Nakamichi DR1 Reel Motor problem

[MeerKat]

Hello all,

This machine suffered from the usual belt that had turned to black tar. I replaced the belt but now the it stops during playing, this can be after a few seconds or minutes. You can get it to work again for a while by turning the reel motor a bit. FF and RW work OK.

There appears to be a dead spot somewhere on the com/brushes that cause the fault when the motor is turning slowly, when it's motoring fast no problem. I'm pretty sure it's the motor as I monitored the volts on it, they die shortly after it stops but the electronics cut the power quick when they detect the reels have stopped, it's that quick you only get about an inch of tape outside the cassette.

I have the motor out now, that was a mission! I carefully prised the end cap off and found a black deposit on the ends of the leaf spring brushes and on the com where the brushes don't touch, see picture of the brushes.

Can anybody shed light on this?. Should that black deposit be there?

The machine does not appear to have been stored in a hostile enviroment.

[Welsh Anorak]

The problem is that the motor runs at a low voltage (ISTR 2-3v) in play and that causes the build up of carbon you're seeing as I don't really think those motors were meant to run at such a low voltage.

Cleaning does help - for a while. Trouble is, these motors are like gold dust. I eventually got one from America, for a hefty price, and that was second hand.

[knobtwiddler]

Glyn is bang on. This is a much-debated subject, and it affects all non-DD motors used in Naks. Here is one thread as an example: https://www.tapeheads.net/threads/na...rebuild.55498/

I suggest searching the TH site.

https://www.youtube.com/watch?v=ilXWu6d52Zw

https://www.tapeheads.net/threads/so...l-motor.65309/

The guy who runs TH sells the 'pacific stereo motor', but I am not sure if it is compatible with your model. On top of that, if you find a replacement, you may need to do a little metalwork to get the new motor in (as Glyn shows - a drop-in replacement is likely to be secondhand, and if you want a new one you'll need to do some mods to fit it).

Good luck!

[TIMTAPE]

I see on the TH forum one person recommends just running the motor for some time minus any load to allow the commutator and brushes to polish each other clean.

My guess is the initial problem which is reportedly fixed by this free running technique is an overcurrent situation in a motor which is running in "too high a gear". Especially at the end of a cassette tape side, the motor is now running at its slowest but the torque demand is at its highest.

It might also be related to those flat type brushes. In the 80's I had to service over 1000 cassette players on a rotation basis, the TBC1 Audio Telex talking book cassette player which ran at standard and half cassette speeds.

The original Jeco brand Japanese motors used brushes very similar to those in the OP's photo. These motors became notorious in our workshop for brush failure problems. For replacement motors Telex eventually supplied Canon brand motors. They didnt use the flat type brushes as above but a block of brush material on a spring arm (pictured), as per the block type brushes I used to see in my younger days in vehicle generators and starter motors.

I figured that these brushes were better on two scores.
The larger surface contact area meant less pressure per unit area so the brushes lasted longer, but also as the brush was wide enough to traverse the gaps between the commutator segments, there was less chance of very high spot current leading to overheating, sparking, deposits. Whereas the Jeco flat brushes had more of a point contact area with the commutator.

Whatever the reasons, the Canon motors had very few problems with slowing, stalling, or electrical noise induced into the audio amplifier which was the other symptom with the Jeco motors. The Canons seemed better at slow speed, high current "lugging". The brushes and commutator seemed to self clean. But I suspect these brushes and the arms they were attached to were more costly to make than the flat, thin type brushes which seem common in many small cassette deck motors I've encountered over the years.

My guess is these flat type brushes would work better running at a higher motor speed and reduced motor torque, which would reduce current at the brush/commutator contact point. But that would mean redesigning the reel drive system for lower gearing.

[MeerKat]

Thank you guys for the replies and links although I had carried out the repair before they were published.

Success I think but time will tell as it’s my own machine and a keeper. It was given me free and the prices they command surprised me.

I cleaned the com and the brushes, on inspection of the brushes it is apparent one of them was hardly touching the com, see picture, some wear on one but virtually none on the other, it’s nice when you find a fault!

I carefully reassembled the motor, to keep that process gentle (those brushes are delicate) I filed notches 10 degrees from the stake points, 6 places, so the brush plate dropped in easy, it didn’t want to turn though so I filed a chamfer from my notches up to but not encroaching on the stake location. The plate turned easy then and locked nicely at the stake points. I replace the brushes then and adjusted the dodgy one. A couple of bits of wire bent in to a ‘U’ shape inserted in the 4 slots in the plate served to ensure the brushes cleared the collar as the plate went on, once the plate was in position out they came.

On bench test the motor turned reliably at low speed, about 100 rpm with 0.2 V applied for a good hour with out stopping so reinstalled. FF and RW good and I’ve played many tapes now and no stopping. Once all appeared well I applied a spot of epoxy to the plate/case to make sure no movement would occur in service, as you know the brush angle is important.

I have to say I’m disappointed to find brush motors in a high end deck such as this and not even proper brushes at that!

A good point I haven’t seen in any reviews is that the capstans stop when the machine isn’t playing, I wonder how many hours the average home machine is on and not playing a tape with the capstan motor gaily spinning away.

[MeerKat]

Just had a thought - carbon brush motors usually like turning one way only!

[Analogue man]

Originally Posted by MeerKat

Just had a thought - carbon brush motors usually like turning one way only!

Permanent magnet ones are easily changed just be reversing the wires to them but motors with a field coil can be reversed by either changing the wires to the commutator or to the field coil (but not both).