OK,
The "Mickey Mouse Ears" ABR is as good of a "solution" as you could expect considering the company's financial and developmental resources.
If you look at the total number of SP/AMI/TD machines produced over its roughly 20 year life, multiply by the prices they actually got for their products and divide by 20, you won't get a very large number. Then look at their overhead; this wasn't a large business by any means.
I'm amazed at the precision / construction / build quality of their machines. Ingenious take on constant tape tension. All of which with simplest, practical "solutions". Believe they only had one machinist. Quality of the individual mechanical parts is simply amazing.
The ABR "works". Forget anything about a tight wind. Re wind / rewind the tape on your home machine after using - but for location work the Sp can do the job.
The Sp's used mostly 5/6 pin DIN (European standard) connectors - 7's had "twist lock" securing; 8's went to a "screw-in" barrel. All are still available but you have to do some looking - I can supply some Mouser numbers for the "screw in" types. Microphone inputs were originally DIN, but they went to "standard" 3-pin XLR's later on. All the SP machines (except for very few four channel) were designed/built for 2 channels. When sold for mono use, they used internal "jumpers" as necessary. Unfortunately they had a few different methods of doing this and you have to figure out which method for a particular machine if trying to convert a mono machine to stereo.
Because I have a bit of time, I''m "finally" doing a COMPLETE rebuild of a parts machine I got in trade. For any repair, I start by getting the transport going - down to bearing replacement if necessary. Determine this by holding/spinning the particular part next to my ear and LISTENING - motor also. Finished transport runs so quiet during play/record that you have to get your ear up to a few inches from the machine to hear anything mechanical. Doing a COMPLETE re-capping, which I've never done before, as getting to some of the caps is nearly impossible. See attached photo. Some caps are BETWEEN adjacent PC connectors. It's a 7 but since I have a "final" butterfly headblock, I'm going to make an "M" out of it. Upping the bias frequency will be the biggest challenge as there are no "M" bias assemblies left, and will have to "repurpose" a standard (SIZ) plug-in.
Most OEM parts are still available from Switzerland. Some NEW plug-in modules - at least the most -needed amplifiers are now available. Certain OEM machined parts are gone, but these don't break - if you're lucky.
Enough -bored you to death - Bye!