I found a very dirty looking Fostex X15 multitracker (Four track cassette deck) for sale online. It was listed as spares or repair: would not play tapes. I snapped it up for a great price!
The deck is from 1983. It will record two tracks at once. Mixdown playback 4 tracks at once. Has Dolby A noise reduction (always On) It has no Sync capability.
First it needed a CLEAN! I soaked the plastic parts in hot soapy water for a while and brushed them with the toothbrush. This really helped, though the grime was very well stuck to the plastic.
The unit would stay in REC mode even when switched out of REC mode. The REC/PB switches on the PCB needed aligning properly. This was easy to perform although tricky to get the little hook springs back in place!
Rewind and Fast forward had no issues, though going into REC or PB mode sometimes would not take: The tape transport was trapped from moving freely by the wiring loom to the heads. Moving the trapped wiring caused the issue to be fixed (see the wires to the left of the Erase head, near the bottom left of the cassette)
Basic procedures:
Cleaning, Changed the belts, oiled some mechanisms, realigned some switches, cleaned the heads with alcohol, degaussed, re-soldered some loose wiring, reassembled, tested.
Special work:
The red plastic cassette door was covered with shallow scratches. It is the center-piece of the design of the unit. To make the cassette door look lovely again, I sanded it with 800,1000,1200 wet and dry sandpaper, then metal polish to return to clear. This always takes a while, though it is very rewarding when the polish makes the plastic clear again! I could have gone even further with polishing, though then it would not match the rest of the deck, and make that part look worse!
Advanced:
Modified the speed control circuit to allow the tape to be run faster. The faster the tape runs, the higher the quality of the recording. I replaced a trimpot with a higher value in the speed control circuit (double original value). The unit now runs much faster than previously. It is also much more stable. I'm not sure at what speed it runs now. It's unimportant really, faster will get a better recording. This is a low end deck, every little helps!
If I keep the device I will fit a PWM controller board and have a switch to select original motor speed control vs PWM control. The new PWM speed knob will fit close to the analogue tape counter.
Here is a gallery, and a video of the machine playing back some material:
I've realised that the pots were on incorrectly, PAN and GAIN were switched! Here's what they should look like: (No wonder the PAN was weird) The pots are all a bit weird though, they use the old style sometimes found still on compressors, down is up!
Here's a video discussing the above and showing the cassette door just prior to polishing: