Yesterday I picked-up from a flea market a Panasonic RQ-P145, a pretty cheap auto-reverse walkman from late 80's (it still has the square design that I love so much). To my knowledge Panasonic walkman are pretty reliable, so I guessed it just needed a new belt and a bit of clean-up. Indeed the tape transport just needed a new belt and a general clean-up (not a single broken piece, the loosen belt was till in one piece, plastic still very strong and old grease is still soft, Sony do you hear that ?). Fixed the transport, the only problem left was noise from the motor itself. I suspected a dead capacitor, but again Matsushita caps are quite reliable so that was odd to me. None the less I pocked around with a big cap, trying to find what could silence the motor noise. And I have found what appears to be a solution, but I'm not sure I'm fixing the problem. This is the motor control electronics An this is what the IC3 does It is basically a comparator (VR4 sets the reference voltage to adjust motor voltage) which control the transistor Q1 to feed the correct voltage to the motor M. I found out that by replacing the red circled capacitor (1 uF) with a much bigger one (e.g. 22 uF) the motor noise is greatly reduced; it is still there, but it is below tape hiss thus no longer a problem. The role of that cap is to smooth the motor power up (IC3 needs to fill the capacitor before it can properly drive Q1) and to filter the control signal for Q1. So it make sense that a bigger cap will reduce any fluctuation on the signal on pin 7, thus decreasing motor noise. But is that an actual fix or I'm just hiding the problem of an aged motor ? I do not have an ESR meter, but the 1 uF cap I removed was not leaking, it appeared fine and it measured right on the spot ( 1.07 uF).
This is a fix in the sense that it will make the walkman usable again. But the root cause of this motor hum is an aged motor as you already suggested. What I would do apart from changing the capacitor is to re-lubricate the motor in order to improve its lifespan as much as possible. It's why, in my opinion, motors should be lubricated when the belt is changed. C34 along with the NPN transistor after it act as a capacitance multiplier. This is a secondary function to the main one of driving Q1's base. The capacitance "seen" on the collector is multiplied by the gain of the transistor, thus providing the effect of a much larger capacitor (which would be impractical). So assuming a capacitor of 1μF on the base and a gain of 50, when we look from the collector's point of view, we will see 50μF. This mod will work on any motor drive circuit where one of the transistor's base is available externally. So capacitor could have been placed on pin 7 for example (if pin 6 would not exist), but a much larger value would have been needed for the same effect. In regard to the capacitor itself, don't think it's the problem since the hum didn't completely vanish even with a new and much larger capacitor.
Re-lubricating the motor bearing is part of the general clean-up procedure: I have added a drop of clock oil in the metal bearing under the belt pulley, but the bearing on the other side (witch is made of plastic) is inaccessible, I would have to dismantle the motor which is more work that this walkman is worth. I paid it few euros and I just wanted to have it run without buying parts; the belt I have used is one belt I had laying around for years, but it fits properly. I'm interested in the motor fix because I have another Sony WM-22 which has the same problem, but there the motor is basically new, I bought the unit as NOS.
In the application circuit from the AN6612 datasheet the capacitor on pin 7 is 0.47μF. Looking at some other devices that use the AN6612 this value varies between 1μF and 4.7μF. The capacitance value will depend on motor current draw and the current gain of the drive transistor: - higher motor current will require a higher capacitance to obtain the same filtering; - a drive transistor with lower current gain will require higher capacitance to obtain the same filtering; So in the end, it's not unreasonable to simply put a 22μF in there and call it a day. In regard to the motor, if it's simple to disassemble, it's worth doing it even if it won't make improvements in regard to the hum. Please post some pictures with back of the motor as I'm not familiar with this particular one.
A tell-tale sign that something is not fully right with the motor (or the transport) is the fact that with its original setting, tape was running slow, I had to increase the voltage to have correct speed. I tried to run the motor on its own with a battery (it is rated at 1.7V) and indeed is not smooth as it should be. But I was lazy and did not want to take the motor apart and risk damaging it (I had mixed experience with taking apart small motor). This walkman is just a collector item, I will not be using it as a daily driver; it has a passive Equaliser that I do not like at all as it basically just decreases output volume.
A couple of pics of the unit. It is a reasonably cheap entry level walkman from late 80's, the most appealing aspect is the square boxy look typical of that period (before the more "curvy" design of early 90's) No dolby B, nothing too sophisticated, the PCB has a lot of free space and assembly quality is mediocre at best (wire solders are very hastily made). It is indeed made in Taiwan, not in Japan where they would assemble only higher level models.
Indeed if the motor doesn't run smoothly, it means there is increased friction which in turn will increase the motor current. And yes, the tape speed should not require any major adjustment if the correct belt is fitted, which I assume it is. In your case, if device is not to be used regularly it's probably better to leave it as is. There is a chance that it will get better with use, but there's no guarantee. As for the walkman itself, I like it apart from that EQ. Would have preffered a big window to see the cassette instead. I see the back of the motor in the second picture, it's the type that can be taken apart but it's not as easy as the larger ones. For reference (in case anyone reading this wants to service a motor), the steps I follow when opening such a motor are the following: 1. Take the motor pulley out. If it's plastic simply pull, if it's brass heat it up with a soldering iron so the metal exapands and then take it out with a pair of tweezers. 2. Bend back any metal tabs that are holding the back cover 3. With a finger on the back cover, push the shaft inwards making sure the rotor and the back plate remain one piece. Otherwise there is a risk to damage the brushes, especially on small motors with very fine brushes. 4. With a fine pair of tweezers, retract both brushes from the commutator and remove the rotor. Pay careful attention as it's not uncommon to be a washer beneath the commutator which has larger diameter than the commutator itself. This washer usually remains stuck to the rotor and has the potential to damage brushes if they're not retracted far enough. 5. Clean everything with IPA and re-lubricate the botton bearing with watch oil. Commutator can also be lubricated with special grease. 6. Retract the brushes and re-install the rotor onto the back plate. 7. Re-install the rotor/back-plate assembly into the case, holding a finger above the top bushing in order to prevent rotor from separating from the back plate. This step is very important, as the stator magnets will have a tendency to pull the rotor out of the back plate. 8. Bend the holding tabs back or put a couple of drops of super glue insetad. 9. Lubricate the top bearing and re-install the pulley. If pulley is brass, re-install the pulley first and add oil when the motor is at room temperature.
Today I had a bit of spare time so I decided to open it up again; if you are interested here a couple of pics of the tape transport I re-added a bit of grease to silence the central gear: it originally had grease, but I remove it while cleaning thinking it was not needed. As you anticipated, with use the transport is improving, today I re-adjusted the speed and it was indeed running a bit faster. Anyway i did not dare to take apart the motor, not really worth the time. Very common entry level tape transport used on many Panasonic walkman's from late 80's to mid 90's. I have another walkman with the same transport but higher quality brass flywheels and indeed it sounds better.
