SWR SM400 bass amp problem

[Randall]

Someone help me please. This SM400 is working except for the gain clip and limiter LEDs. The gain knob does as it should and will go into clipping when set too high, but LED does not come on. The limiter threshold knob is also working, but no LED. Both LEDs come on when tested in circuit on my Fluke.

Both LEDs are connected at a leg, which is connected to a wire that runs down to the V+ of an op amp in the input section. Each of the other legs run via their own wires to something farther down the board. What exactly, I don't know.

The problem I am having is I cannot make sense of the schematic. Components not numbered, generic op amps, and what is O/L '1'. O/L '2', and O/L '3' ? I don't see the LEDs anywhere. This seems more like a sketch than a finished schematic to me.

Can someone enlighten me please?

http://schems.com/manu/swr/swr_sm400_bass.pdf

[g1]

The led's are shown on page 5 of the pdf. Do you have +15V at the anodes of both diodes? The only other thing that looks to be common to both circuits is the IC, an LM339.

[Jazz P Bass]

Check the voltage labeled 'B' on page 1 or page 5.
You should see a 'low' voltage that will rise as the threshold is reached.
(a high & a low are logic levels. For a comparator circuit, the actual values are not as important as are the differences in them.)

To turn on the LED, the comparator that drives the LED needs to see a higher voltage on the - input (Point 'B'), as compared to a semi high voltage on the + input.

The comparator output pin is an open collector output (to ground), so ensure that the LED itself has voltage on one leg.

Note: The 'OL' designations (OL 1, 2 3) are inputs for the OverLoad LED circuit.

[Randall]

I see. Literally. You guys are great!

I do have 14V on one leg of each LED. I was going to guess at the LM339, because it appears the wires run from the LEDs over to that IC, but it's always better to have a sound explanation. But, I bet that's it.

Thank you!

[Randall]

To follow up, it was indeed the quad op amp IC LM339N. It was in upside down, courtesy of the local guitar store repair guy. I see a lot of his work. Local supply house had one in stock and it's now up and running. Thanks again.