Mesa MkIIB - Relay Buzzing

[acorkos]

The channel switching relay buzzes with the channel footswitch engaged in Normal mode (the LED on the switch lights when in Normal mode)

When I clipped the LED, the buzz is gone, the switch still functioning.

The buzz is definitely coming from the relay itself....little plastic rectangular box with a see-through top.

I replaced the LED and the 680R resistor in the switch...same buzz.

Removing the footswitch and using the switching fiunction on the amp front panel (pull switch on the Lead channel gain pot)....no buzz.

So it seems something inside the amp in the switching circuit doesn't like the LED in the footswitch, There's a diode and 3uF cap between the footswitch jack and the relay....maybe it's one of those components or the relay itself?

http://www.schematicheaven.com/boogi...ogie_mkiib.pdf
the switch circuit is at the very bottom of the schematic

[oc disorder]

Looks simple enough although doesn't show the internal switch looks like the supply for the relay comes from the heater supply via one diode.
This is then smoothed by the 470uF 16volt capacitor passes through the 39 ohm resistor through the relay coil where it is connected to ground or not depending on the switch.
If the relay is chattering (going on and off rapidly) it would suggest to me there is not enough power getting to it to magnetise the relay enough to hold it. I would check the cable and plug to the f/s which may have some resistance.As the front panel switch energises the relay ok there is no need to check the 470uf/16v cap.
A quick test would be to insert a jack with the tip and sleeve shorted together into the f/s socket. If the relay changes ok like the front panel switch it must be in the f/s itself.

Attaching an early version but has voltages marked on it.
Even though the dropping resistor is bigger the voltage readings should be similar.

[acorkos]

A quick test would be to insert a jack with the tip and sleeve shorted together into the f/s socket. If the relay changes ok like the front panel switch it must be in the f/s itself.

the relay doesn't buzz with a shorted plug inserted into the amps FS jack.
The relay doesn't buzz when I use the footswitch, as long as the LED is removed. the buzzing only occurs in the presence of the FS LED.