Hi guys. Hoping for a small tip here. I've got this amp which blows LEDs for some reason. There are four: one for some sort of tape monitor, another for loudness, a third for subsonic and a fourth for direct mode.
The direct LED can only be lit alone.
The other three will individually blow up if another LEDs is already lit (but the already glowing LED will survive.
Even if a LED is broken or missing altogether (cut out of the amp), if it is supposed to be lit, and I try to lit another (working) LED, the working LED will blow up.
All LEDs can work independently (if replaced).
I cannot understand why it does this. I've never come across anything that blows LEDs. I've traced out their signal path on the schematic and short of replacing everything in the little dedicated LED power supply (which I intend to do), I cannot think of a solution to the problem.
Have a look if you're intrigued.
Big link: http://i.imgur.com/W8qPlZM.png
The direct LED can only be lit alone.
The other three will individually blow up if another LEDs is already lit (but the already glowing LED will survive.
Even if a LED is broken or missing altogether (cut out of the amp), if it is supposed to be lit, and I try to lit another (working) LED, the working LED will blow up.
All LEDs can work independently (if replaced).
I cannot understand why it does this. I've never come across anything that blows LEDs. I've traced out their signal path on the schematic and short of replacing everything in the little dedicated LED power supply (which I intend to do), I cannot think of a solution to the problem.
Have a look if you're intrigued.
Big link: http://i.imgur.com/W8qPlZM.png
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Hard to make out with all the red lines 😉 but I can't see any limiting for the LED current other than the parallel 10k's on the PSU board.
If any LED switch was momentarily open for an instant then the next time an LED is in circuit it will get the full charge of that 220uf 63 volt cap. Zap !
Does that give you any clues ? Breaks in the circuit, intermittent switches etc.
If any LED switch was momentarily open for an instant then the next time an LED is in circuit it will get the full charge of that 220uf 63 volt cap. Zap !
Does that give you any clues ? Breaks in the circuit, intermittent switches etc.
If the power supply measures 38V as noted, and the resistors are good, then the maximum LED current is less than 15mA,
which should be safe for an LED. Measure the four parallel 10k resistors to see is they are 2.5k. You can remove one or two
of the resistors to reduce the current, but the LEDs will dim somewhat.
So you're saying the problem might be caused by something as simple as dirty old switches? That seems like a terrible design. 🙂
I have cleaned them but they feel rough and uneven as I press them.
I have cleaned them but they feel rough and uneven as I press them.
Gotta have more background.
Anything modified or replaced in this amp?
What did you do for troubleshooting? Did you verify the voltages on the schematic?
If the LED PS correctly gives 2.0 VDC, did you measure any AC on it?
Anything modified or replaced in this amp?
What did you do for troubleshooting? Did you verify the voltages on the schematic?
If the LED PS correctly gives 2.0 VDC, did you measure any AC on it?
It does seem like terrible design I agree but that is how the circuit appears to be. The LED's rely on the circuit never going open for an instant. If it does then the cap will reach full charge and then discharge all its energy into the LED/s that are in circuit.
Actually... that is what happens but if you study the circuit you see the switches short out LED's they don't switch them out of circuit. So the most likely cause could be a dry or a break in the print somewhere. Check all the continuity and joints from the cap on the PSU right through the circuit. A dry on an LED would cause the problem for example.
Wait. Two of the LEDs were dead to begin with, and now all are gone. The dead ones measure open. Maybe it's as simple as that? Replace all with functioning LEDs and all will be good?
Make sure that they are receiving a DC voltage, since an AC voltage would still blow them.
The rectifier diode feeding the resistors could be shorted.
Thanks for the ideas, guys. I haven't really done any fault finding yet, since I'm waiting on an order for parts (new LEDs). Just collecting some potential faults before I get cracking. 🙂 Getting to the board in question is a lot of work. You basically have to disassemble the entire amplifier into tiny little pieces.
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If there is no load, (o/c led) when one is switched on, the full voltage from the 220u cap will kill any good led. Remove that capacitor (C523) and no more problems. A constant current supply, (15mA) would be ideal but not shown here.
Get an LM317L and a 680 ohm resistor and make a CCS. Remove those 4x10K and fix the CCS and remove C532.
Gajanan Phadte
Gajanan Phadte
Yes, leave C523 where it is and have a current source regulating the LED current. I guess shorting one of the LEDs leaves a short peak voltage on the other LEDs which destroys them.
It would help if the poster measures voltages.
It would help if the poster measures voltages.
It won't be anything more than that with the possible exception of a dry somewhere. However modern LED's will be like searchlights with 19ma or so which means you may need to alter the parallel 10k's to give something more suitable.
It is because the forward voltage of LEDs are not same. Because there is no current limiter on each LED line, the less LED FV, the more current flows into and eventually it gets destroyed by excessive current . You may even see some LEDs not lit even though it works alone outside.
Solution is to use rezistor at each LED line .
Solution is to use rezistor at each LED line .
Forward volt drop doesn't matter, they are in series. The reason for using such a high driving voltage into the 2.5k (the four 10k's in parallel) is to approximate constant current operation. The LED volt drops, whether its one in circuit or all of them, make so little difference to the current that the brightness is unchanged as you switch some of them in and out of circuit.
The problem of the failing LED,s can only be the circuit going open for a fraction of a second. That could be physical such as a dry or break or it could be one of the LED's being intermittent. Nothing else could zap the LED's.
The problem of the failing LED,s can only be the circuit going open for a fraction of a second. That could be physical such as a dry or break or it could be one of the LED's being intermittent. Nothing else could zap the LED's.
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