Simple question I hope

[Oldmactech]

Replacing an LED that has failed on a Chinese Peavey Classic 30. Simple proceed in theory except I have had to dismantle the entire chassis and pull the PCB just to get at the LED. First attempt failed.

Simple question is this: can I switch power on with the chassis in standby mode with no tubes and no dummy load attached without doing any damage to test the LED function?

thanks.

[Jon Snell]

If all needed connections are made and it is safe to do so, yes.

[g1]

You don't need a dummy load if power tubes are removed.

[Mars Amp Repair]

Only issue I can see is without a load on the power supply, the B+ can possibly rise above the safe operating rating of the filter caps. On many amps the standby switch is after the first filter cap so it will get charged even in stby.
You can decrease the AC input voltage about 10% or so to prevent this from occurring and the rest of the amp would function as normal for you to trouble shoot the LED issue.
Be careful to discharge the filter caps after you power the unit down as they will be charged higher than they normally would and depending on design esp if no bleeders, remain glen

[Oldmactech]

Finally getting around to swapping in a new LED. Need help with the PCB and LED convention. I’m believe the longer lead on the LED is positive, correct? The mounting point for the LED has a circle with a white line that crosses the circle that intersects one of the solder pads. Positive mounting point?

Thanks?

[The Dude]

The long lead is normally the anode lead. That said, you can test it with your meter on diode test to be sure. I have had some cheap Chinese LED's that had the long lead as the cathode. It's best to just test and be sure. On the circuit board, same thing- use your meter to measure voltage on the pads where the LED solders in and see which pad is more positive than the other.

[Pixel]

If you can see the guts inside the LED the bigger side is the cathode.

[catalin gramada]

..

[Oldmactech]

Thanks. I will test the LED but I have this thing completely apart just to get to the solder pads and I’m not comfortable turning it on so that something doesn’t short against metal casing. I want to avoid the tedious reassemble only to find out it’s backwards.

The graphic on the PCB looks similar to the red flat-spotted graphic on the file attached. Odds are that would be the correct orientation?

[g1]

The graphic on the PCB looks similar to the red flat-spotted graphic on the file attached. Odds are that would be the correct orientation?

Yes, flat side denotes cathode.

[Oldmactech]

Hate to be a newbie but would be interested in the science behind this LED.
The original LED was clear.
The new LED is white and it seems the voltage drop is higher on solid white.
Will this cause initial failure or early failure.
Hopng not to disassemble this thing again.
Should I look for clear?
Any other tips?
thanks.

[mhuss]

Different LEDs will drop different nominal voltages, grouped by color. Did the original/clear LED emit white or another color? (This for anyone familiar with the amp in question.)

[Oldmactech]

I don’t know for sure as it as under a jewel cover when it failed.

[Helmholtz]

What is the color of the jewel? Not much sense having a white LED behind a red jewel.
The forward voltage of a white LED is typically higher by at least 1V than that of a red LED.
A red LED can have a clear mold/case.

Please post amp schematic and if possible the datasheet of the LED.

[g1]

The LED is D1, in series with 1K2 1W running off 36VDC (edit: 36V is negative).
It is described as WHT T1 3/4 LED S-BRT (peavey part # 30450879)

schematic attached