Anticlockwise overspeed on power-on

[Jon Stanley]

Hi there.

I've got a CD player that's in need of some attention. When I power up
the player it immediately starts the disc spinning the wrong way and
seems to accelerate slowly up to a pretty scary speed! When I press
play there is the sound of movement from the pickup and the cd slows
down to a stop momentarily, until giving up and spinning backwards
again. Before it started this strange behaviour, the player had
trouble spinning the disc up to read the TOC, until it got to the
point where you'd have to give it a spin with your finger to get it
going...

I've replaced some resistors that had burnt out and gone open-circuit
and also replaced some transistors that had shorted out. I believe
they're in a push-pull configuration, with the input coming from an
op-amp. The spindle motor receives a negative voltage from this
config, which I'm assuming is what's causing it to spin backwards!

Has anyone come across this before, and can offer any pointers? I'm
gradually working my way back from the motor, replacing components as
I go...bit trail and error. Is this likely to be a fault limited to
the spindle control section, or could there be a problem with the
pickup?

Thanks,
Jon.

 

[Dave D]

"Jon Stanley" <jon@kbs1.demon.co.uk> wrote in message
news:a9c99773.0307141527.7657903f@posting.google.com...

Hi there.

I've got a CD player that's in need of some attention. When I power up
the player it immediately starts the disc spinning the wrong way and
seems to accelerate slowly up to a pretty scary speed! When I press
play there is the sound of movement from the pickup and the cd slows
down to a stop momentarily, until giving up and spinning backwards
again. Before it started this strange behaviour, the player had
trouble spinning the disc up to read the TOC, until it got to the
point where you'd have to give it a spin with your finger to get it
going...

I've replaced some resistors that had burnt out and gone open-circuit
and also replaced some transistors that had shorted out. I believe
they're in a push-pull configuration, with the input coming from an
op-amp. The spindle motor receives a negative voltage from this
config, which I'm assuming is what's causing it to spin backwards!

Has anyone come across this before, and can offer any pointers? I'm
gradually working my way back from the motor, replacing components as
I go...bit trail and error. Is this likely to be a fault limited to
the spindle control section, or could there be a problem with the
pickup?

Thanks,
Jon.

Firstly, a make and model would be useful ;-)
Check the motor driver section is getting both + and - supplies, and that
they are both equal. If that's OK, you'll need to find out why the driver
stage is swinging negative. Is the op-amp output at 0V? Did you replace the
components with similar devices? Is it possible you subbed an NPN for a PNP
transistor or vice versa? Something else very likely caused these components
to smoke, are the + and - voltage regulators supplying the correct voltages?
(often + and -12V for the motor)

Dave

 

[Jon Stanley]

"Dave D" <someone@somewhere.com> wrote in message news:<bevru4$k6o$1@sparta.btinternet.com>...

"Jon Stanley" <jon@kbs1.demon.co.uk> wrote in message
news:a9c99773.0307141527.7657903f@posting.google.com...
Hi there.

I've got a CD player that's in need of some attention. When I power up
the player it immediately starts the disc spinning the wrong way and
seems to accelerate slowly up to a pretty scary speed! When I press
play there is the sound of movement from the pickup and the cd slows
down to a stop momentarily, until giving up and spinning backwards
again. Before it started this strange behaviour, the player had
trouble spinning the disc up to read the TOC, until it got to the
point where you'd have to give it a spin with your finger to get it
going...

I've replaced some resistors that had burnt out and gone open-circuit
and also replaced some transistors that had shorted out. I believe
they're in a push-pull configuration, with the input coming from an
op-amp. The spindle motor receives a negative voltage from this
config, which I'm assuming is what's causing it to spin backwards!

Has anyone come across this before, and can offer any pointers? I'm
gradually working my way back from the motor, replacing components as
I go...bit trail and error. Is this likely to be a fault limited to
the spindle control section, or could there be a problem with the
pickup?

Thanks,
Jon.

Firstly, a make and model would be useful ;-)
Check the motor driver section is getting both + and - supplies, and that
they are both equal. If that's OK, you'll need to find out why the driver
stage is swinging negative. Is the op-amp output at 0V? Did you replace the
components with similar devices? Is it possible you subbed an NPN for a PNP
transistor or vice versa? Something else very likely caused these components
to smoke, are the + and - voltage regulators supplying the correct voltages?
(often + and -12V for the motor)

Dave

Hi Dave.

The player is an Arcam Alpha 6 (same as the 5 with a different DAC, i
think).
The op-amp supplies are at +11V and -11V, with the push pull stage at
~+9V and -11V. I'm fairly certain I subbed the correct transistors (a
BC547 and BC557). As far as I can make out the sequence of events was
that the transistors burnt out and went short circuit after they got
hot with the spindle motor spinning backwards. After they failed the
only thing between +ve and -ve were a pair of .25W 3R3 resistors that
were on the collectors. These got pretty hot (evidenced by black marks
around the body of the resistor) and then went open-circuit.
Obviously, replacing these components means I still have to find the
original problem!

I think the op-amp's in a negative feedback configuration, with a
capacitor in parallel with the feedback resistor (integrator?). I'm at
work at the mo - I'll double check those voltages and the op-amp
circuit config when I get home.

Thanks,

Jon.

 

[Dave D]

"Jon Stanley" <jon@kbs1.demon.co.uk> wrote in message
news:a9c99773.0307150500.27130f8b@posting.google.com...

Hi Dave.

<snipped>

The player is an Arcam Alpha 6 (same as the 5 with a different DAC, i
think).

Oh, that's a nice player! I can see why you want to fix it!

The op-amp supplies are at +11V and -11V, with the push pull stage at
~+9V and -11V. I'm fairly certain I subbed the correct transistors (a
BC547 and BC557).

Check the suffixes, IIRC there's a couple of pinout variations on those
devices.

As far as I can make out the sequence of events was
that the transistors burnt out and went short circuit after they got
hot with the spindle motor spinning backwards. After they failed the
only thing between +ve and -ve were a pair of .25W 3R3 resistors that
were on the collectors. These got pretty hot (evidenced by black marks
around the body of the resistor) and then went open-circuit.
Obviously, replacing these components means I still have to find the
original problem!

I think the op-amp's in a negative feedback configuration, with a
capacitor in parallel with the feedback resistor (integrator?).

Electrolytic? If so, change it.

I'm at
work at the mo - I'll double check those voltages and the op-amp
circuit config when I get home.

OK. It would be useful to scope the op-amp output, or even disconnect it
from the driver stage to see if the motor stops.

Dave

 

[Jon Stanley]

<snip>

Check the suffixes, IIRC there's a couple of pinout variations on those
devices.

I've checked the pinouts and they're an exact match...

I think the op-amp's in a negative feedback configuration, with a
capacitor in parallel with the feedback resistor (integrator?).

Electrolytic? If so, change it.

it's not an electrolytic, but i checked the value and it was smack-on.

I'm at
work at the mo - I'll double check those voltages and the op-amp
circuit config when I get home.


OK. It would be useful to scope the op-amp output, or even disconnect it
from the driver stage to see if the motor stops.

Thanks for your continued help Dave! I've disconnected the op-amp o/p
from the transistor stage and the spindle motor does stop, I'm
assuming this means the problem is further up the chain, either with
the op-amp itself, or the error signal it's receiving. I double
checked those voltages and the op-amps at +10.94/-11.24. The
transistor stage at +10.96/-11.27, which is pretty equal. The op-amp
stage outputs a steady +5-odd volts, with a few mV of ac ripple at
50Hz (mains freq). I'd have thought the positive voltage would have
turned it the right way! With the motor connected the voltage starts
around 3 volts and climbs to around 5-6V as the spindle accelerates, I
guess it's placing less load on the transistors as the acceleration
slows at max speed. It doesn't seem to be doing it now (maybe I've not
connected it back up properly!) but I'm sure the voltage was dropping
to zero, or thereabouts, when I pressed play. I guess this would
indicate it was 'trying' to adjust the speed, but couldn't overcome
the large bias?

The op-amp is a JRC NJM4560D 8-pin DIL package
(http://www.njr.co.jp/pdf/ae/ae04053.pdf) I'm not sure what the other
side is being used for, but there appear to be a number of resistors
of varying values all lined up in close proximity.

I'm not quite sure what the next step should be, my knowledge of cd
player internals is quite limited! Any ideas?

Regards,

Jon.