[Mellotronists] Chamberlin M1 guts
2003-04-18 by Gene Stopp
And now a report from our embedded reporter in Gene's garage: The pictures in Frank's Mellotron book, Mark Vail's "Vintage Synthesizers", and www.mellotron.com pretty much cover what it looks like. (Same pictures at those three places, by the way.) In addition there is a good under-the-lid shot at the web site. The M1 I got does not have the remote unit and therefore does not have that extra add-on control box or external wiring harnesses. The cabinet is inexplicably short. I'm not sure what the intention was, to put it on a low table or maybe on top of the road case that it comes in, or what. Perhaps it was done that way to avoid blatantly copying the M400? Much in the way that the Mellotron M400 is really one assembly that drops into the wooden cabinet, inside of the M1 there is an assembly which is pretty much the entire machine. It exists in the center of the unit with airspace on all four sides. One major difference from the M400 is that the two motors are not part of this assembly but rather are mounted separately and independently. Yes I said two motors - one is a fixed speed AC motor (looking much like a reel motor from an old reel-to-reel). This has a pulley with a belt over it, to the big flywheel on the capstan (consult the picture at Mellotron.com), and a spring-loaded idler pulley near the motor. The other motor is on the right side of the machine (as you look at the keyboard) and has four skinny rubber belts running up to a fairly small (compared to the other one) pulley on a shaft that runs along the length of the keyboard under the keys themselves. This shaft holds the reels where the tapes are stored. The shaft spins in the direction that rewinds the tapes after a key is let up (opposite of capstan rotation), and there is one reel per tape (total of 35). The reels slip on the spinning shaft, with enough friction to be moved by the shaft if the tape is not taut. There's a setscrew in each reel to set this friction point but the tape has to be off of it (i.e. in the take-up bin) to get to it. So this is how the M1 returns the tapes, rather than using springs. Very compact, but it seems that this method takes longer to rewind the entire tape than the M400 method. This is a fixed-speed return, whereas the M400 snaps the tapes back however fast they will travel under spring tension. All over inside of this thing are paper labels created on a typewriter, taped on to various surfaces, containing all kinds of useful information about motor oiling, tape threading, take-up reel friction adjustment, pinch roller and felt pad adjustment, and so on. There is a tape-threading tool which consists of a long skinny brass "tape" about 6" long. There is an allen wrench behind a little clip which is used to adjust the take-up reel friction setscrews. There is also an extra motor pulley. On top of the main assembly sits the keyboard, which is mounted on a square aluminium frame. This is a wooden 35-note keyboard, much like the Mellotron, except that the low "G" is a real low "G" in that it has the cutout in it for the F-sharp rather than being like a "C" in shape. The cheekblock next to the low "G" has the corresponding shape to match. Interestingly, the high "F" has a straight side where the F-sharp would be, so all I can say is go figure. Again the pictures on the Mellotron.com site show this clearly. The keyboard can be removed by unscrewing four sheet metal screws which mount it to the main assembly. No knurled knobs, get your screwdriver buddy. The track selector at the right end needs to come out at this time as well, since it covers the fourth screw. As soon as you unscrew the keyboard, it pops up from spring pressure below. When you lift it off, you will see that there are no pinch rollers or felt pads on the keys, just the front and back adjustment screw shafts poking downwards. The pinch rollers and felt pads are attached to spring-metal "fingers" that are mounted to a bar that runs across the front of the chassis. Removing this bar exposes the tapes. At 1/2" each, the tapes are almost touching each other with AGO key spacing. There are comb teeth separating and aligning the tapes, but they are very skinny. The tapes appear to be riveted to both the take-up reel and the anchor bar at the rear of the chassis. I have not yet examined these rivets to see if they are perhaps simple press-fit things that can be removed (hopefully), or if they are the kind you need to drill out so don't even go there and ship the whole thing back to Upland. Like that's gonna happen.... The heads in the headblock are 2-track heads. For any given head each gap goes to a separate output, so there are 2 outputs. The track selector has six positions: 1 and 3 2 and 4 3 and 5 4 and 6 5 and 7 6 and 8 Output electronics is under the left-hand cheekblock. Four pots, tone and volume for both outputs. On/off switches for each output also. Under the cheekblock is a linebox, inside of which is the circuit board with some passives and a single chip (haven't ID'd it yet). The circuit board is made out of vectorboard. Definitely not a mass-production effort here. On the right-hand cheekblock is the track selector lever (once again seen at the Mellotron.com picture), power switch, and pitch control. Hey wait didn't I say that the capstan was driven by an AC motor? Yes indeed, behind the pitch pot there is another piece of vectorboard with some electronics on it (including a 3-terminal SCR-type thingey) so there is definitely some AC waveform monkey business going on here. The pitch pot behavior seems to be stable pitch for some of the pot's rotation, then a slight drop off, then the motor drops to half-speed or so and then stalls. So I suppose that the motor is intended to be slightly sharp and the pot set somewhere in the "slight drop off" portion. This theory is reinforced by the existence of the spare motor pulley with a note next to it that says "use this pulley for A-440". So, tuning is set by pulley size, eh? I've not yet been able to get a good listen to the sounds yet. The sounds on this tape set is an assortment of strings, horns, flute, and vibes. Standard set? Don't know. When I started to work on this machine there were two main problems - first, all the keys were out of adjustment so I could not get a well-played note anywhere. Second, the electronics seems to be gummed up or otherwise dirty so there is a high-pitched squeal if the volume is turned up at all. Long road ahead... I'll try to take pictures along the way. Best Regards, - Gene