Lead 100 Mosfet: Bass, Middle, Treble Knobs Non-responsive

[Searching4Tone]

Not too long ago, I bought a 1986 Marshall Lead 100 Mosfet solid state head and applied Deoxit contact cleaner to the pots as they were scratchy. Now that I can hear the amp clearly, I noticed there is no difference in the tone when I change the Bass, Middle or Treble knobs in any settings. Zero or 10 yielded the same result.

Can you help me solve this issue? Thanks in advance

 

[ampmadscientist]

Not too long ago, I bought a 1986 Marshall Lead 100 Mosfet solid state head and applied Deoxit contact cleaner to the pots as they were scratchy. Now that I can hear the amp clearly, I noticed there is no difference in the tone when I change the Bass, Middle or Treble knobs in any settings. Zero or 10 yielded the same result.

Can you help me solve this issue? Thanks in advance

This type assembly can oxidize, disabling the functions...
any place that there is metal to metal electrical contacts (like plug-in connections between boards, jack contacts/ switches, ground connections ) which becomes oxidized, can fail.

You may go thru and clean a bunch of connections, maybe still not get anything. You may resolder besides...it's hard to tell where a connection has failed.

But, on the other hand, I have cleaned these amps connections and they worked again too...

 

[mickeydg5]

Um, the switching for the Master EQ or a connection/contact could be malfunctioning.

I think that is a greater possibility than all of the EQ potentiometers going sour.

 

[Searching4Tone]

This type assembly can oxidize, disabling the functions...
any place that there is metal to metal electrical contacts (like plug-in connections between boards, jack contacts/ switches, ground connections ) which becomes oxidized, can fail.

You may go thru and clean a bunch of connections, maybe still not get anything. You may resolder besides...it's hard to tell where a connection has failed.

But, on the other hand, I have cleaned these amps connections and they worked again too...

Thanks ampmadscientist. I'll go through and clean the connections first, before resoldering or replacing the potentiometers.

 

[Searching4Tone]

Um, the switching for the Master EQ or a connection/contact could be malfunctioning.

I think that is a greater possibility than all of the EQ potentiometers going sour.

Thanks for the response mickeydg5. The knobs were extremely scratchy when I got the amp, in fact none of them work properly until a contact cleaner is applied.
I'll definitely check the electrical contacts first before resorting to getting new potentiometers.

 

[ampmadscientist]

YOU DON'T NEED TO REPLACE THE POTS...it's almost always a bad connection somewhere...

 

[FleshOnGear]

As said above, it's probably a bad connection. When the tone controls sound like they do nothing, but you still get a full sound, it indicates that the tone "stack" isn't connected to ground at the counterclockwise tab of the mid control. That ground connection is needed to dip the mids and bass, otherwise every control sounds like it's stuck at 10, and even higher at the mids.

The Lead MOSFET has some unusual switching happening at the base of the tone stack that might be acting up. Also, I vaguely recall that the tone controls might be bypassed in boost mode. Do they work normally in clean mode? I haven't played one in 20 years and I've never been inside of one these amps, maybe somebody has better insight...

 

[scott-colorado2016]

I have the very same amp. The bass, middle and treble controls only work for the non-boosted channel unless the gain knob is pulled out. If the gain knob is in, only the tone knob by the boost channel volume is active. This is true whether you are using a footswitch to channel select or running it without a footswitch where the two channels are both active (and blended using their respective volumes). Do a search, the manual is out there and explains it.

 

[BowerR64]

I had one also and when i first got it it was full of bad connections, mine had a nasty humm. I first cleaned all the input jacks and pushed in a cable and the humm got a little quieter, then i did all the pots and it was a little better finally i cleaned the reveb tank connections and it was golden. My amp was littered with oxidization

I will agree with scott, this amp and its tube version are REALLY weird amps how they work and how the channels stack/layer/cascade what ever they do. You can get it into a mode where the EQ knobs do nothing you just have that one "tone" knob or "shape" knob that really limits how it can be set.

I will also add my amp was a very subtle when using the EQ knobs there was very little change in the boost/cut of the EQ knobs. COmpared to other amps ive owned

The subtle change can be a good or bad thing IMO, other amps that have drastic changes in EQ can get you introuble boosting to much at certain volumes

That amp is one that has the classic "it only sounds good cranked" because of the subtle EQ controls you really cant set it wrong since they are so subtle