Marshal MG100 hbk Volume Jumps?

[Soupaman91]

Hello everyone,

I'm sorry if this is a bit of a newby question, but my knowledge of how things work is pretty rudimentary!

Anyway, I have a Marshal MG100 hbk solid state amp that suddenly developed a problem. I've noticed that the volume is cutting out, when working properly everything is well, but I will be playing and after a couple minutes I will have a drastic reduction in power (not sure if there's a tone difference). This jumps back and forth as I play and is obviously quite frustrating. I'd say the volume equivalent on the knob is: when working 9 o'clock, when not working 3 o'clock.

Any thoughts on what is wrong (and whether it is easily fixable) would be much appreciated!

Thanks!

[J M Fahey]

Start by plugging a guitar cable from the Line/effects_loop/preamp_out jack to its matching Line/effects_loop/preamp_in jack .
These jacks have an internal contact which often rusts/oxidizes and causes intermittent signal loss as you describe.
If so, clean it with a squirt of Deoxit and working the plug back and forth a few times to mechanically clean it.
If not, keep asking.
Good luck.

[Soupaman91]

Thanks for the reply! I'll try it out once I get home from work, just to clarify for cleaning, would you recommend removing the electronics panel (don't know what it's official name is) from the back of the amp to clean it or just use something like a pipe cleaner?

Thanks again!

[phatt]

He means use the plug jack itself. :tu:
You can pull the whole thing apart but a lot of these 6.5 chassis sockets are encapsulated anyway and as such are almost impossible to get to the actual problem.
A lot of *in the Know* users just leave a short lead permanently inserted in those efx loops and this saves a whole lot of messing about.
Phil.

[Kaz Kylheku]

Pick the fruit from the low-hanging branches first, then proceed to the more difficult fruit.

Though I think that this problem is probably not caused by the mechanical connection in an effects loop jack, or between a jack plug, or the wiper contact in a potentiometer, it's not difficult to rule these out.

If you believe that a gain or volume potentiometer is causing this, you may be able to restore it by cleaning. Blow aerosol contact cleaner into the potentiometer and give it a few good turns back and forth. You may have to repeat a few times. If that doesn't help, find a potentiometer with the same physical dimensions (diameter, shaft length) and electrical resistance, and replace it.

Bad jacks can be replaced also (and I second the idea that if you have a jack-switched FX loop that you are not using, it may be worth it to just bridge it with solder connections so you have one less tone-sucking mechanical connection in your signal path).

If you rule out jacks and potmeters as being the cause, the next thing that might be it is a sliding connector inside the amp (if it has any). If there are any such connectors between the amp's circuit boards then (with the power off), gently unplug them and plug them back in. Be sure not to exert any pressure or tension on circuit boards. They are fragile, and sometimes the connections are hard to unplug or plug in. You may have to squeeze a little tab that opens a latch to allow the connectors to separate.  Sometimes unplugging and re-plugging a connector can restore a poor contact. (If doing that fixes your amp, I would consider rebuilding or replacing that connector.

If it's none of the above, the next thing we suspect is a failed solder connection on a circuit board, or possibly on some off-board component like a potentiometer.

I would proceed by using some non-conductive tool (wooden stick) to gently prod or tap the components (resistors, capacitors, transistors, hookup wires) with the power on. If any component causes popping, crackling or basically any noise when gently poked, it almost certainly has a bad connection. You may be able to repair it simply by melting the solder and allowing it to cool.

How can poor connections cause a fluctuation between two volume levels? Pretty darn easily.

For instance, between one amplifier stage and the next there may be a voltage divider (pair of resistors in series, going between the output of the previous stage and ground. The actual output to the next stage comes from the connection between these resistors). This voltage divider takes a fraction of the signal voltage to the next stage.  If the voltage divider's ground connection is disconnected, then the signal path still exists, but has a different level.  If the ground connection is flaky, then there may be a fluctuation between two levels as the voltage divider cuts in and out.

In the amp there may be fixed voltage dividers made with resistor components. But controls like gains are also voltage dividers. A bad ground connection to a potentiometer can thus do the same thing. Voltage dividers can be in feedback paths to; there, a flaky ground will cause level fluctuations by varying the gain.

Speaking of gain and feedback, in an emitter-follower amplification stage, a flaky connection on the capacitor that bypasses the emitter resistor will cause gain fluctuations. When the capacitor is in, negative AC feedback is shunted to the ground and the stage has a high gain. When the capacitor cuts out, AC signal flows through the resistor, inducing negative feedback, and reducing gain.

You don't have to understand any of that to find the component with the loose connection.

Please read the website's safety topics before doing any poking in live equipment. Don't touch any exposed conductors with an electrically conductive probe. If you do any repair, turn off the amp first, and wait a few minutes for all of the big power filter caps to discharge. You can still get a shock from the capacitors even if the device is unplugged from the power grid.

[joecool85]

If you do any repair, turn off the amp first, and wait a few minutes for all of the big power filter caps to discharge. You can still get a shock from the capacitors even if the device is unplugged from the power grid.

Lots of amps don't discharge the power caps with the amp unplugged so it's a good idea to measure the voltage across them with a voltmeter, if there is more than 1v or so you should drain it.