Nakamichi CR-3E repair and mod

I'd like to share my recent repair experience with a Nakamichi CR-3E tape deck I was given for free (wasn't working). The most immediate problem I found was a melted capstan belt and weak back tension belt. I got the kit from EBay (seller: Revox) as I wouldn't invest in a kit from Marrs without knowing if this deck would work fine afterwards. This was a smart decision, as the deck turned out to have a lot of other issues and, well against what I've read around, the after-market belt kit I got is working very well. So, I then tested and the tapes would stop playing after a couple of minutes, wind-up reel would stop. I researched: it would be the typical blind spot problem of these Sankyo motors. I tested, holding reel with hands and in some places it would simply stop and not start again, which confirmed it. Decided to recondition the motor and did this quite successfully. Deck now was playing the entire tapes. BUT: quite a lot of wow (I think this is how it's called - with a sine wave test tape, the waveform was "floating around the zero line" a lot).

After some tests and investigation, and taking everything apart again, I noticed very clearly that the flywheels of the capstans showed some bad signs of age and wear - surface wasn't smooth, it made the belt "dance" and one of them even has fine cracks in the metal! I guess, if they were available as spares, they would need to be replaced. As they're not, I decided to try and recondition them... with very fine sandpaper and extreme caution I managed to polish their surface. After reassembly, the belt is now going smooth and that extreme wow in the waveform was gone! So I thought that now the deck was fine. Did usual cleaning and fine tuned the speed of the capstan (was a bit on the low side) and decided to keep it for myself, as the sound is amazing, never listened to tapes with this sound quality.

But then, a few days later, after it had been sitting around a bit as I didn't listen much to tapes at that time, I wanted to play a tape and it suddenly stopped again.
Was back to the sudden stop problem! I decided that this couldn't be normal and decided to measure again the voltage and research on the internet to find valid values. Oh well, it had some problem in the driver circuit! The reel motor was only receiving 1,8 volts during play! But it got the normal 5,3V during forward and rewind, which both worked fine. I tried the torque pot, but it was almost at max, the voltage didn't reach more than 1,9V at its max. So, electronic problem. Started to investigate the circuit (I have the service manual), but all componentes measure fine, still the voltages are incorrect! :(

Then I had a sudden and absolutely crazy idea :D: instead of wasting hours to try and find the problem, why not build my own motor controller, which would only be activated during play/record, on rewind and fast forward the original controller would work, being my controller circuit bypassed. I decided to go for a very simple PWM circuit based on 555. Yes, crazy. It needed a lot of tweaking and changing, as in the beginning the frequency was way too low, making the motor whine, I then settled at about 35 kHz (well outside audible range), also the voltage turned out to be insufficient, in the end I opted for 9V and regulating the duty cycle very much close to the low end, so that motor wasn't too fast and didn't give too much torque, but still receiving higher voltage, hoping this would help to not oxidate and stall so soon again. It worked!!! the motor didn't stall. Or at least seemed so. Then it was quite a challenge to get the right logic signal to switch the bypassing (which was done in the most clean way possible with two relays, to avoid that the circuits could interfere, also it means this mod is totally reversible cutting a few wires and taking the circuit out and soldering the orignal wires, in case one day I'd find the defect or something), I finally managed with the signal which is output by one of the logic ICs which drives the Play LED on the front panel. So, my circuit was working fine! Problem now was the adjustment of the torque. It started to stall again!!! I drove it up in speed, but results were bad: too much torque, tape was getting pulled by reel motor now, clearly noticeable in bad sound, with "oscillation" on top of the sound and higher pitch. So I was going too far. But why would the motor stall if lower speed? Maybe it was beyond repair and I would really have to exchange the motor?

I decided not to give up on it yet and opened up the motor again. Oh well, it had oxidated quite a bit again! Maybe due to little use and the defective driver circuit which had it playing tapes at only 1,8V! I did a very thorough reconditioning until it was shining new. Reassembled. Yes, now I was able to lower speed until the capstans were in control again and sound got brilliant again. No stalling anymore! At least until now, fingers crossed!!!

Made recordings, wow and flutter improved, at least as far as I can see without having proper equipment. In general, it now sounds much better! Let's see if the motor manages. Unfortunately I have no torque tape so to be able to configure the perfect torque. As I don't know how the motor will do, I'm still planning to order a Mabuchi RF-510T as long as they're available, in case it starts stalling again.

So, this was my journey so far, never had modified a tape deck, or even heard of using a PWM controller to driver a reel motor, so feel free to call me crazy, but it's in fact working fine and solved this problem! If anyone wants more details or the schematic, I can try to provide it, I only have a sketch, don't have much time, but if necessary I can do this. Attached goes a photo of my circuit in place.
 

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Ended up exchanging the reel motor.

Well, a quick update to this: the motor continued on its unavoidable path to motor-heaven, starting to stall every now and then if not used for some time (and I already had done a second rebuild)... so I decided to get a substitute on ebay, after some research and asking for advice on tapeheads forum, I ordered a couple of RF-500TB-12560. As they are cheap, I figured it would be good to have one extra spare in case they won't be available anymore twenty years from now (yes, I sometimes think about the future haha).

Wasn't very easy to substitute, especially those small plastic washers (is this how you call them?) which hold the two reel hubs. I managed to get them off and back on, later, but I figure that without the proper tools they wouldn't survive this operation more than twice. I'm glad this was the first time for them. Then, it was also difficult to remove the gear from the original motor, had some very bad luck as it suddenly took off, making the tool pierce my finger :eek: (quite bad, but no ambulance needed, though, lol :mummy:) and make the gear and all piece jump far away (spent about ten minutes on the floor looking for them, found all except for the spring, which I managed to remake from another spring I had somewhere). Hadn't seen that coming... But in the end all went well, new motor installed and running nicely, never had any more tapes stalling, my pwm controller was easily adjusted to good torque by common sense and ear (I set it to minimum torque that would not cause the most difficult tapes to stall when starting close to the end), sound improved again quite a bit (which kind of surprised me) and there's one not so perfect change, though: rewind and forward is about 1/4 of speed as before, seems this is not a perfect electrical match (though it's a perfect mechanical match - shaft length is optimal and the mounting holes wasn't a problem, either) - possibly they sold me motors with wrong voltage rating. But I don't mind, I can wait, what I need is perfect playback and that is possible now. Checked the playback of 400 Hz test tone and got a really good result. Attached is a pic.

So, in the end this seems to close this long story - lesson learned: when the motor's dying, it will die, a rebuild will only be a temporary fix. Now I'm glad I did come up with this crazy PWM controller, as the new motor probably wouldn't have run at all with the original controller with its hard to find problem. Also, this can make the motor have a longer life, as it is running a higher voltage, closer to its specs.

Btw, these instructions were moderately helpful for changing the motor: Scott's Nakamichi FAQ Page)

They didn't mention the difficulty of getting the gear off the motor shaft safely, though. Nor do they give any tips about what kind of tools to use to get off the plastic washers. I guess we're all on our own for this... unless we have professional tools...
 

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