Nakamichi 500 Dual Tracer

 The Goodmans SCD100/Nakamichi 500 Dual Tracer

Under the bonnet, this Goodmans SCD100 is 99% apparently a Nakamichi 500 Dual Tracer.

So what follows in this blogger page concerning the Goodmans SCD100 can also be applied to the Nakamichi 500, except that the '500' has a built in 400Hz tone generator for level calibration.

I bought the SCD100 second hand, it wasn't working properly. Since then, I've done quite a lot of work on this, and lot's more to do yet! I've got the machine operating in playback, record, fast forward, and rewind. Sounds very promising!  However, many minor repairs are required.

Initial Issues

1. Internally the oils, and grease had dried up. (fixed)
   
2. Mechanically, most functions were difficult to operate, and Pause had seized! (fixed)
3. Motor speed was very inconsistent. (temporarily fixed)
4. Noisey audio playback. (fixed)
5. Intermittently - right channel extremely loud on playback (diagnosed - very probably a contact issue with the Record/Playback multi-switch not returning to Playback mode!?)
   
6. VU Meters were moving sporadically. (fixed)
7. Dolby instability/oscillations in left channel. (fixed)
8. The SCD100 was recording 180° out of phase between left and right channels! Playback from pre-recorded tapes from third party machines were in phase - ie, normal.
   (fixed)
9. Auto-stop not functioning. (Repaired using external circuit - 16/05/2022)

Later Considerations

1. Professionally lap the record head?
2. Audio alignments?
3. An alternative motor controller, perhaps based on the AN6651 controller? 
   

When I get the time, I'll write about how I fixed (or not) the most challenging issues, ie, 3,5,7,8 and 9. 

 

Issue No 9: Auto Stop Fix

The original problem was that the auto-stop failed to work. After studying the circuit diagram from the service manual and opening up the auto-stop circuit many times, I concluded that the best solution would be to remove as much of the original circuit leaving only the working section in situe, and build an external circuit to drive it.

Take-up Spool motion detector and timing shown in light brown.

The inclusion of a flyback diode 1N4148 (or similar) was merely a precaution to protect Q601 when the reed switch closed, a reverse voltage (Vc601~ -15v) appears across the PN junction of the Q601 transistor.  Examining oscilloscope traces suggested that this 'spike' was at least 7.5v.

All transistors Q601/Q602/Q603 were replaced with 2SC2001.

The circuit was extensively tested 'offline' before applying it to the front end of the default relay switching circuit as operated by Q604.

Circuit Operation

Note the value of R604 has been increased to a maximum of 100kΩ
to help reduce an erroneous auto-stop state if the take up reel turns too slowly.
The auto-stop function takes about 2.5 seconds at R604 ~ 100kΩ.

When Play/RW/FF or Record+Play are pressed at least two events  happen ...

Mechanically: The take up spool rotates and the reed switch will continuously open and close, 

Electrically: Q603 is switched ON via R605 10KΩ and 1R606 KΩ biasing, and regarding autostop timing - a potential of 15v is offered to the auto-stop circuit. 

At the point of t=0+Δt seconds, current through C601 (22uF) is at a maximum and Q601 is quickly switched ON.

This exponential decay current initiates an ON-period for Q601 for as long as Vbe ~ 0.6v or greater.

The graph above illustrates the exponential decay curve of the current through resistor R602 (10KΩ), capacitor C601 (22uF), and R601 (3.3kΩ). Of particular interest is the potential difference across the 3.3kΩ load, ie Vbe of Q601.

The approximate value of Vbe vs Time is plotted above - this is a reasonable approximation. In reality the base-emitter junction of the Q601 possesses capacitance, and dynamic resistance, and so will also draw current. Typically PN junction capacitance is of the order of several pF - a tiny amount. This should not impact on the basic model I have used. The dynamic resistance however, may give rise to an increase in current at the R601/Vbe node.

There's potential drop across 10KΩ to be considered. The value of Vbe for this short duration of time begins at approximately 3.7v (but could be as low as ½ of this figure?) and at sometime close to ½ second or less, Q601 will be turned OFF.

When Q601 =ON, then Q601 Vce ~ 0 (probably <0.2v), then C602 (47uF) is drained of charge quickly - this largely 'empties' C602.

When Q601=OFF, the collector-emitter of Q601 can be considered as an open circuit, and soon capacitor C602 begins to become charged again via R604. If the charging of C602 were not to be interrupted, it can reach a potential close to 15 volts.

During the time when Q601=OFF, the voltage across C602 rises until at about 0.6v + V(R606) this process then switches Q602=ON, which 'shorts' the Vbe PN junction of Q603 and forces Q603=OFF. 

Finally, now that Q603=OFF, the relay circuit and in particular the 'plunger' (that forces a mechanical STOP), is activated.

Transistor Q603 effectively switches power transistor (not shown) Q604 ON and OFF by controlling the current through R608, and hence Vbe of Q604.

STOP Action & Activation of the Solenoid Plunger: 

 

Not yet illustrated, is the circuit of a relay which activates a plunger - this pulls the Stop or Play/RW/FF release bar

 

When Q601=ON, Q603=ON, Q604=ON, Relay cannot force an OFF state.

When Q601=OFF, Q603=OFF, Q604=OFF, Relay can invoke an OFF state.

 

When the take-up spool is moving at a sufficient angular rate, the reed switch perpetually opens and closes, and so the charge on capacitor C602 never develops a large enough voltage to eventually activate the relay and force a STOP state.

 

Motor Speed

 

Initially, motor speed consistency was problematic. My initial thoughts were that brush to commutator contact was partially failing.

 

Internally, the motor had many tiny carbon deposites, all of which were removed. The commutator was also cleaned and roughed up with very fine emery paper. The spring action of the brush's sprung copper arms was a little suspicious, so I pushed them more towards the centre before reassembly.

 

The electrolytic capacitor was replaced, and the commutator was sprayed with Servisol Super 10. 

 

Note: great care and thought must be made when diassembling and re-assembling the motor as the permanent magnet ('field magnet') will pull and jerk the rotor and may damage the brushes. 

 

So far motor speed is consistent, but I'm not 100% confident as wear on the brushes is obvious.

29/05/2022: Possibly more on this deck to follow at a later date.