Samson Servo 260 Idle Light

[stoneattic]

I've jut replaced the output relay in a Samson Servo 260 that had individual channels cutting in and out and it seems to have solved the problem.

But I've noticed that the "Idle" lights on neither channel seem to do anything. According to the manual:

"These lights go on whenever signal is present at the corresponding input. They go off when no signal is present."

A call to Samson gets me the same explanation so I'm still not sure what it means. Does that mean that if there's audio going in the Idle light should be on? That makes no sense to me because if it's amplifying signal it's not Idling. Maybe it's my automotive background.

Regardless, signal or not, sound or not, etc, the Idle lights never come on. Neither do the Clip lights, but it's used in a studio and is never near clipping, so those may or may not work.

I've tested the LEDs themselves and they all work (both channels, Clip and Idle) and I read 50 VDC on pin 1 of each connector.

Can anyone explain how the Idle light is supposed to work and if mine isn't working, any idea what might be wrong?

Schematic attached.

Samson Servo 170_260_550 Schematic.pdf

[Jazz P Bass]

The idle (signal) led gets turned on by the speaker output.
Is there any voltage on the anode?
Can't read the schematic too clearly but it appears to be the B+.

[stoneattic]

The schematic could definitely be better, but that's the best I've been able to find.
The B+ on the anode of the LEDs are 50VDC on both channels at the pins on the main board with the LED board disconnected.

[Jazz P Bass]

Yes but is the B+ reaching the led anodes, through the connector.
That is the only item common to all of the LED's.
The idle signal comes from the speaker.
The clip signal comes from the top of each channels LTP.

[stoneattic]

When I took the 50Vdc measurement yesterday I tried to get a measurement on the LED board and got something much lower than the 50Vdc (something like 10Vdc or so). I only measured 1 channel on the LED board. I figured it was giving me a false reading because of the circuits on the other side of the LED. But I guess if the LED is not lit then that circuit should have no affect. I will will check that tonight.

I would be strange that both channels cables and/or connectors would be bad, but I guess if it's a crappy design maybe it should be expected.

[Jazz P Bass]

The B+ side (anode) of the LEDs should not change, regardless of the circuit that is 'turning on' the LED.
Q120 L_R (clip) & Q121 L_R (idle) simply provide a resisted connection to ground for the LEDs when 'on'.

[stoneattic]

I measured the B+ at the LEDs and they are both 50Vdc. So either unplugging and plugging the connector cleaned them off or I just didn't get a good reading yesterday. So instead of testing with the usual studio monitors I plugged into the PA speakers and found a few things out:

1. If I use 90° pancake speaker plugs and the plugs touch the right channel is distorted. With straight plugs, no problem.

2. The idle lights work but the amp needs to be at last at 50% of the full range on the amps output knob. The Idle light seems like a pretty useless function. It indicates that the painfully loud sound coming from the speakers is because the amp works. Duh.

3. One of the Clip lights works if it's REALLY cranked. I swapped LED boards and used the diode tester on my DMM and it appears that one of the Clip LEDs is dead.

So I need to find a replacement LED. I'll give Samson a call tomorrow hoping to get an exact replacement. If not I'll measure it (its rectangular) and order two from Mouser or where ever just so they have the same brightness.

Thanks for the help!

[Jazz P Bass]

If you feel the idle LED lights too late in the power curve, you might want to change out C117 L & R.
Being that they are E Caps & the amp is getting along in years, they may be upsetting the function of the circuit.

[stoneattic]

I'm sure I have some 10uF E caps so I'll give it a go.

Any chance you've seen LEDs shaped like these before:



I'm having a tough time finding them with this shape. They are 5mm x 4mm at the base and 5mm x 2mm in the thinner section. About 9.6mm tall.

[Jazz P Bass]

Green LED: NTE3161 - Green LED - Rectangular - VETCO.NET
Red LED: NTE3160 - Red LED - Rectangular - VETCO.NET
Read the datasheet to confirm the size.
They look like the ones.

[Enzo]

They are common enough. The narrow front end size is common, though I usually see them that width all the way back. Yours are wider at the back so they can have the little shoulders. The shoulders keeps them from poking farther out through the panel, which may or may not be an issue.

The ones JP linked ought to work, or leaf through Digikey or Mouser.

[stoneattic]

Thanks guys.

I had searched my usual suspects (Mouser, Newark, Digikey) without any luck. The ones Jazz P Bass posted the links to are very close. Close enough for sure.

The board is held in place by little snap in tabs and it looks like the LED shoulders are what keeps the tension on the tabs so I probably need the shoulders.

I have a couple of other projects (4 digit 14 segment alpha-numeric display in a Behringer Virtualizer with a bad segment & a clip light coming on an AB Systems 410C amp when it doesn't sound like the amp is clipping) that I will likely need to order parts for once I diagnose them. I will place the order for the LEDs with those parts. Kind of silly to pay $18 with shipping, etc., for a $0.40 part.

[Enzo]

Or call Samson and get the exact part from them.

[stoneattic]

Or call Samson and get the exact part from them.

I tried them first. They said they didn't carry parts for items that old. I've had good luck getting parts from them before, but it was always for more recent/current products.

[Enzo]

Other than the ones Jazz linked, which ought to work, another alternate is to just use the common flat ones. And if you need the shoulders to position them, glue a litle hunk of plastic to the side of each one