-27volts from a PC power supply
 

Hi all, this may sound like a rookie question, but here goes:

I have a faulty Mustek r100a DVD recorder. Having traced the problem to
the power supply (where the majority of problems in these machines
are), I quickly proceeded to fry several components in my efforts to
repair it. At this point, I am thinking it might just be easier to
substitute a PC power supply for the original one.

This should be **fairly** easy to do, as the supply rails are quite
similar. The DVD recorder uses the following rails: +5v, +12v, +3.3v,
and -27v. Obviously the first two can be handled straight from the pc
supply, but the others will require some circuitry. I assume the 3.3v
can be handled by simply using a series diode of the right value, but
it is the -27v that I don't know how to create.

If someone could give some pointers or a circuit suggestion it would be
much appreciated.

Regards,
Jonathan

-27volts from a PC power supply
 

Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

-27volts from a PC power supply
 

fredrickew wrote:
Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.



The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

-27volts from a PC power supply
 

"James Sweet" wrote in message
news:9os7h.95$mM1.63@trndny08...
fredrickew wrote:
Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.



The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

Likewise, the -27v is also for the VFD. The 3.3v will be the CPU core
voltage, so must be properly regulated. If you have done damage beyond the
original PSU fault, it's probably not worth wasting more time and effort on.
Just what additional components do you think you have fried ?

Arfa

-27volts from a PC power supply
 

Thanks for your input. Here is what happened:

After testing the output voltages on the PSU and finding them to be a
bit off-target, I did a touch of soldering to eliminate the dry joint
possibility. Unfortunately I think I may have killed a couple of the
more sensitive components (all soldering was done on the low-voltage
side of the board). Upon plugging it back in there were several flashes
followed by a blown fuse. On closer investigation with my trusty
multimeter, I found the live side to be completely shorted. Once I
desoldered and tested a few components, I found three of the four
rectifier diodes were shot. I imagine it must have been a pretty
catastrophic problem on the low-power side to have caused this! (and
no, there are no shorted tracks) I **think** I may have seen the
L7812cv rectifier IC flash in the process.

Just to add to the problem there is a signal diode that has shattered,
and is now unreadable. Other than that there are only a couple of
sensitive components, so I guess it may well be easier to try and fix
it. Does anyone have a schematic for this? I realise that these units
aren't really made to be repaired, so there is a good chance that one
doesn't exist.

Anyway, thanks again for your help.



On Nov 18, 1:48 pm, "Arfa Daily" wrote:
"James Sweet" wrote in messagenews:9os7h.95$mM1.63@trndny08...

fredrickew wrote:
Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.Likewise, the -27v is also for the VFD. The 3.3v will be the CPU core
voltage, so must be properly regulated. If you have done damage beyond the
original PSU fault, it's probably not worth wasting more time and effort on.
Just what additional components do you think you have fried ?

Arfa

-27volts from a PC power supply
 

fredrickew wrote:
Thanks for your input. Here is what happened:

After testing the output voltages on the PSU and finding them to be a
bit off-target, I did a touch of soldering to eliminate the dry joint
possibility. Unfortunately I think I may have killed a couple of the
more sensitive components (all soldering was done on the low-voltage
side of the board). Upon plugging it back in there were several flashes
followed by a blown fuse. On closer investigation with my trusty
multimeter, I found the live side to be completely shorted. Once I
desoldered and tested a few components, I found three of the four
rectifier diodes were shot. I imagine it must have been a pretty
catastrophic problem on the low-power side to have caused this! (and
no, there are no shorted tracks) I **think** I may have seen the
L7812cv rectifier IC flash in the process.

Just to add to the problem there is a signal diode that has shattered,
and is now unreadable. Other than that there are only a couple of
sensitive components, so I guess it may well be easier to try and fix
it. Does anyone have a schematic for this? I realise that these units
aren't really made to be repaired, so there is a good chance that one
doesn't exist.

Anyway, thanks again for your help.




Did you create a solder bridge somewhere? That could pop things like that.

Your best bet is probably to look on ebay or craigslist for an identical
unit with some other problem, I'd guess the optical drive is the most
common failure.

-27volts from a PC power supply
 

Yep, did check for a solder bridge but there were none. The machine
itself isn't worth much at all, this is really a learning exercise more
than anything else. I don't really want to spend any more on it (aside
from minor component replacements). But thanks for the suggestion.

On Nov 18, 2:53 pm, James Sweet wrote:
fredrickew wrote:
Thanks for your input. Here is what happened:

After testing the output voltages on the PSU and finding them to be a
bit off-target, I did a touch of soldering to eliminate the dry joint
possibility. Unfortunately I think I may have killed a couple of the
more sensitive components (all soldering was done on the low-voltage
side of the board). Upon plugging it back in there were several flashes
followed by a blown fuse. On closer investigation with my trusty
multimeter, I found the live side to be completely shorted. Once I
desoldered and tested a few components, I found three of the four
rectifier diodes were shot. I imagine it must have been a pretty
catastrophic problem on the low-power side to have caused this! (and
no, there are no shorted tracks) I **think** I may have seen the
L7812cv rectifier IC flash in the process.

Just to add to the problem there is a signal diode that has shattered,
and is now unreadable. Other than that there are only a couple of
sensitive components, so I guess it may well be easier to try and fix
it. Does anyone have a schematic for this? I realise that these units
aren't really made to be repaired, so there is a good chance that one
doesn't exist.

Anyway, thanks again for your help.Did you create a solder bridge somewhere? That could pop things like that.

Your best bet is probably to look on ebay or craigslist for an identical
unit with some other problem, I'd guess the optical drive is the most
common failure.- Hide quoted text -- Show quoted text -

-27volts from a PC power supply
 

"fredrickew" wrote in message
ps.com...

Yep, did check for a solder bridge but there were none. The machine
itself isn't worth much at all, this is really a learning exercise more
than anything else. I don't really want to spend any more on it (aside
from minor component replacements). But thanks for the suggestion.

A power supply is just a device. You can repair it like most other things.
Sometimes you can't get the parts you need but often you can substitute.
However you do need to have some experience and this may be a tough one to
learn on.

-27volts from a PC power supply
 

James Sweet wrote:
fredrickew wrote:

Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

3v could possibly come from 5v via 2 power diodes, which will drop apx
1v each. However the exact drop depends on the diodes and current draw,
so feeding a cpu from this would be chancey. A 3v reg would be rather
safer. Easier though would be a regulated 1.7v dropper, basically a
transistor multiplying resistors/diode on the base side. Easy to
implement.

The vfd filament is a single fixed load, so this can be run from the
smpsu transformer output via a cap or resistor. IOW you need to either
measure the vfd filament current draw, or start with a high value R and
move down until you get 3.9v on the vfd. IIRC with vfds start by
assuming 20mA, then adjust upwards as required.

VFD -27 rail will be very low current, so you can get this by voltage
tripling the ac output from the smpsu transformer. It does not need to
be 27v, 24-36 is fine. Because the smpsu runs at high freq you should
be able to get above 27v no problem. Capacitor size determines how much
of the notional '36v - diode drops' you get, so again start small and
increase them to get enough V.


NT

-27volts from a PC power supply
 

Thats a great help, I'll see how I go. Thanks for the tips.

wrote:
James Sweet wrote:
fredrickew wrote:

Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

3v could possibly come from 5v via 2 power diodes, which will drop apx
1v each. However the exact drop depends on the diodes and current draw,
so feeding a cpu from this would be chancey. A 3v reg would be rather
safer. Easier though would be a regulated 1.7v dropper, basically a
transistor multiplying resistors/diode on the base side. Easy to
implement.

The vfd filament is a single fixed load, so this can be run from the
smpsu transformer output via a cap or resistor. IOW you need to either
measure the vfd filament current draw, or start with a high value R and
move down until you get 3.9v on the vfd. IIRC with vfds start by
assuming 20mA, then adjust upwards as required.

VFD -27 rail will be very low current, so you can get this by voltage
tripling the ac output from the smpsu transformer. It does not need to
be 27v, 24-36 is fine. Because the smpsu runs at high freq you should
be able to get above 27v no problem. Capacitor size determines how much
of the notional '36v - diode drops' you get, so again start small and
increase them to get enough V.


NT

-27volts from a PC power supply
 

"fredrickew" wrote in message
ps.com...
Thats a great help, I'll see how I go. Thanks for the tips.

wrote:
James Sweet wrote:
fredrickew wrote:

Sorry, just noticed that there are two more rails also, a -12v and an
AC3.9v. I'm not sure if the AC is refering to Alternate Current or
accessory. I'm pretty sure it would be a dc rail though.

The AC voltage is for the heater in the VFD display. You'll spend a lot
more time re-engineering a PC power supply to work than you will just
fixing the original.

3v could possibly come from 5v via 2 power diodes, which will drop apx
1v each. However the exact drop depends on the diodes and current draw,
so feeding a cpu from this would be chancey. A 3v reg would be rather
safer. Easier though would be a regulated 1.7v dropper, basically a
transistor multiplying resistors/diode on the base side. Easy to
implement.

The vfd filament is a single fixed load, so this can be run from the
smpsu transformer output via a cap or resistor. IOW you need to either
measure the vfd filament current draw, or start with a high value R and
move down until you get 3.9v on the vfd. IIRC with vfds start by
assuming 20mA, then adjust upwards as required.

VFD -27 rail will be very low current, so you can get this by voltage
tripling the ac output from the smpsu transformer. It does not need to
be 27v, 24-36 is fine. Because the smpsu runs at high freq you should
be able to get above 27v no problem. Capacitor size determines how much
of the notional '36v - diode drops' you get, so again start small and
increase them to get enough V.


NT


Before getting into this any deeper though, I would strongly suggest that
you get a suitable isolation transformer to run it from whilst you are
working on it. Switch mode power supplies are potentially lethal - and I
mean that in the literal sense of the word. Even when run on an isolated
supply, they are still dangerous, so please be very careful. It should also
be noted that they are extraordinarily fickle - not for the faint-hearted.
They can have the most experienced of us tearing their hair out ...

Good luck with it !

Arfa

-27volts from a PC power supply
 

"fredrickew" wrote in message
oups.com...
Hi all, this may sound like a rookie question, but here goes:

I have a faulty Mustek r100a DVD recorder. Having traced the problem to
the power supply (where the majority of problems in these machines
are), I quickly proceeded to fry several components in my efforts to
repair it. At this point, I am thinking it might just be easier to
substitute a PC power supply for the original one.

This should be **fairly** easy to do, as the supply rails are quite
similar. The DVD recorder uses the following rails: +5v, +12v, +3.3v,
and -27v. Obviously the first two can be handled straight from the pc
supply, but the others will require some circuitry. I assume the 3.3v
can be handled by simply using a series diode of the right value, but
it is the -27v that I don't know how to create.


3.3V is supplied natively on ATX power supplies- no need for trickery,
unless you want to use an old AT supply. -27V or close enough can probably
be derived from the winding on the transformer which supplies the -12V rail.
Voltage doubling may be necessary though.

If someone could give some pointers or a circuit suggestion it would be
much appreciated.


Seriously, this is not a very wise idea- there are safety issues to
consider. While it may well be made to work, until there's a problem
somewhere- PC PSUs are capable of supplying massive currents and will dump
30+ amps into a short circuit before shutting down or self destructing. In
the event of a fault on your DVD recorder's main PCB this will vapourise PCB
tracks, explode ICs and maybe melt wiring etc, not good unless you like
fireworks from your AV gear!.

Frankenstein projects like this are best avoided, if it's worth fixing then
I'd advise you get it done properly. If not, scrap it or sell it for spares
on eBay. It is quite possible that the power supply damaged the main PCB
when it blew anyway.

If you're sure there were no solder bridges, I suspect the original fault
was due to high ESR/failed caps, and when you soldered around the PSU the
heat affected them causing the catastrophic failure. I've seen this happen
before.

Morse

-27volts from a PC power supply
 

Morse wrote:

Seriously, this is not a very wise idea- there are safety issues to
consider. While it may well be made to work, until there's a problem
somewhere- PC PSUs are capable of supplying massive currents and will dump
30+ amps into a short circuit before shutting down or self destructing. In
the event of a fault on your DVD recorder's main PCB this will vapourise PCB
tracks, explode ICs and maybe melt wiring etc, not good unless you like
fireworks from your AV gear!.

Oh, so we can add visualisations to our older gear? :)


NT

-27volts from a PC power supply
 

wrote in message
ups.com...
Morse wrote:

Seriously, this is not a very wise idea- there are safety issues to
consider. While it may well be made to work, until there's a problem
somewhere- PC PSUs are capable of supplying massive currents and will
dump
30+ amps into a short circuit before shutting down or self destructing.
In
the event of a fault on your DVD recorder's main PCB this will vapourise
PCB
tracks, explode ICs and maybe melt wiring etc, not good unless you like
fireworks from your AV gear!.

Oh, so we can add visualisations to our older gear? :)


Yes, but leave the cover off or you won't get the full benefit! ;-)

Morse