20 Most Recent Marshall Mg100dfx 100-Watt Combo Guitar Amplifier - Page 3 Questions & Answers

Typically these input jacks have 3 or 4 "tails" that solder to the circuit board. The best thing is to get an exact matching jack. You can search for 1/4 inch jacks with matching profile at Digikey.com, Mouser.com, and try www.tubesandmore.com/

In the future ALWAYS loop the guitar cord through the handle so plug is not yanked sideways when people trip over the cord.

A educated guess points to either broken soldering or a broken ground on the input jack itself. This is a CHRONIC problem of amps because people trip over the cord and it yanks the cable and jack sideways breaking it or the solder where it attaches to the circuitry.

To avoid the problem, loop your cable through the handle of the amp so when cable is yanked it doesn't yank the jack so hard.

Here's a link:http://www.marshallamps.com/downloads/files/MG50_250%20hbk%20Eng.pdf Although not exact should be close enough.

Maybe. The cost involved wouldn't reward you enough. Here are some links for you:http://www.drtube.com/marshall.htm and http://duncanamps.com/index.htm and http://www.ampix.org/ If you have all the tools, parts, schematics and above all experience. You may end up with something you like. Personally I would look for a vintage Marshall that needs a rebuild and start there. You will end up with a great amp and the experience you need to mod the MG100DFX if you really want to do it. Hope this helps.

One, You may Have a bad 1/4" jack on the Input, or Two you may have unballenced Bias on the Output Tubes. This is a technical adjustment in the Amplifier.

The way you troubleshoot this is to put a 150 watt lamp in series with the input power cord. The lamp acts as a resetable fuse and will limit the current while you look for the bad component. Start with the capacitors and rectifiers connected to the power transformer.

Yes, they are a pain to have to remove the knobs etc. to get the board out.

If the resistor has burned or browned the board, resoldering may not be good enough as the traces to the pads and the pad MAY have unbonded from the board. In this case you may need to install eyelets to get adequate support for the resistor. The other method is to install "turret" terminals and attach the resistor to those... the turret terminals kind of rivet into the board to create a secure mechanical connection.

There are some possibilities. One, your speaker may be blown or disconnected. Make sure the wiring is still connected to the speaker and if you have one, try measuring the resistance(impedance) of the speaker with a volt ohm meter.Do this unplugged from current and disconnect one of the wires from the speaker or you will get a reading from the output stage. Two if there is a heaphone or line out jack it may have become faulty. If you have sound with the headphones plugged in your amp is still producing sound. Some jacks are designed to interrupt the signal to the speakers allowing you to practice without bothering anyone. If your amp has that feature and you still have sound with the headphones and your speaker tests OK then your problem is the jack. Hope this helps.

If the amp is quiet with cables unplugged at the same settings as observing the buzz, the amp is OK.

In this case, cables and/or shielding and wiring in the guotar is suspect.

Guitar pickups are sensitive to magnetic fields and you can search out these fields by moving and orienting your guitar to find maximum buzz.

If the amp has objectional buzz with teh cables UNPLUGGED then the amp needs servicing. Filter capacitors among other things are suspect.

The fuse would be internal. The fuse blew for a reason, and unless you had an accident with the amp such as shorting a speaker lead, there is likely internal damage requiring servicing. Replacing the fuse without curing the original reason it blew will likely cause damage to further components.

Take the unit in for repair.

That should be a 320K pot. <br /> <br /> Here's a link to the schematic. <br /> <a href="http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf">http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf</a> <br />

This is likely to be an internal problem. Check the +5 volt power to the DFX board as a first step.

Probably the bias is set wrong... Please note that when installing MATCHED pair tubes, the tube pairs usually go at opposite ends of the strings, that is one on either side of the center of a set of four.

There are MANY more likely components than the transformer that will cause the fuse to blow.

Most common are filter capacitors, rectifier diodes, and shorted components in the power amp section.

Transformer failure is very UN-likely. Have a competent electronic tech troubleshoot the unit.

When testing the unit, put a 150 watt light bulb in series with the power line... saves fuses during troubleshooting. This limits the current so one can analyze the real cause of the problem.

Use a SPEAKER RATED cable which will have good quality 1/4 inch jacks. Go from a speaker out on the head to one of the jacks provided on the speaker cabinet. NOTE: some newer speaker cabinets use SPEAKON connectors and if those are provided, they are more secure than 1/4 inch plugs/jacks.

PLEASE loop the speaker cables through a handle of the cabinet at each end to avoid them being yanked out while hot as the amplifier can be damaged if it happens.

OBSERVE the maximum load. If the amp says 4 ohms minimum, put no more than two 8 ohm cabinets on the amp as two 8 ohms parallel to 4 ohms.

DO NOT use an instrument cable.

Replace the fan with an IDENTICAL part. Do this before using the unit again.

There is a possibility that the fan is controlled by temperature and the failure is actually with that electronics rather than the fan itself.

NEVER disconnect a speaker with the power on the amplifier !!!!!! Good way to blow the amplifier.

If the speaker rattles while you are sending audio through it, it is likely damaged. It should be inspected as there are several failure modes, some of which as hazardous to your amplifier.

First test is to gently move the cone and listen for any rubbing of the voice coil. If you hear rubbing, then the speaker needs replacing OR re-coning.

Sometimes the magnet breaks off the back of the speaker.

Investigate this before further use.

What you're hearing is FEEDBACK caused by the nearness of the guitar to the amplifier speaker. Always mute it when not in use, or don't move it close to the guitar amplifier speaker when unmuted.

Usually, even though you hear a high-pitched screech, the problem is actually caused by low frequencies, sometimes below the human hearing threshhold (i.e., subsonic) because they are more omnidirectional and thus more easily easily can loop between input (the guitar pickup, especially on a hollow-body guitar) and output (the speaker). The screech frequency is determined by the distance between input and output, determining how quickly the output reaches the input (then loops around through the amplifier). Therefore, reducing the BASS a bit will often alleviate some of the problem.

This is basically the same type of feedback heard when a microphone is turned up too loudly or the mic is pointed at the PA speaker. Avoid those actions and your feedback will be minimized.

Only time and experience will allow you to automatically take actions to avoid feedback. I worked in the San Francisco Bay Area as a manager and sound man for rock bands and several nightclubs for 22 years. My college major was Physics (which includes acoustics).

Keep on rockin'!

The fuse blows because of a short in the Power Supply. The power transformers primary windings are probably shorted. next in the chain would be a bridge rectifier or a quad of diodes forming a bridge rectifier with a couple surge diodes off of the filter caps. so if you can disconnect the power from the amp, on the power tranny disconnect the primary wires ( the input AC power wires) usually white and black and take an OHM reading with a mulimeter accross the white and black wires. you should get a reading of 15 ohms or higher up to 30 or so. if so the primary windings are good. so then check the secondary (output windings) red and red yellow, etc. ohm reading and you should get a relatively low ohm reading 2-6 approx. if so its a good tranny. on the primary side if you get a real low reading for ohms , like 1 or something, its toast. once you rule out the tranny, switch your digital mulitmeter to read diodes (in the ohms area looks like a diode symbol (a line and a solid triangle with the line being at the tip end of the triangle). desolder one leg of the diode and put one probe on one side and the other probe on the other side. it should only ring out in one direction. you will be testing approx 6 diodes in this power grid on the pcb. you can also desolder the filter caps after you safely discharge the voltage with a 1 ohm 1 watt resistor to ground and use an analog volage meter to see the needle rise in one direction and not in the other (switching the test probes from one side to the other on the cap). usually what i do is just spend 20 bucks on all those parts from mouser.com and instead of desoldering to test i desolder to replace with new. diodes usually part number 1N4003 or 1N4004 somewhere in that ballbark. hope that helps. i think its your power tranny primary winding that is shorted, be aware that this might domino effect into the rectifier and caps, so expect to replace them. its super easy!! you can email me from my website yostamplifier.com