by Andrew Bayfield
I ran a 16 Track version of one of these in my studio for fifteen years starting in the mid 1980s. Here's their strengths and weaknesses:
They are a hard working machine. I still have it but it's no longer operable. I keep it though, just in case ( ... see below).
The DC servo capstan is the most agile I have ever seen outside an AMPEX VPR3 or something. With Time Code on track 16 and connected to an Audio Kinetics Pacer synchroniser it would make a better than 35us lock to an external Time Code source (like a VTR or another audio machine) in slightly less than 2 seconds from a standing start. My DAW can't do that.
The speed of spooling is fantastic. New visitors who were used to Ampex MM1100 etc. would stand back for fear of something flying apart. (in never did). It would spool from end to end of a 2500' reel in one minute and smoothly auto locate to a standing stop. (XT24).
The other great thing was (is) the ergonomics. The remote control was easy to use and sessions were fun. The auto sync switching is perfect. The drop in speed is quite fast. There is a gap left when exiting record but 3M minimised this by placing the record and erase head as close as possible on the LHS of the isoloop. At 30IPS it was rarely a problem.
To achieve this lock to code performance, I made an outrigger board that rectified the wildly surging capstan control voltage and used it to apply more take-up or supply tension as required to prevent the machine from throwing a loop of tape.
I also did have to remake the two stepped pinch rollers and had the capstan motor's brushes replaced and the commutator turned. When reassembling this there is a cylindrical motor pulley that MUST be remade as well. This needs to be pressed onto the capstan motor shaft and then ground in situ to maintain the published W&F spec. The runout of this pulley on the motor with new bearings was <1/10,000". My machine came in comfortably under 0.03%RMS.
The ceramic capstan is about 40 mm in diameter and sits at the top of the isoloop. My machine would click randomly in replay. This turned out to be static discharge, the surface of the capstan being an insulator. This became a problem after the capstan bearings were changed. You can solve this (as I did) by earthing the bottom of the capstan shaft to the deck with a carbon brush (like the sort of thing you'd find in an old car spark distributor).
The two design flaws in this machine both compromise head to tape contact:
• the 7 degree tape wrap over each head, as opposed to the about 15 degree wrap found on most anything else. This was presumably done to save head wear and the design engineers might have thought the tape tension they achieved with their differential isoloop system ("differential" as the exit pinch roller drags tape out of the loop very slightly faster than the entrance pinch roller supplies it) meant they could get away with reduced wrap.
• the turnaround roller at the bottom of the isoloop needs to be vertical to very fine tolerances, especially on a 2" machine. Pretty soon when loading a reel of tame someone will strike the roller and bend the spigot. This immediately reduces tension on the top tracks and increases it on the bottom.
The above two effects mean the stability of recording and reproduction at the top end of the frequency band is very poor unless everything is working for you. An arthritic old Ampex with well worn (but not worn out) heads will recover 15kHz at 0VU with the meter dithering about maybe 2 dB. An old 3M M79 would be lucky to hold this tone in a 6 or 8dB window.
My power supply eventually failed on switch on one day. The pass transistors (2N3055 or similar from memory) went short circuit and the 24V rail went to an unregulated 40V. This damaged the system control card (but not the signal electronics). I have fixed the power supply but the system control card awaits my attention 20 years on.
On the whole one of the good if not great machines of the era of tape. Not on balance in the same league as Studer or some Otari machines but in selected areas out stripping them all. I was well glad to have run one commercially. Considering the machine was at least 15 years old when I purchased it I have to give it high marks (the machine has a 1969 date stamp in it as I recall).
Lastly if you are thinking about using one, I haven't found replacement heads to be either cheap or plentiful.