see what happens so I'll just touch them first to see how sensitive they are and if there is any kind of static or any kind of noise there so what I'll do is I'm just gonna grab this and just shake this and look at that that's just from just ever so cent so slightly just moving this very sensitive movement of this you can see that II can just see spikes and everything so I can see where the owner of this but definitely have problem so I'll move one of the controls actually look at that Wow very very dirty controls right so a lot of the times the plating in the controls and this just happens over time the plating tarnishes inside the controls or they get some form of corrosion in there and this is the kind of effect that you get you can see just how incredibly touchy that is so I can get this to settle off we'll move on to the next one you can see I'm not even touching it so you know that would be incredibly aggravating if you're listening to a music program so let's move this one around you can see they are all just really really dirty so this one is just a standard VR so the carbon track in the VR is going to be dirty so we can already see that's the balance control that's pretty smooth so you can see this here is the mode so if I move this off of here it's just gonna disappear you can see that so you can see we can tell already that this thing just needs a complete cleaning so all the switches will get cleaned and all the vrs and everything will just get cleaned inside this thing and then I'll revisit this and we'll see if we can get rid of all of this horrible noise here so I've gone through all the dr's and all the switches on this this is just one pass so far of contact cleaning and it goes quite a bit beyond that as well whenever you do something like this I'll explain that here in just a moment to give you an example going through and cleaning this and doing everything to this point right now it's two and a half hours and that's only one pass I'm going to do another pass again tomorrow and that's very very important to wash the residue out of the VRS and the switches so just to give you an example here all right so this is turned up to half volume everything I've got this set already I'll turn this up here so we can see a trace and I'll turn up the amplitude and if you'll watch the oscilloscope here as I turn the amplitude up you can see it's nice and clean and watch what happens nothing is happening when I'm moving these VRS like before you see a slight change in amplitude as I'm moving this but that's it as you can see everything is nice and clean this is only one pass this is the base control here not gonna see anything because there's not much of an amplitude change when you're moving these around if I move the volume around he'll you know see it move up and down huge but you're moving these other controls not much happens what we're looking for is spikes and what we saw before an intermittent control that was horribly intermittent as you can see that's solid right now no problems now when you go through and contact clean these for the first time this again is the first pass yeah thoroughly spray contact cleaner in there move the controls around and you want that to spread around inside the VR if it's a carbon track you want that to move all over the carbon track and if there's contacts you want it to spread to every contact and get the tarnish off of the contacts so you have to move the VRS around a lot you also want to inspect the wiring you want to make sure that the shafts are tight a lot of the times just from sitting from years the grease that they use from the factory especially in a dual control like this because there's a shaft inside of a shaft here so we have an outer shaft and an inner shaft this was incredibly stiff this was very very hard to move in fact it was so hard to move the retaining nut that holds this set of VRS solid in there but the whole VR cluster would move back and forth with this because it was just so sticky so that own leads to be cleaned out and lubricated things like that you really need to spend a lot of time again about two and a half hours into just one pass tomorrow I'll go over this and I'll just wash every VR out again so there's not going to be so much time in there expected all the wiring and things like that as well so when somebody says they're gonna spray contact cleaner inside one of your units if they do a proper job it is a very time can zooming process at least to do it properly and again it does have to be revisited the next day what happens is is all the tarnish and corrosion and dust and grit and stuff that's inside the controls get softened up by the contact cleaner because it stays inside there the next day you want to hit it again with contact cleaner to flush all of that stuff out and by doing that you get a really really solid device after that you don't have to worry about it getting crackly and you know basically starting to pop and hiss again after a couple of weeks that second pass is very important again you can't just do this once and move things around put the lid on it and forget about it it has to be done a couple of times just to be safe and that requires every switch every VR everything in here to be cleaned you'll go through a lot of paper towels when you're doing that because this stuff is it just runs over everything so you have to tuck paper towels in here of course this is done with the unit off there's no power to it and all the caps are discharged and it is a very very time-consuming process to get that kind of result you can see there's just no movement there at all very very solid and this has been checked on both channels since the owner wanted the original caps left in I checked every cap which is almost basically one more D solder and I could replace them all so I checked every single capacitor for capacitance I checked them for ESR and I also checked them for parallel resistance as well which is also known as leakage all the caps are great they're absolutely great all within spec very surprising again it just yells quality so this thing is is an incredible piece of equipment if you have the chance to purchase a McIntosh piece of gear you always get quality stuff great piece of equipment thanks for stopping by the lab today I hope you enjoyed this episode involving this McIntosh c29 preamplifier if you did enjoy the video you can let me know by giving me a big thumbs up and hang around there'll be many more episodes come like this in the near future we're going to be taking a look at pre amplifiers amplifiers test gear all sorts of very interesting electronic devices vacuum tube and solid-state alike so if you haven't subscribed now would be a good time to do that as well if you're interested in taking your electronics knowledge to the next level and learning electronics in a different and very effective way you might want to check out my patreon page I'll put the link just below this video in the description and I'll also pin that link at the top of the comment section so if you click on the link it'll take you right there right now we're building some very specialized test gear for the test bench stuff that you just don't normally find on the market so if you're into audio or just repairing electronic stuff in general there's some pretty interesting stuff going on all the plans and everything for these pieces of test gear have been released on patreon right now so if you get a chance definitely check it out alright take care see you next time bye for now you

Channel: Mr Carlson's Lab

Views: 192,623

Keywords: McIntosh audio, McIntosh stereo, repair McIntosh, restore McIntosh, fix McIntosh, diagnose McIntosh, McIntosh tube, McIntosh solidstate

Id: bXUvlVPWF2I

Length: 23min 30sec (1410 seconds)

Published: Sat Aug 25 2018