Panama Fuego pilot light issue

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910
I recently picked up one of these are a good price. The amp sounds great but has an issue with an intermittant pilot light (LED). I had read in some older posts that this was an issue with these amps, but couldn't locate a fix for it. I tried re-flowing the solder joints. That worked for a couple of minutes, then the light went out again.

Anyone remember these amps, and have a possible fix?
 
Messages
910
It's the blue LED next to the standby switch. The red channel and gain lights work 100%. And the amp works 100% as well. It's just the blue LED that is being stubborn...
 

HotBluePlates

Member
Messages
13,876
... intermittant pilot light (LED). ... I tried re-flowing the solder joints. That worked for a couple of minutes, then the light went out again. ...

I have not been inside any of the Panama amps, and they apparently refuse requests for schematics (according to other online posts). Below would be my thought-process for trying to fix this given no schematic, so take this with a grain of salt.

You reflowed solder; does the amp have a p.c. board backing right up to the LED, or are there wires from the LED/holder back to the board? Is there a voltage you can measure present where the LED connects to the board? Is it steady/missing/intermittent?

LEDs usually have a current-limiting resistor (that also drops a too-high voltage down to an appropriate forward-voltage for the LED). In some LED packages, the resistor might be integral to the package; at other times (or with older circuits), the resistor is a separate part. Where is this resistor (in the LED package, or soldered to a leg of the LED, or mounted on the board)? Could a connection to that resistor be intermittent or cold-soldered?

P.C. board traces to the resistor/LED could be faulty. I worked a summer at a p.c. board manufacturing facility, and even well-made boards have some small percentage that are defective. It could be a cracked trace (especially if board substrate is thin & flexible) or a short from one trace to another. You might be able to detect a board fault by checking for the voltage that should be fed to the LED to turn it on. That said, board faults are also a hassle to find, and are almost never repaired except on some military gear (where the cost/effort is justifiable).​

Of course, if the amp works you might decide you don't need a little light to tell you it's working (especially if the channel LEDs function). I notice that some Panama Fuego amp photos show a blue pilot light, while others don't...
 
Messages
910
You reflowed solder; does the amp have a p.c. board backing right up to the LED, or are there wires from the LED/holder back to the board? Is there a voltage you can measure present where the LED connects to the board? Is it steady/missing/intermittent?

LEDs usually have a current-limiting resistor (that also drops a too-high voltage down to an appropriate forward-voltage for the LED). In some LED packages, the resistor might be integral to the package; at other times (or with older circuits), the resistor is a separate part. Where is this resistor (in the LED package, or soldered to a leg of the LED, or mounted on the board)? Could a connection to that resistor be intermittent or cold-soldered?

P.C. board traces to the resistor/LED could be faulty. I worked a summer at a p.c. board manufacturing facility, and even well-made boards have some small percentage that are defective. It could be a cracked trace (especially if board substrate is thin & flexible) or a short from one trace to another. You might be able to detect a board fault by checking for the voltage that should be fed to the LED to turn it on. That said, board faults are also a hassle to find, and are almost never repaired except on some military gear (where the cost/effort is justifiable)​

Yes, there are short leads from the PCB to the LED/panel. I was measuring about 1.5v on those leads. It seemed low to me, but that was when the light was working. I didn't think to check the dropping resistor. Thanks for the idea! I'll try re-flowing those connections as well. Since the amp is otherwise working, I'm not going to try to trace any PCB faults.

Of course, if the amp works you might decide you don't need a little light to tell you it's working (especially if the channel LEDs function). I notice that some Panama Fuego amp photos show a blue pilot light, while others don't...

I chuckled when I saw your comment. ;) From what I had read, it's a fairly common problem with this amp. Thanks for your help!!!
 

HotBluePlates

Member
Messages
13,876
... Yes, there are short leads from the PCB to the LED/panel. I was measuring about 1.5v on those leads. It seemed low to me, but that was when the light was working. ...

Typical forward voltages are anywhere from 1.5 to 24vdc. That's from one lead of the LED to the other, just in case there's some gimmicky wiring that has an actual ground between the two leads. IOW, 3v forward voltage with that gimmick wiring would read 1.5v from each lead to ground.

Your LED might have been getting exactly what it needed. The LED should clamp itself to its required forward voltage, and draw whatever current is required to get it there. This is why the LED is fed from a current-limiting resistor, which is chosen to give the desired operating current when the supply voltage is dropped down to the LED's forward voltage.​

Pulling a random LED, here's a data sheet. It says typical Forward Voltage is 3.5v, and that maximum Power Dissipation is 98mW, while maximum Steady Current is 30mA. 98mW / 3.5v = 28mA of current, so we can see where the limits come from. Catch is, an LED gets brighter with higher current, and you might not need a full 28mA @ 3.5v to light up the LED. Maybe you only need half that, or 14mA.

Let's say there is a 6.3v supply handy (like maybe our heater wiring), this means we might tap off that to power the LED. 6.3v - 3.5v = 2.8v to drop in the limiting resistor. 2.8v / 14mA = 200Ω, so we might have a 200Ω resistor in series to one leg of the LED (or if 100Ω resistors are handy, a 100Ω resistor in series with each leg of the LED, then attaching to the 6.3v wiring).

AFAIK, it doesn't matter that the 6.3v is a.c. and not d.c. After all, the LED is a diode. Technically, the LED will flash with each cycle of the a.c., but 60 times per second still looks like steady light (a movie is typically 24 or 30 frames per second, and they seem to do alright :D). I don't think there's a noise penalty for doing it this way, but I'll let someone else chime in on that. I know I've lit LEDs from heater wiring to create a light-up faceplate, and didn't have any hum or buzz issues in that amp as a result.
 
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