[Amps] Screen Supply for 4CX250 series

[David Kirkby]

David Lisney wrote:
> Hi, a couple of people emailed me off list for the circuit diagram of my 
> transistor stabilised screen supply, sorry I had a hard-drive failure (how 
> do they possibly claim such amazing MTBFs) 

The MTBF's on hard drives are quoted in an odd manner (well odd to me 
anyway), but I guess it has some logic to it.

A decent SCSI hard drive will have an MTBF of over 1 million hours, but 
this does *NOT* mean if you take a load of hard drives and run them 
until they break, the mean time before failure will be 1,000,000 hours 
(114 years)

The MTBF is based on the mean time for a random failure to occur, 
assuming that you replace the disks periodically in a preventative 
maintenance schedule. You need to replace them at the end of their 
service lives (5 years typically for SCSI, less for other disks), even 
if the disks are still working.

So if the service life of a disk is 5 years and the MTBF is 114 years, 
you will (on average) need to replace the disk 22 times over before a 
random failure will occur that will mean a disk fails under 5 years old.

Of course, few of us bother replacing disks that are not broken, which 
is why we don't see average lifetimes close to the MTBF's.

If you make heavy usage of disks (lots of DVD ripping or similar) then 
SCSI are a lot better. I knew someone who was hammering IDE disks very 
hard 24 hours/day. These would often fail  three or four times during 
the 3 year warranty period. I suggested he switched to SCSI, which he 
did and has not looked back. They are built much better, and have more 
intelligent drive electronics that minimises head movement. Of course, 
the downside is that they are 2 or 3 times more expensive for the same 
capacity.

-- 
David Kirkby,
G8WRB

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/