Mk llc + ..odd footswitch fault.. any ideas?

[sharkinabus]

Hi all,

I'm new to the forum.. great to find you!

Any help with the following would be appreciated..

I built a footswitch for my mkllc+ combo (twin footswitch latching type)

Initially I plugged into the mono socket under the chassis by mistake.
Then I found the correct stereo socket.

Then things got weird...

I had no output! There was signal getting to the speaker as it crackled when I turned the presence knob and I could hear a radio talk-back show....
otherwise I had nothing whatsoever! even after removing footswitch.. and trying the old voltage interrupted recalibration..

I powered down and opened her up to see if I could see any obvious damage/faults.. all looked good.

Reassembled, powered up and hey presto! It worked again..

Then I tried the footswitch again and It worked for EQ, though popped very badly when switched.
Reverb did not work, though this could be the reverb itself..
I was still picking up a radio signal with the footswitch plugged in..

Any comments ideas on this scenario?

Cheers!

Sharkinabus

 

[Octavarius]

The mono input you plugged in is the poweramp/preamp junction. (poweramp in or preamp out).
I'm not sure exactly how it works, but I'm guessing that you somehow just cut the signal path between the preamp and poweramp sections when you used a shorting relay.

By the way, could someone explain how one jack input can be a poweramp in and preamp out at the same time? How can one input share this function? How does this work?

 

[Boogiebabies]

The cable needs to be a stereo shielded type.
The tip/ground is for the reverb and the ring/ground is for the EQ. To attempt to reset the reverb, use a 1/4 in tip to ground plug and see if the reverb engages. The EQ pop is a common problem. The EQ circuit did not have a resistor to bleed surge voltage to ground, unlike the other pull or footswitch circuits. The repair is a 62K resistor from the top ground trace on the fader board to the ground plane of the main preamp board. The large area between V3 and V4 is the common area to place the end lead for ground reference.

The left socket on the back is the preamp out.
A feature that is seldom used, but if you use a 1/4 inch footswitch in this jack it will function as a preamp mute when engaged.

Any use of speaker cables or causing a loop with switching function circuits will basically act as a tube powered radio antenna. Hence, picking up Aboriginal Free Radio.

Good Luck and say hi to my Aunt Matilda.

 

[Boogiebabies]

The mono input you plugged in is the poweramp/preamp junction. (poweramp in or preamp out).
I'm not sure exactly how it works, but I'm guessing that you somehow just cut the signal path between the preamp and poweramp sections when you used a shorting relay.

By the way, could someone explain how one jack input can be a poweramp in and preamp out at the same time? How can one input share this function? How does this work?

The terminology is that you can use a power amp input into this jack. Basically, the same as the preamp output into the power amp input. It's a slighly different feel than the slave which takes it's signal off the speaker output taps.

 

[sharkinabus]

Thanks!

Any idea why I temporarily lost sound though?

Even with footswitch unplugged I could get nothing...

I also understand you can use a volume pedal on the preamp socket to act as a preamp gain volume control..

 

[Boogiebabies]

The preamp out shorting jack may have been hung open. If there is no short, no sound.

 

[sharkinabus]

Cool, thanks..

This is my only gigging amp and I'd hate to have an intermittent fault..

Cheers!

...oh and oddly enough Boogiebabies, as I write this post there is a TV show on our cultural TV station about an Aboriginal radio station... Spooky...

 

[Octavarius]

The mono input you plugged in is the poweramp/preamp junction. (poweramp in or preamp out).
I'm not sure exactly how it works, but I'm guessing that you somehow just cut the signal path between the preamp and poweramp sections when you used a shorting relay.

By the way, could someone explain how one jack input can be a poweramp in and preamp out at the same time? How can one input share this function? How does this work?

The terminology is that you can use a power amp input into this jack. Basically, the same as the preamp output into the power amp input. It's a slighly different feel than the slave which takes it's signal off the speaker output taps.

So you mean it's only a preamp out? Wow, now I'm confused. This is from the IIC+ manual from the Mesa site:

A Pre Amp Out-Pwr Amp In jack is located on the bottom of the chassis and
provides access to the junction of the preamp and the output power amp. With it, another Boogie can be
linked up (Slave Out of the first amp to Pwr Amp In of the second), or it can allow you to use the Boogie
as a preamp only.
For weaker incoming signals, the Effects Return can be used as an input and then the Graphic
would be functional.