Nakamichi CR-4 repair reel-motor driver LB1649

On 3 Aug 2005 12:23:34 -0700, "Jens" put finger to keyboard and composed:

The LB1649 appears to be just a dumb power amp. I'd be looking at the speed feedback from the motor or from some encoder wheel. There may also be some logic which detects motor current.

- Franc Zabkar

On Thu, 04 Aug 2005 08:07:49 +1000, Franc Zabkar put finger to keyboard and composed:

Oops, I just realised the problem is with the reel motor, not the capstan motor. The reel motor does not appear to be speed controlled, assuming it follows the datasheet example. If the IC inputs are OK, then I'd suspect the IC or the motor itself.

- Franc Zabkar

Your problem is due to carbon build-up on the motor commutator. You just need to take it apart & clean the carbon out, or replace the motor.

-- Stephen Sank, Owner & Ribbon Mic Restorer Talking Dog Transducer Company

I´m just trying to repair my NAK CR-4 but I can´t trace the failure. I would be very happy, if someone could help me.

Symptom: The reel motor stops working after some time. It seems that the temperatur has an influence, since the tape works fine without housing.

During normal operation, the motor voltage (out1 -> out2 of motordriver LB1649) is around 2,5 Volts. In case of malfunctioning, the voltage drops down to 2,0 Volts.

Now, the question is, which component forces the motordriver to reduce the voltage? With respect to the LB1649... where could I find further documentation? The official datasheet isn´t detailed enough.

I would be grateful for any advice...

Greetings from Germany,

Jens Kupich

I had an MR1 with a bad reel motor a while back. I was able to fix it by unsoldering one terminal, and applying about 6 or 7 volts DC to the motor and letting it run all night. Seems these type motors fail more from disuse. Sure beat the heck out of trying to mechanically repair the motor. This of course would not work on a motor with a shorted brush, but these types tend to open up instead.

Mark Z.

Hi Franc,

.=2E.that=B4s the question. The LB1649 has for each channel four inputs. These are

IN1,IN2 direction VCC power supply VZ1 voltage adjustment for motor

The VCC and VZ1 have a direct affect on the voltage that is applied to the motor through out1 and out2. According to the service manual, I have checked the measuring points related to these pins. The result:

IN1 and IN2 (coming from =B5C) are perfect VCC and VZ1 are different to reference values BUT both in case of failure and normal operation. That means, even if the tape is working correctly the measuring points have although wrong values. Must I understand that ???

What I will do now is to replace the capacitors and transistors involved in this circuit and, as recommended, to clean the motor, whereby I doubt that the brushes are the failure cause.

Thank you for answering that fast... and if you still have a guess, I really appreciate it.

Jens

Hi Stephen,

according to your address, you have an indepth experience in servicing NAK VCRs. Please let me pose one more question to you: The tape works fine, when the case is open and the electronic circuits are cooled sufficiently. Isn=B4t that an indication that an electronic component is overheating and causes the malfunctioning of the reel motor? Couldn=B4t it be that e.g. a capacitor is depleted and affects the Input values of the motor driver IC?

Best regards,

Jens

(snip)

Hi,

I=B4ve applied 6.5 Volts to the reel motor and let it run the whole day. Now, I could hear four tapes in series without any problems.

It seems the motor and not the driver was malfunctioning.

So thanks a lot to all people who helped me solving my problems.

Jens