Question about Marshall Mg15dfx 15-Watt Combo Guitar Amplifier

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Input clean is very low almost not there. switching to OD works fine.

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  • elmerj1 Mar 06, 2009

    thanks for the input, fixed it with Cramolin R-5 spray on the input switch and the input jack! This stuff is amazing! Use it on anything electrical. elmerj1

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If memory serves me correctly, these units use a 4558 OpAmp Ic. Both inputs pass thru this IC. One side provides a clean amp stage and the other is set up to run in a higher gain mode. This IC needs to be changed, It should be a small IC with 8 pins.

Dan

Posted on Mar 06, 2009

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Have a xga 4000 amp only channel 2 works fine . Channel 1 very very low


Switch input connection pairs and if the 'fault' doesn't changes from one channel to the other it is almost certainly an amplifier or output fault.
Likewise switching the speakers. If after swapping the speaker connections the fault changes to the other speaker it is an amplifier fault. If not a speaker fault.

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Are you trying to use the mic input on the back or plugging it into the instrument input on the front? If you use a low impedance mic with a quarter inch instrument input cord, you would need a transformer adapter to make the mic work on high impedance. Even if you put it on line in, you would still have to convert it. Only the XLR socket on the back will work on a low impedance mic.

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1 Answer

High/low gain not working when guitar plugged in


Switch may be dirty. Clean it with some contact cleaner. Also check to make sure you have the right length plug tip and conductor count for the jack. There are 5 lengths in use.

It is a solid state amp so it is possible there is a faulty gain stage in the preamp/driver section. If the amp is an old one more than 18 years old (give or take) it is possible there is an electrolytic capacitor in the gain stage of the "High/Low", switch. It would be in the vicinity of the wiring from the High Low switch where they solder to the main board. It like is one of a few low noise Op Amps, 4558, 387, TL08X/TL07X/TL06X LF353 series. In the 8 pin IC the pinouts are the same, the characteristics are similar. The most basic feedback configuration is just one resistor connecting an input to the output.

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1 Answer

I turn my fx off but can still here it and the od sound is lower than the fdx button when i push it in to turn it on


2 things 1: there is no off button for the built in digital effects, you just turn the level down to minimum, the fdx button you speak of is actually fdd (frequency dependant damping) this is supposed to mimic how a tube amp sounds

2: when you press the od switch the amp goes into hi gain mode and will hiss with no input, the point is if you plug a guitar in and play it the volume of the guitar is way above the hiss and you don't hear it

so in conclusion having bought and sold many mg series amplifiers the on you have is perfectly ok

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1 Answer

My deville has just stopped producing any sound what soever. All the tubes glow and the red power light is on but no sound. I have tapped the sides of all the tubes and they all register a sound except the...


The first thing i would do is not leave it on in this way for too long. You could blow out a very expensive Transformer if its a faulty speaker. The worst thing you could do to a TUBE amp is run it without a speaker . I would test the speaker first. Just remove the wires going to it. And connect the speaker to either a car stereo or house system at low volume very low. And see what you get. if it works ? Good! Now check your guitar chord/wire. Try different ones try different guitars. If that doesn't work . Look into the input jack on the instrument input hack on the amp.. They do come loose off the board.Either get a new input jack or resolder the old one. if you only have one guitar look at the guitar output jack. Loose wiring on the guitar could cause this also. Dirty In and out put jacks could cause this. A pencil with Very fine sand paper wrapped around it will clean a dirty input/output jack. Try these things. Just remember poking around your amp is Dangerous. The electrical components hold a charge even unplugged that can kill you. No joke..

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My hot rods clean channel sounds great, but as soon as I go to overdrive, there is a loud squeeling sound that is only produced when the drive is on and makes different sounds the more i play with the...


I need you to run a few tests... one at a time.

Plug a set of headphones into the preamp output and simultaneously a set or a guitar or other low impedance device into the power amp input jack (this disconnects the power amp from the preamp)

Now, does the squealing noise appear in the headphone when in OD?

I also want you to test if the normal/bright switch has any effect when in normal configuration in OD?

If the normal bright switch has little effect on the squealing that helps isolate which stage is allowing the feedback.

I suspect that the source of the feedback is either a bad filter capacitor in the power to the preamp OR one of the contacts in the two realys that are for OD are making poor contact. These relays are switched "dry" which means there is little current to clean oxides off the contacts so they can make poor or no contact. An open contact would leave a grid floating as a high impedance "antenna" to pickup and feedback.

The first thing I would do if you had an oscilloscoppe would be to see that C36, a 22mfd 500 volt cap does not have high frequency ripple across it. One could try paralleling a new 22mfd cap across it to see if problem goes away... you will GUARANTEED have trouble finding a 500 volt one of these and have to settle for a 450 volt one which SHOULD be OK judging from the circuit. You can probably get one from Antique Radio supply... use Google to find them. Remember this amp has lethal voltages in it so if you are not adept at working on electronics of this type, best to take it to a shop that is.

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1 Answer

Hey, the clean channel on my Marshall 250dfx Combo amp isnt working the distortion and everything is working as normal, but the clean channel makes no sound at all, i was wondering if you can help?


The switching is done electronically by an analog gate chip. On IC9, check if the signal on pin 8 changes with switching between clean and OD... This is the actual signal that changes the gate. If you don't see a change there of say 5 volts, then the problem is in the circuits that drive this gate. If the signal is there, then the problem is in the clean channel audio op amps or the components around them. Let me know the results of the test.

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1 Answer

FENDER HOT ROD DEVILLE 410 POPPING SOUND


I have answered some of this before, however I have reveiwed the schematic and as an electrical engineer I want you to do a couple tests.

First, plug in a set of headphones into the preamp out jack. Note that due to the impedance mismatch what you hear will not be very loud. Now change the state of the clean OD. Did you hear a faint pop?

Next plug a LOW impedance device OR even an instrument cable into the power amp "input" jack. Do this with the power off then warm amp up.
Now, change the state of the clean OD. Did you hear a faint pop?

Now for the analysis. If you did hear one in the first test and NOT in the second, the pop you are hearing is the "NATURE OF THE BEAST".

I can tell you from the design that the pop you hear is from coupling within the clean and OD relays within the unit. There is no cure for this within the unit other than find another brand...

I would recommend that you use an EXTERNAL pedal for the effects you want rather than trying to use the clean OD in this amp. I suspect it was never intended to be shifted during playing as I can guarantee it will pop when you shift while having anything other than dead silence (not even a bit of hum from your guitar). There is also statistical noise in the preamp that the laws of physics will not let us get around that may be enough to create a bit of a pop.

You could also use a volume pedal between the preamp output and the power amp in to throttle the volume to zero before shifting to OD.

In general I think you will find that what you are hearing is the nature of this beast.

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1 Answer

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The preamp section has a problem. It could be anything in the preamp but is OFTEN the input jack itself. They take a lot of abuse.

Also the insert jack contacts that forward the preamp to the power amp if no external FX are used can go bad.

Additional things are circuit board cracks caused by rough handling and also controls that have had the knobs mashed in which can destroy the potentiometers they are connected to.

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2 Answers

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There are 2 channels, Which means there are 2 sets of knobs ... One for clean and one for crunch. Start out with a low master volume and set up the crunch, Then the clen and switch between the 2 and make adjustments until they match volume wise ... Then turn the master up.

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