Mark Zacharias wrote:


Maybe we could share some "war stories" of cool repairs we have done in the
past.

I was called in unofficially to have a look at an X-ray machine in a
university crystallography lab, there was an intermittent fault which
shut it down after a few seconds of operation. The running time was
getting shorter and shorter and the manufacturers had given up on
finding the fault.

On the way there, I mentally ran through what I could remember about
X-ray machines (apart from the obvious dangers) and realised that most
of what I knew had come from reading my father's hand-written course
notes in the 1950s; they dated from when he was trained as an army
radiographerat the outbreak of WWII. I knew what an X-ray tube looked
like as a symbol, but what did one look like in reality?

On being introduced to the faulty machine, I glanced around the room and
saw a number of copper-and-glass objects on a shelf - and concluded that
they must be spare tubes. Luckily, the manufacturers had furnished a
full set of circuit diagrams and the lab had managed not to loose them,
so I knew what I was dealing with, even if I didn't initially know how
most of it worked. The circuits were all discrete components with
intermixed transistor, diode and relay logic.

By the end of the first day, I had gained a fair idea of how the power
supplies and safety circuits worked and had been instructed in the
necessary safety drill by the technician, so I was able to fire the
machine up and watch what happened. The fault showed up, but it all
happened so quickly that I wan't able to spot what was going on.

Luckily, on the morning of the second day, I happened to spot the tube
current meter flicker downwards and the voltmeter kick upwards just as
the fault occurred. Careful monitoring of the primary of the mains
transformer showed unstable mains voltage, which the control loop had
been over-compensating and then tripping out on over-voltage.

The cause was a burnt contact in the main contactor, so I stripped it
down and sandpapered the contacts, much to the amusement of the staff.

Fault cured - machine saved from the scrapheap.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk