I've heard/seen that certain members have been having speed/torque problems with their decks. I looked in the manual(s) I have and it appears that all that's there are schematics; with no mention of how things work or any troubleshooting tips. So I spent some time "doping out" the circuitry and the resulting "analysis" appears below. Unfortunately, any intense troubleshooting will require certain test equipment, most notably a DVM and oscilloscope and the ability to use and interpret the measurement results. A modicum of circuit analysis ability also helps.
Looking at the schematics reveal that there are similarities between the operation of both the capstan and reel motors. For both sections there is a (separate) 50kHz oscillator (TR931 and 932 for the capstan) and TR791 and 792 for the reel) that generate a 50Khz squarewave? into the ?position detector coil primary? of the motor windings. This waveshape is sensed in a three-phase manner by the ?position detector coils secondary? and fed to a series of transistors which then generate three phase control signals to the motor armature coils. One thought to troubleshoot these sections is to put a scope on the output of the 50Khz oscillators (collector of TR792 for the reel motors ? collector of TR 931 for the capstan motor) and see if they are working. You could also look at the output signals produced by the three sets of armature coil driver transistors (A1, A2 and A3) to see if the waveshapes ?look? similar - if say one is "missing, then one or more of its transistors is probably bad. Unfortunately, because the circuitry appears to be working in a three-phase manner, to see if the ?correct? phase relationships are being maintained, you?d have to use a multi-channel scope to be able to view all three phases simultaneously. Actually, it makes sense that if the motors turn at all, this portion of the circuitry should be OK and a speed control problem is undoubtedly somewhere else.
The control signals for what I?ll call the motor ?drive circuits? appear to come into TR947 for the capstan motor; TR747 for the supply reel motor and TR748 for the takeup reel motor and may be ?viewed? on test points TP911 and 912 ? capstan motor; Is ? supply motor and It- takeup motor. You should see what I?d call ?similar type? of signal here (don?t know the actual waveshape), but a LACK of a signal signifies something wrong ?upstream? allowing the motor to run in an "uncontrolled" manner.
For the reel motors; when playing/recording a tape, the control signals come from the circuitry on the right hand side of the schematic which is labeled ?torque control?. As both the supply and take up motor control circuitry seems to be identical, I?ll describe the supply portion. A signal from Phase 2-2 of the position detection coil is detected and amplified by transistors TR701 and 702 and sent to TR801 which is called the ?Waveform Shaping? transistor. This signal is somehow compared with a current corresponding to the selected speed (through switch S3) and a resulting what I?ll call ?torque control signal? coming from TR817 is fed to the ?drive circuits? mentioned above. To troubleshoot a potential problem in this circuitry, I?d compare signals between the upper supply motor section and the lower take up motor section ? they should ?look? the same. If you lose a signal somewhere, proceed "upstream" till you find it and see what component blocks it's passage. In thinking about this, it makes sense that if either the supply or take up motor control circuit has a problem, then the tape reeling would get all screwed up, with the "bad" motor running (or attempting to run) at a totally different speed than the other.
The speed of the capstan motor looks to be ?locked? to a crystal-controlled oscillator called IC903. Apparently a sawtooth-shaped waveform is available "downstream" at TP907 that should have a period of 256hz if S3 is switched to 15ips (128hz for 7.5 ips). S2 (before TP907) selects the output from either the crystal oscillator or a variable-frequency oscillator controlled by the (front panel) pitch control, so you should see a similar-shaped signal regardless of S2's position if both oscillators are working properly. The ?actual? speed of the capstan motor appears to be sensed by an internal coil labeled FG which is then amplified, shaped and divided by the circuitry on the upper left-hand and center portion of the schematic. If this circuitry is working correctly, a ?higher? frequency squarewave? should be available at TP903 (frequencies aren?t listed) and a ?lower? frequency which should be ? of the ?higher? frequency should be available at TR902 (misprint) ? schematic says 64 hz, whatever that means. The ?actual? and ?desired? frequency signals appear to be compared in a differential amp circuit at the bottom right of the schematic and a control signal fed to the motor control transistors via TR947 and available for viewing at TP912 as mentioned above. Again, for troubleshooting I?d look for an absence of a ?meaningful? signal at the various testpoints and if you spot something untoward look ?upstream" from there.
Of course you should also first check that the various circuits are getting the correct power supply voltages. All the circuits use 5, 15 and 21 volts from the main power supply; with the 21 volts used in the motor drive circuits, the 15 volts used in what I?ll call the analog control circuits and the 5 volts used in what I?ll call the digital control circuits. Makes sense that a failure of the 21 volts should not allow a motor to operate at all. A failure of either the 5 or 15 volts could allow a motor to operate but certainly in an uncontrolled manner.
Hope this helps.
Anyone with more info, corrections to the above or troubleshooting tips PLEASE contribute.
Charles
