----- Original Message -----
Sent: Thursday, November 15, 2001 12:12 AM
Subject: Re: [M]: Bottom plate screws :The two in the middle...what do
they do?


> Roger,Bruce:
>
> Many thanks for your explanations.
> I believe (please correct me if Im wrong) this
> explains a runaway in both RA\Dec drives I've
> experienced with our club's 12".
> I installed a new PCB after the original one got fried
> by some of the club's observers who werent aware of
> the dangers of hot-plugging connectors.
> I tested the scope OK on the working table (I think I
> missed those two small screws but not certain about
> it...damn).I mounted it on its superwedge, powered and
> wow!the two motors a running without control. Powered
> off\on several times-nothing. Played with the
> conenctors (including the ribbon behind the
> panel)-nothing. I took the scope back o the working
> table, removed the bottom plate-no change. Played with
> PCB and all visible wires, components that could be
> loosened- nothing.
>
> Could it be that I've killed a new PCB by ignoring
> those screws? can it be fixed or should I go for a new
> PCB (errrr....)?
Urgh!...
I have heard in another thread a while ago, that some scopes, can
intermitently display runaway, if the 'bracket' is not earthed properly to
the case. Also the piece of metal sits _very_ close to some of the
components below, and if it 'bent' away from the bottom cover at all
(which without the screws, could have happened), it may have shorted in
this area (careful inspection time). Realistically, if the only failure
was lack of sufficient heatsink, then it would take some time before the
drivers overheated, and the chips would probably have started to make some
fairly 'hot' smells before failing. I would suspect something else is
wrong (getting one of the 'strip' connectors misplaced by one pin for
example). Lack of heatsink, would really only 'kill' the drivers
themselves.
The most likely failure is that something is touching round the area of
the drivers. This can easily cause 'dual drive runaway', and is often not
fatal. I'd start with the bottom off, and bend the drivers slightly away
from the board, and have a very careful look with a torch and magnifier.
Also have you now replaced the screws?. If so, are the 'new' screws as
short as the old ones?. Otherwise these could be touching, and causing
similar problems.
Did you originally 'test' the unit before closing up the base?. You can
leave both clutches disengaged, and simply run the assembly on the bench.
This would at least have confirmed that the 'service kit' was working OK.
Otherwise it is possible that it was faulty from the start...

Best Wishes

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