In order to keep the speed constant with varying battery voltage, the drive motor is regulated using a centrifugal switch on the motor shaft, which controls a transistor which in turn switches the current to the transistor on and off. Although advanced for its time, this type of regulation proves to be less than perfect as the constant opening and closing of the regulator contacts cause wear and subsequent loss of regulator precision. Also, early versions of the MT5 (and MT4) have a design flaw in the regulator circuit causing loss of regulation after a while.
Primarily a battery-operated machine, it uses up batteries. Lots of batteries, especially during fast wind. Initially not having bought the original power supply, my father later had his machine modified to accept an external one, or the battery costs would have ruined him. (As a young boy I could never understand how a machine which ran on 8 batteries, which to someone who'd just learnt to count meant 12 volts, could run on a power supply rated only at 6 volts...)
Another thing, common with older Butoba models, is that there is no record interlock button. Press the single record button and the machine starts to record with no questions asked. With the record button being on the extreme left and the play button at the other end of the row of buttons, one quickly learns the difference between left and right!
A related issue is the fact that on most machines where the motor and flywheel are not constantly rotating (and for reasons of battery life, this includes most portable machines), it takes a few seconds for the machine to come up to its proper speed. This is especially true of the MT5 with its heavy flywheel. On this machine this has been solved by having it automatically enter pause mode as soon as the stop button is pressed, so that the machine is paused when record or play mode is subsequently engaged. After starting, the operator moves the pause button to the left after a couple of seconds, and the machine starts on a dime. The pause button can also be used during ordinary play, oddly enough however it is not latching.
The machine required a bit of renovation before it would operate properly. In particular, the rubber rim on the flywheel had gone hard, and had to be replaced. I used an O-ring for this, which seems to work quite well, although I think it is harder than the rubber originally used. In fact, I actually used the flywheel I'd saved from my father's old recorder, because the one that came with depicted MT5 had an odd circular crack in it, and I feared it would wobble when rotating. The motor speed was slightly off, owing to aging and the probably slightly different diameter O-ring compared to the origninal rubber rim, necessitating adjustment. Also, the motor suspension needed adjustment for the same reason. A bad capacitor in the regulator circuit caused induction of motor noise in the amplifier, and was replaced. Furthermore, not surprisingy, the felt brake pads were worn, which required replacement and consequent adjustment. The bias oscillator also required some adjustment before it would operated correctly.
As with most battery-operated machines, tape tension is a critical point - these machines have low tape tension in order to save on motor supply current and batteries. Changes in tape path friction due to aging and wear can cause tape slippage. The pinch roller rubber tends to go hard with age also causing slippage, or wandering of the tape across the tape path if the pinch roller is not parallel to the capstan. The drive clutch for the takeup reel in the MT5 is a spring belt which seems to either loose its tension over time or develope a smooth surface - at any rate, loosing takeup tension, necessitating replacement of the belt, or, as I tried, shortening it. It's quite fiddly to get it all to work properly.
One oddity is that the tape lifter lifting the tape from the heads during fast wind is missing; I'm not sure if it has been removed at some point or was never mounted at the factory.
Inside there are a couple of differences, the most important one being that at least early production runs of the MT4 lack the internal sub-chassis which in the MT5 supports the motor/flywheel assembly. Since the (heavy) flywheel on these machines is at the bottom of the machine, the 10cm long capstan shaft running through a support tube (with bearings at each end), any knocks to the machine have a tendency to bend the support tube and/or capstan shaft. I guess this was discovered after the MT4 had been in production for a while, the design then being updated. The downside of this is that it makes servicing more awkward as the sub-chassis tends to get in the way when servicing the amplifier PCB or adjusting the motor speed.
As for the exterior, I must say I think the casing of the MT4 is downright ugly. Apart from the mechanical problems, most likely sales where down because of the (lack of) design; especially the small "speaker grille" on the front does very little to enhance the appearance of the machine. The perforated plastic wrapping used on the MT5 greatly enhances the looks of this machine in my opinion, and underneath the perforations, the holes in the wooden case to let the sound out when the machine is operated with the speaker facing down are bigger, so the MT5 sounds almost as good when operated 'reels up' as when standing on its back. The small grille in the MT4 in contrast causes a marked loss of bass when the machine is operated 'reels up'.
Back to my Butoba page.