Experiment only, not Recommended.

Aiwa AD-F770 Capstan Motor

Regulator Replacement
(Just an Experiment)

Quite recently I decided to return the original troublesome '12v 2400rpm CCW' capstan motor back to the Aiwa F770 cassette deck for further investigation. Previously, I had replaced it with a probable Chinese copy of a Mabuchi DC motor. 

I chanced my luck and replaced two internal electrolyic capacitors hoping that the onboard regulator would work correctly. It wasn't long before the transport (playback) speed of the deck began to show the same subtle variability as before - the very reason I replaced it!

Backplate of flywheel - note the regulator circuit inside the motor circuit.

Front view

I again examined the original regulator circuit and ordered some 2SA684 (PNP) transistors. There were also zener diodes in this old regulator circuit, and I soon realised that I may need to replace all the components to be sure this would work again!? 

While I waited for the transistors to arrive, I mulled over the idea of designing a simple, but highly stable voltage regulator, centered around the popular National LM317.    

The original voltage regulator sits on the back of the dc motor.

Desoldering the old regulator circuit from the original motor I needed to find out what voltage the motor operates at the 'correct speed'? - the result was approximately 5.5v. With this in mind, I built and tested a circuit to deliver trimmable motor voltages between 4.7 and 6.0 volts, approximately.

The LM317 Based Voltage Regulator Circuit:

Note: A tantalum capacitor was initially added across effective-R2 as recommended by National Semiconductor (to reduce ripple effect), but this made the output slightly unstable - approximate deviation of about ± 2% when stable, so I removed it. I also added a small 10uF tantalum capacitor at the output with the addition of a protection diode across the LM317, and another across the motor. Probably not needed, but I put them there.

 
Note that as Vout = 1.25(1+R2/R1), the effective value of R2 is given by ....

R2= 1200(1200+Rv)/[2400+Rv]

Where trimmer Rv is approximately 2600Ω (by measurement), at maximum setting.

Simulating the 'new' regulator circuit.

Using stripboard ("veroboard"?), the said circuit was then assembled, and tested again. Later the circuit was encased, and secured into a simple plastic insulating case. All connections to the F770 were resumed.

The motor needed to be returned to its casing - all previous regulator components (except an 8-pin chip) were removed, and a new lead was soldered in.

Correct Playback Speed Alignment: Using a full track width reference tape, and an external digital source I aligned the motor speed by listening to the two sine waves 'beating' together until there is zero, or a very slow drift between the two.  (17/04/2020)

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18/4/2020 ... This morning I ran some more tapes through the F770, and again the speed began to wobble slightly - it's very subtle, but noticeable over a period of about 15 minutes.  It's so subtle that you can be forgiven for thinking "did I hear that pitch wobble?...".

Conclusion: Now that the regulator issue may have been eliminated, the motor is probably partially worn? I suspect brush-commutator contact is partially failing?

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New Mabuchi Motor Fitted EG-510ED-2B2 (18/4/2020)
After realising that the old motor was showing its age, I decided to discard with the idea of a new stable regulator - it worked well, but the motor wasn't behaving itself! 

I've now fitted a genuine Mabuchi motor into the Aiwa AD-F770, after putting the parts back together, the deck is now working very well.

The Aiwa supplies 12.5v to the dc motor, and the motor draws an average of about 58mA when in play mode, and a little less when just running free.  Of interest too, when the old motor was in circuit, it drew only around 40mA - perhaps there was an issue within the old motor were brush/commutator contacts were more resistive due to dirty or worn contacts?

The old motor dissasembled is shown below.

From close-up inspection, I could see why the motor was varying its speed - the commutator brushes had partially broken. There was also a small build up of carbon deposites on the commutator.

General Comment about the AIWA AD-F770

This is a very good performing machine, but personally I wouldn't recommend buying a F770 due to the complexity of the internal circuit layout. They are awkward to fix or service, so lot of patience required!

7/05/2020: Judging the feedback from another website, I think it is important for the reader to understand that the use of the LM317 voltage regulator (as stated above) is an *experiment* - it is not recommended. 

Dedicated DC motor controllers typically sample the change in motor shaft speed due to a change in armature current, or terminal voltage.

If the load torque 'T' increases during an interval of time 𝚫t, the motor will slow down, the back EMFwill decrease (since EMF∝ speed) , and so armature current increases. 

(Note: T ∝ armature current). 

The controller then (typically an AN6651 in circuit) increases the terminal voltage by a proportion so that constant speed is maintained.  The reverse is true if the load decreases.

8/05/2020: I have just ordered some AN6651 controller chips after managing to decipher the AN6651 (and other similar) datasheets. I hope to devise a general solution to the problem of ageing DC motors in cassette decks by using and modifying third-party DC motors that can run at 2400rpm.

I have begun a new article here ... https://cassettedeckman.blogspot.com/2020/05/external-motor-controller-experiment.html

Over the next few weeks I hope to establish a reliable working modification that can be used for other third party DC motors. 

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(Note: www.blogger.com has editing/software bugs, and so I may need to revise or edit the articles without notice. It's very irritating for me to do, and for you to read - so apologies.)