W
WT
- Jan 1, 1970
- 0
The patient: SONY CFD-758 portable AM/FM dual cassette and CD player,
manufactured 1993.
Rescued from neighbor's garbage. Very nice shape, clean, no visible damage
or abuse.
Unfortunately, I did not recover the IR remote control.
Symptoms: Plug in AC cord, press ON/OFF button on front - nothing happens.
No sound, nothing shows on the LCD display. This unit is supposed to have a
clock display, but it doesn't appear either.
Okay, this looks easy - must be some kind of power supply problem. Open the
unit up. Power supply module mounted on back of enclosure. Probe some
connector pins - see a nice steady +16VDC, always on. Hmmm... maybe this is
a bit more complicated.
I have no schematic, but am able to trace out a fair amount of the circuitry
related to power control. The "main board" has a type BA3924 voltage
regulator chip. I download datasheet from the Rohm website. This big linear
regulator is designed specifically for CD/radio cassette players like this
one. It has a "Standby" pin which can be used to turn most of the voltages
on and off. This pin is in the OFF state, and the regulator does have the
raw +16V on its input pin. I trace back from the Standby pin and see that it
goes back through a 2-transistor buffer to the main microcontroller chip, a
Sony CXP50716. I force the transistors on through a limiting resistor, and
lo-and-behold, the LCD backlight comes on, along with the power LED. Nothing
on the LCD itself, but at least it suggests that the basic power control is
okay.
I download the CXP50716 datasheet, and start probing its pins (carefully!).
This processor chip has a steady +5V (actually closer to about 4.7V) on its
power pins. Its crystal oscillator is running (good news!). No other
activity, so I guess that it is waiting to be told to turn on. Pressing any
of the control button on the front of the unit cause various DC voltages to
be applied to pins 35 through 40 of the cpu, which are analog-to-digital
converter input lines. Looks like this design uses different voltage levels
to differentiate which button is pressed. I haven't yet mapped out which
button causes what voltage on which pin, but I might have to. Pressing the
ON/OFF button causes cpu pin 39 to drop, so the cpu does see the button
being pushed.
So, I see a couple of possibilities as to why this unit doesn't "wake up":
1) The cpu chip is dead or bad, even though its oscillator is running,
2) The cpu is okay, but either did not start-up from reset okay, or is being
held in a stopped state for some reason.
3) The cpu is okay, but is looking for a signal which says "it's okay to
turn on power - AC power or battery power is available".
If 1) is true, then I'm out of luck - I doubt that I could get a replacement
cpu chip.
Personally, I think 3) is the most likely. That would mean the cpu normally
runs in some kind of low-power mode, watching for a turn-on command from
either the IR remote receiver, or from the front panel ON/OFF button. The
cpu gets its +5V power from either the AC power supply, or the installed
D-size batteries (10 of them), or from the 4xAA backup battery. However, I'm
guessing that it will not try to turn on the main regulator unless it
detects either AC power or main D-size battery power. And I am guessing
that on this unit, that circuitry isn't working.
If anyone has worked on this type of unit or a similar one, and can confirm
or explain the way in which the unit powers up, I'd be ever grateful.
Walter
Winnipeg, Canada
manufactured 1993.
Rescued from neighbor's garbage. Very nice shape, clean, no visible damage
or abuse.
Unfortunately, I did not recover the IR remote control.
Symptoms: Plug in AC cord, press ON/OFF button on front - nothing happens.
No sound, nothing shows on the LCD display. This unit is supposed to have a
clock display, but it doesn't appear either.
Okay, this looks easy - must be some kind of power supply problem. Open the
unit up. Power supply module mounted on back of enclosure. Probe some
connector pins - see a nice steady +16VDC, always on. Hmmm... maybe this is
a bit more complicated.
I have no schematic, but am able to trace out a fair amount of the circuitry
related to power control. The "main board" has a type BA3924 voltage
regulator chip. I download datasheet from the Rohm website. This big linear
regulator is designed specifically for CD/radio cassette players like this
one. It has a "Standby" pin which can be used to turn most of the voltages
on and off. This pin is in the OFF state, and the regulator does have the
raw +16V on its input pin. I trace back from the Standby pin and see that it
goes back through a 2-transistor buffer to the main microcontroller chip, a
Sony CXP50716. I force the transistors on through a limiting resistor, and
lo-and-behold, the LCD backlight comes on, along with the power LED. Nothing
on the LCD itself, but at least it suggests that the basic power control is
okay.
I download the CXP50716 datasheet, and start probing its pins (carefully!).
This processor chip has a steady +5V (actually closer to about 4.7V) on its
power pins. Its crystal oscillator is running (good news!). No other
activity, so I guess that it is waiting to be told to turn on. Pressing any
of the control button on the front of the unit cause various DC voltages to
be applied to pins 35 through 40 of the cpu, which are analog-to-digital
converter input lines. Looks like this design uses different voltage levels
to differentiate which button is pressed. I haven't yet mapped out which
button causes what voltage on which pin, but I might have to. Pressing the
ON/OFF button causes cpu pin 39 to drop, so the cpu does see the button
being pushed.
So, I see a couple of possibilities as to why this unit doesn't "wake up":
1) The cpu chip is dead or bad, even though its oscillator is running,
2) The cpu is okay, but either did not start-up from reset okay, or is being
held in a stopped state for some reason.
3) The cpu is okay, but is looking for a signal which says "it's okay to
turn on power - AC power or battery power is available".
If 1) is true, then I'm out of luck - I doubt that I could get a replacement
cpu chip.
Personally, I think 3) is the most likely. That would mean the cpu normally
runs in some kind of low-power mode, watching for a turn-on command from
either the IR remote receiver, or from the front panel ON/OFF button. The
cpu gets its +5V power from either the AC power supply, or the installed
D-size batteries (10 of them), or from the 4xAA backup battery. However, I'm
guessing that it will not try to turn on the main regulator unless it
detects either AC power or main D-size battery power. And I am guessing
that on this unit, that circuitry isn't working.
If anyone has worked on this type of unit or a similar one, and can confirm
or explain the way in which the unit powers up, I'd be ever grateful.
Walter
Winnipeg, Canada
