Miele Vacuum HEPA TT 2000 / S591 (?) - pulsing motor, fixed.

[EHT]

I just fixed this Miele vac; i thought worth reporting because it's worth fixing IMO. The machine is very well engineered and quite easy to take apart. It is also possible to buy various spares, although annoyingly I can't find the front panel which has broken (hence gaffer tape in pic..). The model number plate has worn off on mine but it seems like it is an S591 or similar. This model has an electronic motor speed controller.

The fault on mine was that at more than about 1/3 of max speed it would pulse on/off at about 1Hz. To start with I thought it could be physically blocked but turns out it was the controller board.

The controller seems to be an ST ST7LITE, however, in my model it is an 18-pin DIP so not exactly the same as this one but the circuit is pretty similar to the application circuit on p9.

The motor is regulated by a BTB16 600BW triac mounted on a small PCB on the motor. Failure of this is apparently common. I expected a failure of the TRIAC would make the unit completely dead; i checked it anyway using a bench PSU, LED and resistor on the gate. It was fine. New motors are available but of course not economically viable as a complete unit.

Next, with ref to the schematic on p9, I found the zener in place of ZD1 is a 24V one. I checked with a scope (isolated scopemeter) and found the voltage on the zener was not 24V, and when the unit started pulsing it was dropping quite low making the VCC supply to the micro go to low, then it would presumably brown-out and restart.

Fault was the 220n X2 cap providing the power, it was reading about 100n. Due to the way it was mounted it was necessary to find the exact same part, which is KEMET Part Number: R46KN322040H1M.

There are some showing how to disassemble Miele vacs.