Pretty easy to work on, fully published schematics. Fix four so far, most of the problems were in the H-Bridge MOSFETS shorting out. I used a UDB1105S Mini Function generator, definitely a must for servo systems. Made troubleshooting a lot easier than trying to use a regular function generator. The UDB1105S is around $60 the lowest cost DDS function generator I've seen. It takes a bit to learn curve to step thought the menus but once you get past the learning curve it is very easy to use.
The H- Bridge uses two parallel MOSFET per channel (4 Channel total) to get the current high enough for the SIM motors. MOSFET when they fail they will usually short (most of the time) there is also a internal diode to the power MOSFET to dissipate inductive kick from the large motors, best way to check the MOSFET is out of circuit with an ohm meter, I just check it in circuit and take out the shorted pair. I've also noticed that they have different part numbers on the JAGS depending on model, so keep an eye on it when swapping (steal from Paul to pay Pete) when in a bind. If you can order new from a reputable parts supplier like mouser or digikey.
So far 4 out of 6 were MOSFET shorts. One JAG was the SMPS for the 5 volt rail, impossible to solder with a regular iron. I will have to use the IR SMT reflow on that one, there is a heat sink connection on the bottom and there are no VIAs to solder on the copper side. I bypassed the 5 volt SMPS with a bench PS and it came back to life so it's definitely the IC. The last JAG I could not find the problem, totally dead.
BTW you need to install the jumpers on the limit switch. Both vertical. the manaul and schmatic are here http://www.ti.com/lit/ug/spmu130c/spmu130c.pdf
Next time I get a few more of these I will post a trouble shooting video, I had to get them out ASAP so I could not take a video this time.
UDB1105S Mini Function generator and JAG