360 Shortcut

[Dave Plowman (News)]

I've got a Shortcut 99 which has problems switching on. Leave it powered
up and it usually does after a couple of minutes, then works normally. But
not too reassuring, given the tasks it's sometimes needed for. ;-)

It would appear to be a PS fault - but all the caps etc check out ok and
I've re-flowed the entire board. As do the voltages when it's working.
Anyone come across this and got an educated guess?

--
*A dog's not just for Christmas, it's alright on a Friday night too*

Dave Plowman London SW
To e-mail, change noise into sound.

[Rick Ruskin]

On Thu, 01 May 2014 14:52:55 +0100, "Dave Plowman (News)"
> wrote:

>I've got a Shortcut 99 which has problems switching on. Leave it powered
>up and it usually does after a couple of minutes, then works normally. But
>not too reassuring, given the tasks it's sometimes needed for. ;-)
>
>It would appear to be a PS fault - but all the caps etc check out ok and
>I've re-flowed the entire board. As do the voltages when it's working.
>Anyone come across this and got an educated guess?

As far as I know, 360 Systems is still in business. Give them a call.
It's probably something they've dealt with before.

[Scott Dorsey]

Dave Plowman (News) > wrote:
>I've got a Shortcut 99 which has problems switching on. Leave it powered
>up and it usually does after a couple of minutes, then works normally. But
>not too reassuring, given the tasks it's sometimes needed for. ;-)
>
>It would appear to be a PS fault - but all the caps etc check out ok and
>I've re-flowed the entire board. As do the voltages when it's working.
>Anyone come across this and got an educated guess?

Something has gone wrong with the kickstart circuit on the power supply.
On simple switching supplies, this will be just an RC constant that supplies
power to the pwm oscillator for long enough to get some voltage out of the
secondary.

Get the datasheet for the PWM chip that the power supply uses, look at the
sample circuits. Look for the kickstart circuit which almost certainly will
be identical to the sample circuits on the datasheet. Check the value of the
resistors and replace the small capacitor.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Dave Plowman (News)]

In article >,
Scott Dorsey > wrote:
> Get the datasheet for the PWM chip that the power supply uses, look at
> the sample circuits. Look for the kickstart circuit which almost
> certainly will be identical to the sample circuits on the datasheet.
> Check the value of the resistors and replace the small capacitor.

It's a daughter board with an ASTEC AS3842 which is sadly a mixture of
surface mount and through hole stuff which makes repair impossible for me
- I don't keep any SM spares.

There's plenty room inside the case so I might just fit a slimline PS from
a desktop PC - I can buy a new one for about 30 gbp.

--
*The closest I ever got to a 4.0 in school was my blood alcohol content*

Dave Plowman London SW
To e-mail, change noise into sound.

[Scott Dorsey]

In article >,
Dave Plowman (News) > wrote:
>In article >,
> Scott Dorsey > wrote:
>> Get the datasheet for the PWM chip that the power supply uses, look at
>> the sample circuits. Look for the kickstart circuit which almost
>> certainly will be identical to the sample circuits on the datasheet.
>> Check the value of the resistors and replace the small capacitor.
>
>It's a daughter board with an ASTEC AS3842 which is sadly a mixture of
>surface mount and through hole stuff which makes repair impossible for me
>- I don't keep any SM spares.

The 3842 is a very common controller. You can get the datasheet for the
Astec version at http://diagramas.diagramasde.com/otros/datasheet%20AS3842.pdf

Get that datasheet, look at Figure 14 and the description of the bootstrap
circuit. When you turn the thing on, you should see a nice pulse on pin 7
as that bootstrap cap charges up. If you don't, look at the R and the C
involved.

If there is a constant voltage on pin 7, but there is no square wave coming
out of pin 6, check the voltage on pin 3. If the voltage on pin 3 is too
high or too low the device will shut down assuming that the output is
overcurrent. (A real overcurrent problem usually causes a constant cycle
where it tries to start and shut down repeatedly every second or so.)

>There's plenty room inside the case so I might just fit a slimline PS from
>a desktop PC - I can buy a new one for about 30 gbp.

Those are FLAKY.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Dave Plowman (News)]

In article >,
Scott Dorsey > wrote:
> In article >,
> Dave Plowman (News) > wrote:
> >In article >,
> > Scott Dorsey > wrote:
> >> Get the datasheet for the PWM chip that the power supply uses, look at
> >> the sample circuits. Look for the kickstart circuit which almost
> >> certainly will be identical to the sample circuits on the datasheet.
> >> Check the value of the resistors and replace the small capacitor.
> >
> >It's a daughter board with an ASTEC AS3842 which is sadly a mixture of
> >surface mount and through hole stuff which makes repair impossible for
> >me - I don't keep any SM spares.

> The 3842 is a very common controller. You can get the datasheet for the
> Astec version at
> http://diagramas.diagramasde.com/otros/datasheet%20AS3842.pdf

Ah - that's a much better data sheet than the one I found. Thanks.

> Get that datasheet, look at Figure 14 and the description of the
> bootstrap circuit. When you turn the thing on, you should see a nice
> pulse on pin 7 as that bootstrap cap charges up. If you don't, look at
> the R and the C involved.

> If there is a constant voltage on pin 7, but there is no square wave
> coming out of pin 6, check the voltage on pin 3. If the voltage on pin
> 3 is too high or too low the device will shut down assuming that the
> output is overcurrent. (A real overcurrent problem usually causes a
> constant cycle where it tries to start and shut down repeatedly every
> second or so.)

After the last post I decided to simply replace both the through hole
electrolytics since I had spares and their ESR measurement wasn't good.
And it seems to have sorted it.

> >There's plenty room inside the case so I might just fit a slimline PS
> >from a desktop PC - I can buy a new one for about 30 gbp.

> Those are FLAKY. --scott

I've bought one from the same UK supplier before - CPC - and it's been
fine. It is used nowhere near its maximum rating on an elderly RISC OS
machine, though.

Thanks very much for your help.

--
*Artificial Intelligence is no match for Natural Stupidity *

Dave Plowman London SW
To e-mail, change noise into sound.

[[email protected]]

Hi. I have an old Shortcut SC-182 with a noisy HDD. Does anyone have the format HDD instructions for this device. Any information about HDD size and specs would be great.

Thanks Steve

[Scott Dorsey]

> wrote:
>Hi. I have an old Shortcut SC-182 with a noisy HDD. Does anyone have the format HDD instructions for this device. Any information about HDD size and specs would be great.

It should be in the manual on a special colored sheet! If your sheet has
been pulled out, call 360 systems and they'll tell you! I don't remember
what the secret key sequence on power-up is.

As far as I know, just about any scsi drive made in the past decade will work
fine. It needs to be able to deal with 512kB sectors and it needs a certain
seek time, but I don't think you'll find any drives outside of a museum that
cannot meet those specs.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."