Sony STR-DB930 Flashing LED

[David Jordan]

Hi,

The illuminated pointer on the volume control of the amp has started
flashing when you first switch the amp on. The rate changes from
around 2 per second to around 1 per second randomly then after a
couple of minutes the rate slows and the light goes out. Weird Huh?
Banging and tapping the unit makes no difference to the rate of the
flashing. Also, the position of the volume knob has no effect.

If you remove the knob and look down the shaft you can see the LED is
still lit very dimly. The volume control is on it's own pcb, there's a
couple of connectors on the back with lots of audio cables and a pair
of wires that carry almost exactly 30V. On the front of the PCB is a 4
pin connector joining it to the the front panel pcb. The front panel
pcb also has a ribbon going to the digital board.

Removing and cleaning the ribbon has not cured the problem. If I
remove the connector on the back that has 30V on it the LED is still
dimly lit and the motor still turns the knob. I'm not sure of the
purpose of this 30V but it proves that the power for the LED and the
motor come from the 4 pin connector to the front panel pcb.

Before I pull this apart any more (I'm not looking forward to removing
the front panel) does anyone have any suggestions? I'm really looking
for a schematic that shows how the LED is driven, if it's just from a
power supply line or via a logic signal. All of the other indicators
on the unit are working fine so I'm expecting it to be driven by a
separate signal.


Thanks,
Dave.

 

[Mark D. Zacharias]

If the LED is the only problem, I'm not sure I'd worry about it.

Mark Z.

 

[JVC dude]

Most units are just driven from a steady 5v supply line via a dropper
resistor. It would be unusual to drop a 30v line through a resistor for an
LED.
I don't know whether your sony does have a pulsed LED - but some units do-
usually to indicate whether the protection CCT has cut in/out. these aren't
'intermittent' but a regular pulse driven from the micro.

However- just this week I've had a similar fault with an LED (admittedly a
bright blueish backlight one) it flickered rather like a fluorescent, which
became more intermittent as it warmed up- eventually stopping illuminating-
when cooled/frozen it started to work again. Maybe yours does this. Just try
another LED .
AW

 

[David Jordan]

Most units are just driven from a steady 5v supply line via a dropper
resistor. It would be unusual to drop a 30v line through a resistor for an
LED.

I managed to remove the board with the volume control on it and found
the 30V was +/- 15V for some SM devices on the back of the board I
couldn't originally see.

There is no dropper resistor on the PCB for the LED, that must be on
the front panel PCB.

For the 4-pin connector on the pcb, pins 3&4 go to the motor and pins
1&2 SEEM to go to the LED. There's a white silkscreen on the underside
of the pcb that makes it impossible to see the tracks so I'll meter
this to confirm. The wires are all 'just long enough' so it's a bit
tricky to probe onto things while it's powered up

I don't know whether your sony does have a pulsed LED - but some units do-
usually to indicate whether the protection CCT has cut in/out. these aren't
'intermittent' but a regular pulse driven from the micro.

The VFD displays PROTECTION when that happens. I've caused it a couple
of times while probing around and nearly got blown across the room by
a blast of 50Hz from the sub.

However- just this week I've had a similar fault with an LED (admittedly a
bright blueish backlight one) it flickered rather like a fluorescent, which
became more intermittent as it warmed up- eventually stopping illuminating-
when cooled/frozen it started to work again. Maybe yours does this. Just try
another LED .
AW

Interesting. I think this may be the next thing to try. There is a
steady 5V across pins 1&2 so maybe they do go directly to the LED but
the LED is open circuit.

I'll have to see how easy it is to replace the LED, it shines down the
center of the pot shaft so I don't know how to get at it. If it's
possible I may replace it with a blue one, that would look great!


Thanks,
Dave.