Harry Bloomfield formulated on Tuesday :
Following on from problems with the Danfoss HSA 3 valve actuator earlier in
the week, my heating system is now running fine using the compatible Sunvic
SD 2701, which I had in as a spare.

I have just got around to pulling the 2 year old Danfoss HSA 3 unit apart, to
see if it might be repairable for use as a spare and this is what I found -

Internally and externally it was like new, no obvious signs of any wear and
tear at all. The motor also checks out fine. The only problem I could find
was no continuity between the centre (N/O) contact and the common of both of
the switches, when the button is operated. The N/C contacts are fine.

It just seems a little odd that both of the micro switches have suffered the
same failure mode simultaneously. The items fitted are SAIA rated at 250v
6amp.

So its off to Maplin for some replacement microswitches.

In the interests of experiment, I have now got to the problem.

So from the top.....

The N/O contacts on both microswitches had burnt out. I picked up 2x
replacements plus 2x extra spares from Maplin yesterday, then fitted
them in place of the failed ones. Under the plastic half round plate
which holds the microswitchs, is a brass cam plate with has at its
centre a plastic hub which has the return spring wrapped around it.
After fitting new switches, the N/O ones were still not making
reliably.

I drilled a small hole in the half round plate, one directly above the
operating button of each switch, so I could see what was happening.
What I found was that the brass cam ring is just barely low enough to
catch the switch buttons - they are not able to reliably push them in.
Usually the cams are riding over the edge of the buttons. To hit the
buttons square on, the brass cam plate needs to be about 1.5mm lower.
The new microswitches are physically identical to the old ones, so I
strongly suspect that the N/O contact were burnt out as a direct result
of the poor alignment between the cam and microswitch operating
buttons.

My next trick will be to skim 1.5mm off the bottom of the hub and make
a 1.5mm thick washer to get the cam to properly align with the
switches. My lathe is tied up with another project at the moment, so
this will have to wait a while.

You would though think, that manufacturers might spot these simple
problems in the early design stages.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk