SWR sm500 Limiter Not working

[wizard333]

The light comes on as if it were working, but even with the knob maxed out there is no audible limiting. I'm familiar with how an SWR limiter sounds when working and the sound isn't subtle with the knob maxed. Anyone familiar with limiter circuits have a suggestion on what to check?

[mozwell]

While i have never worked on one of these, the circuit looks straightforward.
The limiter depends on the gain pot setting, and is interactive with it.
First, i would check the shorting link works on the lo gain input, as the 82k is shorted to 0V when using the high gain input, and the limiter acts on the junction of the 82k & 1M & input to the op amp.
If that is ok, check you have signal at the output of the op amp with the "gain" pot. Check the wiper of the "threshold" pot, do you have a proportion of the signal from the gain op amp, does it vary with the threshold pot setting.
Check the next two op amps in the string, do you get what you expect ? the first is a buffer, the second is a x27 non inverting stage, with a peak hold on the output (33k, diode, 2u2) is the diode ok ?
With no input signal, meaaure the gate of the fet 2N5457, vary the "bias" pot, does the voltage on the gate vary.
If all this is ok, replace the fet. These are cheap & common parts.
You could also try a J201, 2N5484, instead. All fets have a wide spread in Vgs cutoff voltage, so maybe try a few different fets till you get one that works ok.

There is a test procedure in the sm400 pdf, this may help to check the limiter is working & how to adjust it.

Good luck.