please no smoking
Hello again,
Back on October 18th post #387 p39 I posted about a runaway bias issue. It looked like I may have let some contact happen between MOSFET and heat sink because only one MOSFET had superheated. I ordered new triplet of MOSFETs, installed and got my right channel (the one without problems) tuned up then tried the left channel with the new MOSFETS.
I came on (with the bulb tester) higher (more voltage across a 0.47 source reisitor, 125mV as opposed to 50mV on the good or right side) than the other channel. It seemed to stabilize though. While I was adjusting the P2 pot for offset (which had not effect by the way) it shot up to twice its innitial value so I shut the thing down.
So, it was not a bit of metal that fried the first MOSFET. Something is amiss. From what I have reported so far is there a prime suspect?
I know these things are hard to figure out.
I can and will depopulate and check the entire board if necessary but I wouldn't mind starting with the usual suspects.
Hello again,
Back on October 18th post #387 p39 I posted about a runaway bias issue. It looked like I may have let some contact happen between MOSFET and heat sink because only one MOSFET had superheated. I ordered new triplet of MOSFETs, installed and got my right channel (the one without problems) tuned up then tried the left channel with the new MOSFETS.
I came on (with the bulb tester) higher (more voltage across a 0.47 source reisitor, 125mV as opposed to 50mV on the good or right side) than the other channel. It seemed to stabilize though. While I was adjusting the P2 pot for offset (which had not effect by the way) it shot up to twice its innitial value so I shut the thing down.
So, it was not a bit of metal that fried the first MOSFET. Something is amiss. From what I have reported so far is there a prime suspect?
I know these things are hard to figure out.
I can and will depopulate and check the entire board if necessary but I wouldn't mind starting with the usual suspects.
naughty little TL-431
Swapping in a new TL-431 did the trick!
I checked all the surrounding resistors while I had the board on the operating table but it was just that silly little three legged frustration device. I recommed everyone to buy the Texas Instrument version as it has tighter specs...
Now that all is well I'm stupidly going to ask: can I add a few LEDs to light my front pannel? How best to do this, draw directly from the 24+/- power supply?
The LEDs I'd like to use (4 of them) have a FV of 3 to 3.4 and draw 24mA...
Swapping in a new TL-431 did the trick!
I checked all the surrounding resistors while I had the board on the operating table but it was just that silly little three legged frustration device. I recommed everyone to buy the Texas Instrument version as it has tighter specs...
Now that all is well I'm stupidly going to ask: can I add a few LEDs to light my front pannel? How best to do this, draw directly from the 24+/- power supply?
The LEDs I'd like to use (4 of them) have a FV of 3 to 3.4 and draw 24mA...
Oh there's nothing wrong with FW!
I just don't know the ramifications for tapping the supply. I really need some brightness as I am not just putting in an indicator but lighting a opal piece of plastic from inside the case. I need 4 LEDs at at least 12mA. See photo for fugly front piece...
I just don't know the ramifications for tapping the supply. I really need some brightness as I am not just putting in an indicator but lighting a opal piece of plastic from inside the case. I need 4 LEDs at at least 12mA. See photo for fugly front piece...
Attachments
Hi Andrew, super guy with engineering data 
I'm completely out of my depth here so it is as if I am superstitious: I'm hoping to draw at least 12mA not 0.003mA. I need to tap a not negligible amount of power off the PSU or so it seems to me.
I'd like to run 4 blue LEDs (FV3.2) / channel (in series) at 12mA. Calculations from Quickar indicate a need a 1K 1/4W resistor instead of the standard 10K (in position R23 on the F4 schematic I have). Will this have a deliteroius effect on my amp or am I just a sissy?
I'm completely out of my depth here so it is as if I am superstitious: I'm hoping to draw at least 12mA not 0.003mA. I need to tap a not negligible amount of power off the PSU or so it seems to me.
I'd like to run 4 blue LEDs (FV3.2) / channel (in series) at 12mA. Calculations from Quickar indicate a need a 1K 1/4W resistor instead of the standard 10K (in position R23 on the F4 schematic I have). Will this have a deliteroius effect on my amp or am I just a sissy?
Attachments
ooooops, the m in 0.003mA should not be there.
Using 3 or 4 LEDs in series is OK if you have sufficient voltage to drive the lighting circuit.
But do take account of varying supply voltage. Is it 24Vdc +-0.5V, or +-1V, or +-1.5V?
Calculate your resistor for the nominal voltage. Check at minimum voltage and maximum voltage, checking maximum current does not blow the LEDs nor overheat the resistor and checking minimum current actually turns on the LEDs.
Using 3 or 4 LEDs in series is OK if you have sufficient voltage to drive the lighting circuit.
But do take account of varying supply voltage. Is it 24Vdc +-0.5V, or +-1V, or +-1.5V?
Calculate your resistor for the nominal voltage. Check at minimum voltage and maximum voltage, checking maximum current does not blow the LEDs nor overheat the resistor and checking minimum current actually turns on the LEDs.