VC.
Sorry about the belated reply – never enough time to do the necessary.
I hope you don't get troubles with the electronics of your mill. I got involved with mine maybe 10 years ago now, when I bought a Sieg type lathe and mill from a German importer (I lived Germany then). At first I had little work for the miller, but I managed to burn out the lathe control circuit in a matter of days. I eventually replaced the output drivers (I think this may be what you mean when you refer to the devices bolted to a heatsink). However, they went 'phut' sp many times that I became thoroughly sick of it. I also found out that the mill circuit was almost identical (there were differences in speed compensation circuitry, but they were both built on identical PCBs, with some detail changes – identical output stages.
There have been multiple threads on this issue, and the clearest thing which has emerged from them is that there seems to be several different versions of the controller/motor combination. Predominantly there seems to be what I call the 'old' version, which has two parallel output drivers on heatsinks, and a small daughterboard attached to the main PCB. Someone in this forum posted a circuit diagram and photos of this some time time ago, but beware, there are differences between the circuit and actual PCBs. I obtained a later controller from a German source, using a single out put stage – much better design, and built with SMD components. This was very good, but had no adjustments, and I found that it would not start from zero RPM – the (mill) motor ran at quite RPM when the start button was pressed.
I have also seen other controller variants fitted to these machines. I guess Ketan Swali could also supply some info on these controllers – I believe his company supplies replacements, and I know he has commented on this in the past.
I eventually got sick of the whole story – I bought the machines to do work, not to have to spend my time repairing or modifying them, so I built a foldback current-limited controller. This simply monitors the current taken by the motor, and when it begins to rise sharply (if the motor is overloaded or stalled), it immediatelyreduces current supplied to zero. I have never had a controller problem since this was done.
Whilst I agree to some extent with Ketans statement that these are lightweight machines. If you overload them then you can expect problems, but I would point out that the electronics (and sometimes the gears) apparently fail in normal use – look back through old threads in this and other forums. I believe there is a requirement that articles sold for public consumption should be 'fit for purpose', and it would seem that some variants of these machines are not fit for purpose for one reason or another. I think this is demonstrated by the number of posts asking for help to fix a problem, or the number of websites offering 'updates' to workaround the problem. Whether the machine costs 350 pounds or 3500 pounds (and I wish I had money like that to spare for my hobbies!), while I agree that it is impossible to allow for machine abuse, it should still do what it is sold for.