The syncron motor burnt out in my old V4073A Honeywell actuator so I replaced the whole actuator with another one that works well. In the end the problem also proved to be deposits around the valve facing and stiff operation of the valve, so I replaced the top plate after part draining the system and a careful cleanup around the valve seating.
Rather than throw away the old 3 way actuator (V4073A), I decided to refurbish it. Several problems arose which have not been covered in any of these forums or advice on the internet, so I thought I would write up my findings.
1. Remove old motor and pcb. The motor was high resistance and slightly rusty so discarded. The other 5 components are a) 1N4007 diode b) 12K ohm wirewound resistor c) 270K ohm resistor. d) two microswitches operated by a centre cam. The top microswitch is meant to operate in the centre position (CH+HW) and the lower microswitch just before the valve is fully open (CH only).
The 2 resistors and diode tested good, but the microswitches were leaky, perhaps again due to damp. I replaced the microswitches with near equivalents rated at 240v and bent the actuator arms to match the originals.
Note - basic electronic skills are needed for this, and it is assumed that readers are familiar with the theory of operation which has been described elsewhere.
2. Clean the mechanical parts. WD40 (could be GT85) was sprayed liberally over the centre bearing components when holding the body sideways over a cloth. All metal parts were then swabbed as best as I could and dried to remove all old dirt and grease.
3. Replace motor.
Because the actuator is just a spare, I replaced it with a OEM synchronous motor so I could check operation and repair without too much cost. This was rated 230 volts, 4 rpm
4. Check operation
After reassembly, the actuator opened the motorized valve and after a little adjustment of contact positions the microswitches closed properly.
However, the original fault, that the MV stuck in the centre position, remained. The springs returned the valve from the fully open position, but when the valve was in the mid-position (ie white wire energised only) it tended to remain there when power was removed. This of course would mean that with the controller on hot water only, the radiators would get hot, and the hot water would be too hot.
5. Actuator and Valve Stuck in Central Position - Fix
Inspection and manual operation of the valve seemed to indicate that everything was working correctly, but the tendency to stick in the central position still remained.
Eventually I found that the manual lever was bent up slightly, causing extra friction both on the lever to chassis contact and the centre bearing.
Slight bending of the lever to a central position in the slot, and a little silicon grease on the lever and mechanism, fixed the problem.
6. Motor overheating
The replacement OEM synchronous motor is rated at 230v.. No wonder it overheats when the UK ~250v mains voltage is applied for a long time, as is normal with operation of the valve. The addition of a 6 watt wirewound resistor of 730 ohm in series with the motor (3 x 2 watt 2.2K in parallel) drops the voltage by about 21volts, or 228v ac on the motor which is within its rating. The new resistor is mounted by the connector with short leads so no shorts can occur. Time will tell if this modification increases the longevity of the replacement, but it is a possible solution to the complaint from many people that the replacement motors burn out quickly.
IMPORTANT NOTE
However, again I repeat that basic electrical and electronic skills are needed to fit and test the repairs. The unit has live mains voltage present.
