Circuit Bending a Fostex 4 Track
I’ve been messing around with creating music using multitrack tape loops and so when someone offered me a free 4 track I jumped at the chance to take it.
After a brief play with it, I noticed an immediate problem: the tape plays at double speed, and the pitch control doesn’t slow it down enough. This makes it a good machine to record on, higher speed means more tape passes the record head which means better recording quality, but when you’re playing tape loops that are only 8inches in length, you want the tape moving as slowly as possible, and you’re probably not really caring about audio fidelity.
I’ve opened portable cassette players and there’s usually a trimpot inside to control the speed of the tape motor by simply restricting the voltage to the motor, so I opened the XR-5 up to see if there was a similar mechanism inside.
The long back rod at the top left is the pitch control and below it is a trimpot that acted as a fine control, but even at its extremities it didn’t adjust it very much.
Checking the resistances of both the pitch control and the trimpot it turned out that the lower the resistance the slower the speed and that the lowest speed was already at 0Ω on both. My hope that I could just replace the trimpot with one of a higher resistance wasn’t going to work, and there must be something more complicated happening in this machine than the small portable players.
Using a tape loop with a known constant pitch on it I was able to check what the actual pitch I was getting from the 4-track. Initially, with the pitch control and trim pot turned to their lowest, an A tone was playing almost an octave higher at F♯.
I took a look at the tape deck’s motor to see if it revealed anything and to my surprise it had 4 wires connecting it to the circuitboard instead of the two I expected. Two wires were red and black and connected to terminals marked (+) and (-) respectively, but the other two terminals were labelled (A) and (B). Doing some research I learned that these wires are for the speed governor. Usually a motor has a governor system inside the shell which tries to maintain a constant speed but these terminals allow for devices which require more presice control over the speed.
Squinting at the underside of the motor I could see that (A) and (B) were connected to the brown and purple wires that connect to pins 8 and 9 of terminal J.
I took some readings with my multimeter across the various terminals at the different speeds.
The voltage difference between the (-) and the other terminals at the different speeds were:
(+): 15.3V (slow) 15.3V (normal) 15.3V (fast)
(A): 14.4V (slow) 14.35V (normal) 14.32V (fast)
(B): 11.5V (slow) 10.55V (normal) 9.71V (fast)
As expected (+) was the constant 15V feed to the motor. (A) varied slightly with respect to the speed, but it was clear that (B) was the main speed control terminal. Looking at the values it seems that the closer B is to A, the slower the motor spins, which then makes sense as to why setting the pitch and trimpot to 0Ω resistance is the slowest setting.
I was able to find the service manual for the XR-5 online, and it contained the full schematics. The image to the left is the section that deals with the J terminal block and we can see that pin 8 and 9 are wired in a loop with the pitch control (V901) and the trimpot (V902). With these set at 0Ω, there are two other resistors that affect the input to the motor (M), R915 and R917. You can see these small resistors marked beside the terminal block on the board.
I decided to short R915 to see how that might affect the speed. My thinking was that it might make the voltage at (B) closer to the voltage at (A) and according to the measurements I’d taken earlier, the closer dVB is to dVA the slower the motor spins.
Using some hookup wire jammed into the holes to short R915 did indeed lower the tape speed. The guitar tuner registered my reference pitch as a C♯, 2 tones above the A that I wanted, so it was closer, but still not enough.
I took a chance and used some more hookup wire to short R917 and bingo, the pitch was now registered as G a single tone below the A reference pitch, which was close enough to use the trimpot to bring it back up to the correct pitch.
All I needed to do now was to solder these shorts in permanently.
With some terrible solder skills, I shorted R915 with the hookup wire on the underside of the board because there was lots of space around it, but R917 was too close to other components to short that way, so I removed it completely and put a small jumper cable in its place.
With the shorts in place it was time to put the XR-5 back together, and put it in place beside its big brother ready to make some noise.
It occurred to me as I was doing this, that a future hack might be to replace one of the shorts with a light dependent resistor, possibly hooked up to a switch so that it could add some more random tape flutter (because who doesn’t like tape flutter effects?) in response to the light level but I didn’t have one of those in by box of components, so I’ll maybe do it at a later date.
Thanks to http://synthhacker.blogspot.co.uk/2014/03/portastudio-voltage-control-of-speed.html which explained how the motor was working with the speed governor, and to aoVI at http://www.ambientonline.org for linking me to it.