Hi everyone, I am working on this interesting KT-4168. This is in exceptional condition. I am trying to diagnose a variable speed issue. It takes a few seconds to reach speed (about 100hz rise) and will do the same if I slow it down by touching the capstans. Everything seems to be moving freely, including the motor. This is a fairly complex mechanism so I have cleaned and lubed as much as possible. It’s the same in both directions so I don’t think it is excessive pressure on the pinch rollers. Battery is new, connections are good I believe and I replaced the large filter cap. Motor draws 20mA@800mV with no belt and 120mA@685mV in play mode. The motor seems like it may be difficult to pull apart. Looking for anyone’s input on where to look next or how to remove any variables? As a side note, this unit has an interesting adjustment for the capstan angle that can be adjusted through the case! thank you in advance!
Nice find, this model is a rare beauty! This was one of the first few models I bought when I initially started collecting, and my repair skills were still pretty basic. Mine also had some issues with the motor having poor torque, and a buzz getting into the audio path. I found that if I hooked the tip of a pointed tool under the edge of the motor brass pulley while running and lifted it up slightly, then the torque would improve and the buzz would disappear. I didn't attempt to disassemble the motor, but my assumption is that there was a wear groove or a ring of oxidation around the commutator, causing poor contact and arcing with the brushes. Pulling the axle upwards was presumably shifting the brush contact point onto fresh metal and solving the problem. I came up with a "pragmatic" DIY solution - using a hammer and punch to make a dimple into the back plate of the motor, pushing the bearing upwards to permanently lift the axle position. It seemed to work! Difficult to photograph but you should be able to make out the "dimple" in the photos below. The capstan angle adjustment is indeed interesting... in fact this deserves a topic of its own! Watch this space... As another side note, it's odd that the 4168 contains what looks like a gumstick compartment, but the "door" is permanently shut and serves only as a contact point for the AA battery caddy. I'm pretty sure I've seen a Kenwood-rebadged version of this unit where the gumstick compartment was usable...
Thanks Nutronic, very interesting hack for the motor bearing. I will play around with mine to see if I can replicate it. I did try to push some watch oil in there but it didn’t seem to help. When I first took it apart, it was surprising to see no contact tab for the gumstick! I couldn’t find a downloadable service manual but I did see there is a version where the gumstick works, KT-4968. I am away for a couple of weeks but will let you know how I make out when I get back. Thanks again.
Interesting info about the KT-4968. I'd never heard of that. In fact there doesn't seem to be a single photo of one, nor a mention on any forum, on the entire internet! A rare beast indeed!!
Update: I took a big risk and decided to open up the motor with a very small diamond bit to remove the crimps and a magnet to remove the filings before opening. It still took quite a bit of effort to remove but I think it made it past the stress. It appears that the magnet is painted with a conductive paint and it is now blistering on both sides (see photo). I believe this makes it thicker and touches the spinning coils slowing it down. Nutronic, this makes sense as your 'dimple' pushes the coils away a bit from the magnet and you get stable speed! Interesting. I am going to sand down the blisters and reassemble. This is not a success yet but I wanted to provide my findings. Does anyone know if i need to recoat the magnet with conductive paint? I would not expect it to impact the magnetic properties that much but any wisdom would be appreciated. Jehr
These thin motors do look intimidating to open, but now that I see it opened realise the risk isn't that high. This is a coreless motor, similar to ones used in AIWA walkmans and made by NAMIKI. Also those bruses at 90 degrees is something interesting, don't think I have ever seen that before. As far as that conductive paint goes, seen that on SONY disc-type motors before. And also the buldges, which sometimes have to be sanded down. While I can say that paint isn't necessary for the motor to work, it may have something to do with improving the motor's efficiency.
Nice work dismantling the motor! Did you make any further progress? What's the condition of the brushes and commutator, is there any serious wear / pitting / oxidation? Your thread prompted me to get my 4168 running again, it had been stashed away for several years and I had pinched the belt for something else. I've fitted a new, high quality belt and lubed the motor and capstan bearings. The mechanism is running smoothly and the speed is stable over time, but the W&F is very high. I'm satifsfied there is nothing rubbing in the motor, it spins freely and there is no scraping noise, but I can hear the pitch fluctuating slightly and I an see corresponding fluctuations in the motor supply voltage. I've started studying the motor driver circuit as I suspect this may be having an influence. I've just re-read your first post. You said the motor supply voltage is 800mV with no load, and 685mV with the belt fitted? This does not look right. The driver contains an electronic governor, which measures the motor load current and should INCREASE the drive voltage as the load current increases, to maintain constant speed. Can you confirm you still see the drive voltage DECREASE as you increase the load on the motor?
Hi Nutronic, I sanded down the bumps (likely zinc plating vs paint), cleaned, lubed and reassembled. The magnet needs a very specific alignment so this took some trial and error to get it optimized. Everything was very clean in the motor and the brushes seemed excellent. Unfortunately the problem was not fixed. The speed will slowly go down 50-100hz and may jump up if I give the unit a shake. I fully serviced all the gears and mechanicals which was pretty tricky on this unit. This unit is almost mint so everything seems fine and working very smoothly. I even etched the capstans. Maybe it is electrical as the speed seems to slowly decrease and I can’t think of how that could be mechanical. That being said, shaking it does have an effect. I have taken a break from this one but will try and do some more voltage measurements under load as you suggested. The motor torque could generally be low. This is just a gut feel at this point but just slightly touching a pulley slows things down more than other Walkmans I have worked on.
I had a few minutes to open this up again. You will see that a slight lift of the motor pulley will increase the speed. It will gradually slow down again. Voltages are shown. Please note, the pulley sits back down again immediately due to the magnet, it doesn’t stay up. If you press it down, it won’t slow it back down. This only happens slowly. Seems to be the same issue as you have Nutronic. See video: https://youtube.com/shorts/JEbwxs0Xjfk?si=zIYWeASuwScTU05k A bit of a mystery to me. I think I would need to test it with another motor. Thx
@Jehr , I'm not quite sure what's going on there, to be honest! I can't see any obvious trend in the motor supply voltage as the speed drifts down, so I would tend not to suspect the electronics... so I guess it must be an issue with the motor, presumably some slight variation in the contact resistance of the brushes. Is the speed increase sustained if you keep the pulley lifted (using something like a cocktail stick)? Tying off some other loose ends: I said the capstan angle adjustment deserved a thread of its own. Here it is: https://stereo2go.com/forums/threads/the-key-to-perfect-azimuth-in-both-directions.10735/ I did some further investigation on my own 4168, concluded that most of the w&f was down to the condition of the pinch rollers. One had a faint groove, probably from having been stored in play mode some time in the past; the other had a discernible lump, of unknown origin. I've had a go at reconditioning them by mounting them on a motor shaft and sanding them smooth, with moderate success. They certainly look a lot better now, and the unit is now at least listenable, if not quite best in class for speed stability... I also found that the supply current was considerably higher in one forward playback than reverse, even without a cassette in the unit, which I could only put down to excessive pinch roller pressure. So I have bent the spring of the forward pinch roller to roughly even out the load between directions. This has brought down the supply current and the motor is less audibly under strain, so I suspect this has also contributed to the improvement.
