It's amazing what you can get away with

Thirty-two years ago I made a little unit that switched a couple of CCTV
feeds and a floodlight. It was controlled by a 'normally on' 12V feed
from a sensor which controlled a 12V relay that in turn controlled three
timer relays. Two switched the video, which went to a monitor. The third
switched a floodlight. At the time I was surprised that ordinary relays
could pass baseband video with no apparent ill effects. But anyway, last
week, after all those years, it was decided that a manual over-ride of
the floodlight switching would be useful. The diecast box was opened for
the first time since installation and I peered inside. All the external
connections were via a long 'choc block' strip, and just above that was
a card with the connection details on it. These details were obscured by
the cables, so I gently bent one cable with the tip of a screwdriver. At
this the relays clicked and the timing sequences started. I drew back my
screwdriver in surprise. It turned out that the screw in the choc block
for one of the two sensor leads had never been tightened, and contact
was hit and miss. This piece of kit has functioned faultlessly since 1982.

Bill

Bill Wright wrote:

These details were obscured by the cables, so I gently bent one cable
with the tip of a screwdriver. At this the relays clicked and the timing
sequences started. I drew back my screwdriver in surprise. It turned out
that the screw in the choc block for one of the two sensor leads had
never been tightened, and contact was hit and miss. This piece of kit has
functioned faultlessly since 1982.

Hi Bill

I had a very similar experience with one of our first machines built in
1991. It had worked faultlessly up until last year. There was no obvious
failure so I removed a terminal box lid and a wire fell out! Amazingly
there was no sign of overheating or arcing even though the wire was
carrying 230v at up to 5A. Judging from the depth of dust and cobwebs I
discounted 'fiddling'.

Barry

Bill, it shows that even when a screw is bad it's still pretty good!

"Bill Wright" wrote in message
...
screwdriver in surprise. It turned out that the screw in the choc block
for one of the two sensor leads had never been tightened, and contact was
hit and miss. This piece of kit has functioned faultlessly since 1982.

Bill



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Yes, I built a dark sensor on a bit of blob board back in the 70s as a
temporary job, I put it into a box using double sided tape, and it was there
from 76 to 2011, with no issues. The light sensor was an ORP12 a devider
network on the front of a simple Schmidt trigetr using out of spec
transistors withe an old relay and the psu was also built very simply.
The only thing that was really wrong with it when I took it down was dried
out electrolytics causing dodgy switching due to hum on the supply.

a friend could have fixed it. Oh and some creature had set up home inside,
looked like a spider.

Its that old adage once again. If it aint broke, don't fix it.
Of course I could not have built it now. I was amazed that the douuble
sided tape and the way I'd blobbed it all together survived all that time.
One might call it early surface mount.. grin.
Brian

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From the Sofa of Brian Gaff Reply address is active
"Bill Wright" wrote in message
...
Thirty-two years ago I made a little unit that switched a couple of CCTV
feeds and a floodlight. It was controlled by a 'normally on' 12V feed from
a sensor which controlled a 12V relay that in turn controlled three timer
relays. Two switched the video, which went to a monitor. The third
switched a floodlight. At the time I was surprised that ordinary relays
could pass baseband video with no apparent ill effects. But anyway, last
week, after all those years, it was decided that a manual over-ride of the
floodlight switching would be useful. The diecast box was opened for the
first time since installation and I peered inside. All the external
connections were via a long 'choc block' strip, and just above that was a
card with the connection details on it. These details were obscured by the
cables, so I gently bent one cable with the tip of a screwdriver. At this
the relays clicked and the timing sequences started. I drew back my
screwdriver in surprise. It turned out that the screw in the choc block
for one of the two sensor leads had never been tightened, and contact was
hit and miss. This piece of kit has functioned faultlessly since 1982.

Bill

I blame the vibrations...

Brian

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From the Sofa of Brian Gaff Reply address is active
"Y Dangle" wrote in message
...
Bill, it shows that even when a screw is bad it's still pretty good!

"Bill Wright" wrote in message
...
screwdriver in surprise. It turned out that the screw in the choc block
for one of the two sensor leads had never been tightened, and contact was
hit and miss. This piece of kit has functioned faultlessly since 1982.

Bill



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On 5 Feb 2014 08:14:18 GMT, Barry wrote:

Bill Wright wrote:

These details were obscured by the cables, so I gently bent one cable
with the tip of a screwdriver. At this the relays clicked and the timing
sequences started. I drew back my screwdriver in surprise. It turned out
that the screw in the choc block for one of the two sensor leads had
never been tightened, and contact was hit and miss. This piece of kit has
functioned faultlessly since 1982.

Hi Bill

I had a very similar experience with one of our first machines built in
1991. It had worked faultlessly up until last year. There was no obvious
failure so I removed a terminal box lid and a wire fell out! Amazingly
there was no sign of overheating or arcing even though the wire was
carrying 230v at up to 5A. Judging from the depth of dust and cobwebs I
discounted 'fiddling'.

A little different but this reminds me one time an OR engineer came
out to look at an ADSL speed issue. He found a complete break in my
phone line, fixed it. Afterwards the ADSL speed was completely
unchanged.
--
(\__/) M.
(='.'=) If a man stands in a forest and no woman is around
(")_(") is he still wrong?

In article ,
Bill Wright writes:
It turned out that the screw in the choc block for one of the two
sensor leads had never been tightened, and contact was hit and
miss. This piece of kit has functioned faultlessly since 1982.

So it seems that the contact must have been almost entirely "hit".
--
John Hall "He crams with cans of poisoned meat
The subjects of the King,
And when they die by thousands G.K.Chesterton:
Why, he laughs like anything." from "Song Against Grocers"

If it was a two screw one, ie the other side has a screw as well, then it
might have been holding on via this until it was disturbed. Just as well
there are no earthquakes in the north of England, at least not until the
fracing has started.
Brian

--
From the Sofa of Brian Gaff Reply address is active
"John Hall" wrote in message
...
In article ,
Bill Wright writes:
It turned out that the screw in the choc block for one of the two
sensor leads had never been tightened, and contact was hit and
miss. This piece of kit has functioned faultlessly since 1982.

So it seems that the contact must have been almost entirely "hit".
--
John Hall "He crams with cans of poisoned meat
The subjects of the King,
And when they die by thousands G.K.Chesterton:
Why, he laughs like anything." from "Song Against Grocers"

Barry wrote:
Bill Wright wrote:

These details were obscured by the cables, so I gently bent one cable
with the tip of a screwdriver. At this the relays clicked and the timing
sequences started. I drew back my screwdriver in surprise. It turned out
that the screw in the choc block for one of the two sensor leads had
never been tightened, and contact was hit and miss. This piece of kit has
functioned faultlessly since 1982.

Hi Bill

I had a very similar experience with one of our first machines built in
1991. It had worked faultlessly up until last year. There was no obvious
failure so I removed a terminal box lid and a wire fell out! Amazingly
there was no sign of overheating or arcing even though the wire was
carrying 230v at up to 5A. Judging from the depth of dust and cobwebs I
discounted 'fiddling'.

Ha! By the way we are still selling those washers, 50 at a time!

Bill

Brian_Gaff wrote:
If it was a two screw one, ie the other side has a screw as well, then it
might have been holding on via this until it was disturbed. Just as well
there are no earthquakes in the north of England, at least not until the
fracing has started.
Brian

No the other screw wasn't involved. The bit of bare wire that came out
of the choc block had been folded to make it thicker but was very short.

Bill