SOLVED - Marshall MG 15 DFX - Measuring impedance of speaker

Spykerwolf

Member
Messages
8
Hey guys. I've got a Marshall MG 15 DFX amp that appears to be faulty at the moment. The condition is unknown, because I got it free from a friend. The amp's power lead was from South Africa, so I had to convert it to a New Zealand plug (that's all the modification I've done so far).

When I plug my guitar in, I get no sound - just a humm. Sometimes I get a little bit of a sound when I switch to distortion, and crackling as well. I tested different guitars and leads just in case.

I checked the impedance (ohms) with my multimeter, and I couldn't get a reading using alligator clips (Just stayed at 0). The alligator clips work, because I could measure my pickups just fine. The thing is if I remove the alligator clips, and try to measure the speaker again - then I get a value, but it just keeps fluctuating all the time. It will go down slowly sometimes as well.

Anyone know what else I can do to test it now?
 

UsableThought

Member
Messages
1,650
Why are you measuring resistance across the speaker connection (your meter can't actually measure impedance) if you don't know anything about the condition of the amp? This does not seem a usual starting point for troubleshooting; do you have some reason to suspect a problem w/the speaker or speaker connection - e.g. good sound of out of headphone jack, bad or no sound out of speaker? If not, I wouldn't suggest limiting your focus to the speaker so soon.

Regarding the fluctuating meter reading, I don't know this amp but in my limited experience with SS amps, measuring across the leads of a connected speaker does seem to produce an approximately "correct" speaker resistance (even thought it's not especially useful info). The fact that you obtained a fluctuating reading, rather than reading an open circuit, suggests the meter leads were not as secure as you thought; or perhaps the meter's battery is bad.

You don't say what experience you have with troubleshooting electronics in general or amps in particular. If it is "no experience" or "very little," then probably best to take this into a shop.

If it is "some experience, e.g. I did this or that," then possibly there are things you could do to check at the most basic level for a broken connection somewhere inside the amp - tedious but in theory doable if you are willing to spend the time. You would want to obtain JPGs or PDFs of the schematic & layout, possibly take some gutshot pics as well, and post all that information to this thread. Then someone might be able to advise you on simple steps to check the signal path and/or power path for breaks in either.
 
Last edited:

enorbet

Member
Messages
46
While there is some value in testing the speaker it must be done alone.... IOW disconnected from the amp. This will only really tell you if the spkr is wired properly and works. Actually that is the proper starting point for amps with no sound because proper procedure begins at the end and works backwards to the Input. If you don't have amp knowledge and proper tools it is nearly impossible to get lucky since the best you can do is use the Line Out multi jack to see if the power section works. Plugging a guitar in there will produce a very weak output but at least you'd have an idea it worked.

If you have some other amp, even a HiFi music amp, and can read a schematic ideally, you might figure out how to attempt to drive a known good power section with the Marshalls preamp. You could actually try to drive abother full guitar amp but th opposite will be true. It will be VERY loud and probably some distorted except at very low settings.

The way a tech would test assumes reading the schematic or knowing ICs so well you know each ones pinout - which is input, output and voltage leads. Ideally signal tracing is done with an Oscilloscope and signal generator but there is a quick and dirty method called "The Pop Test". Once you know the speaker works you connect a capacitor to a voltmeter's lead and progress down the signal chain tapping the capacitor (other le4ad goes to ground) to each Input. You should get a weak Pop! at the output that gets progressively louder as you work backwards through the gain stages until you reach the stage with a volume control which will only be louder if it is up - closer to 10.

Unless you think you understand exactly what I'm talking about you should take it to a tech. There are no lethal voltages but it is pretty easy to fry a good amp if you don't know what you're doing. Alternatively, assuming you're in no hurry, you could seek to buy a musical instrument amp manual. An oldie but goodie was written by Jack Daar but there are others. I just don't know how available they might be in New Zealand or at what shipping cost.

Anyway Good Luck but do be thoughtful.
 

UsableThought

Member
Messages
1,650
Actually that is the proper starting point for amps with no sound
Except the OP apparently does get sound - hum, plus sometimes crackling or "a bit of distortion." If the speaker connection were open, the speaker would be dead silent. And a damaged speaker (e.g. bad cone or whatever) might be more prone to blurt or otherwise make more noise than just hum or an occasional crackle.

But this ambiguity was why I asked about the headphone out - if it doesn't work properly either, then probably no point in investigating the speaker; & vice versa, if it works fine, then further investigation of the speaker & related circuits would be warranted. Although the schematic would be helpful before making this call.

Other than that I agree that (a) if the speaker is truly a suspect it can be disconnected & tested separately, and (b) it's often a good idea to either work backwards, or with a SS do a "divide and conquer" as you are suggesting to see if it can be narrowed down to preamp or power amp. And also (c) a tech might be best unless the OP is willing to be very patient & go very slow & get a lot of help. But that will only work if the problem is a simple one.
 
Last edited:

enorbet

Member
Messages
46
ping UsableThought - Frankly it was impossible to tell from OPs description what was the source of that hum or even crackling. It is probably from the speaker but a tech makes no such assumptions based on a non-tech flimsy description. Anyway my recommendation simply reflected the need to be specific and certain, step-by-step before proceeding.
 

Spykerwolf

Member
Messages
8
Why are you measuring resistance across the speaker connection (your meter can't actually measure impedance) if you don't know anything about the condition of the amp? This does not seem a usual starting point for troubleshooting; do you have some reason to suspect a problem w/the speaker or speaker connection - e.g. good sound of out of headphone jack, bad or no sound out of speaker? If not, I wouldn't suggest limiting your focus to the speaker so soon.

Regarding the fluctuating meter reading, I don't know this amp but in my limited experience with SS amps, measuring across the leads of a connected speaker does seem to produce an approximately "correct" speaker resistance (even thought it's not especially useful info). The fact that you obtained a fluctuating reading, rather than reading an open circuit, suggests the meter leads were not as secure as you thought; or perhaps the meter's battery is bad.

You don't say what experience you have with troubleshooting electronics in general or amps in particular. If it is "no experience" or "very little," then probably best to take this into a shop.

If it is "some experience, e.g. I did this or that," then possibly there are things you could do to check at the most basic level for a broken connection somewhere inside the amp - tedious but in theory doable if you are willing to spend the time. You would want to obtain JPGs or PDFs of the schematic & layout, possibly take some gutshot pics as well, and post all that information to this thread. Then someone might be able to advise you on simple steps to check the signal path and/or power path for breaks in either.
Hey thanks for the reply. I am a novice when it comes to amps and electronics in general. The most I've done was replace all the electronics in my guitar, and replace pickups. I will give the headphone port a try today.

While there is some value in testing the speaker it must be done alone.... IOW disconnected from the amp. This will only really tell you if the spkr is wired properly and works. Actually that is the proper starting point for amps with no sound because proper procedure begins at the end and works backwards to the Input. If you don't have amp knowledge and proper tools it is nearly impossible to get lucky since the best you can do is use the Line Out multi jack to see if the power section works. Plugging a guitar in there will produce a very weak output but at least you'd have an idea it worked.

If you have some other amp, even a HiFi music amp, and can read a schematic ideally, you might figure out how to attempt to drive a known good power section with the Marshalls preamp. You could actually try to drive abother full guitar amp but th opposite will be true. It will be VERY loud and probably some distorted except at very low settings.

The way a tech would test assumes reading the schematic or knowing ICs so well you know each ones pinout - which is input, output and voltage leads. Ideally signal tracing is done with an Oscilloscope and signal generator but there is a quick and dirty method called "The Pop Test". Once you know the speaker works you connect a capacitor to a voltmeter's lead and progress down the signal chain tapping the capacitor (other le4ad goes to ground) to each Input. You should get a weak Pop! at the output that gets progressively louder as you work backwards through the gain stages until you reach the stage with a volume control which will only be louder if it is up - closer to 10.

Unless you think you understand exactly what I'm talking about you should take it to a tech. There are no lethal voltages but it is pretty easy to fry a good amp if you don't know what you're doing. Alternatively, assuming you're in no hurry, you could seek to buy a musical instrument amp manual. An oldie but goodie was written by Jack Daar but there are others. I just don't know how available they might be in New Zealand or at what shipping cost.

Anyway Good Luck but do be thoughtful.
Hey thanks as well. I did disconnect the plug from the speaker before I measured it, is that what you mean? Or do I have to unplug something more? Regarding the line out - I just plug a guitar in and see if I get sound? Thanks.
This amp is actually my brother in laws, and have been sitting in storage for about 8 years now. I tried it out about 5 years ago, and it was working fine. It has been moved a lot in their house etc, so maybe it got a knock? Unfortunately I can't read schematics, although now might be a good reason to start learning :)

Except the OP apparently does get sound - hum, plus sometimes crackling or "a bit of distortion." If the speaker connection were open, the speaker would be dead silent. And a damaged speaker (e.g. bad cone or whatever) might be more prone to blurt or otherwise make more noise than just hum or an occasional crackle.

But this ambiguity was why I asked about the headphone out - if it doesn't work properly either, then probably no point in investigating the speaker; & vice versa, if it works fine, then further investigation of the speaker & related circuits would be warranted. Although the schematic would be helpful before making this call.

Other than that I agree that (a) if the speaker is truly a suspect it can be disconnected & tested separately, and (b) it's often a good idea to either work backwards, or with a SS do a "divide and conquer" as you are suggesting to see if it can be narrowed down to preamp or power amp. And also (c) a tech might be best unless the OP is willing to be very patient & go very slow & get a lot of help. But that will only work if the problem is a simple one.
ping UsableThought - Frankly it was impossible to tell from OPs description what was the source of that hum or even crackling. It is probably from the speaker but a tech makes no such assumptions based on a non-tech flimsy description. Anyway my recommendation simply reflected the need to be specific and certain, step-by-step before proceeding.
Thanks again. The amp doesn't mean anything to my brother in law, so taking it to a tech isn't really an option. If it doesn't work, then he'll just throw it away. Obviously I don't want that to happen, so I'm doing everything I can to resurrect it :)
 

enorbet

Member
Messages
46
The Line Out Jack is a stereo jack that is to say it has one ground, a switch, and 2 x signal paths - one In and one Out - . The reason for the switch is that when you are using Line In almost nobody wants other players coming through their speaker so the internal preamp is disconnected from the power amp allowing an external preamp access to the power amp. This is useful if you have 2 amps and wish to drive 2 power amps and spkrs from the one preamp. Conversely if you access the Line Out you don't want to disrupt the internal signal path but rather just add an Output to an additional power amp. It is much like an effects loop but without any gain matching circuitry.

Schematics are incredibly easy to read and I highly recommend you become literate in the "language" which is simply graphically symbolic. For example if you google "Marshall MG 15 DFX schematic" and blow one up (visually of course, LOL) and locate the Line Out jack instead of a physical representation of what the jack looks like you see a representation of it's wiring. The graphic symbol for a switch is a line with a perpendicular line ending in an "arrow" contacting the first line. This represents that motion connecting and disconnecting the 2 lines is possible.

On a simple level what this means is, since the jack has sleeve(ground), ring(1st contact) and tip(final contact) you can access In and Out separately using a stereo plug or a mono plug (like a common guitar cord) can alternately access one or the other. So you would have to either read the schematic to know whether In or Out is first or just try both. Obviously reading/knowing what to expect is a big leg up toward certainty. Without that knowledge you simply feel 2 temporary stopping points - click...click. One is In (to the power amp) and the other is Out (of the preamp).
 

J M Fahey

Member
Messages
2,363
The most I've done was replace all the electronics in my guitar, and replace pickups. This amp is actually my brother in laws, and have been sitting in storage for about 8 years now. I tried it out about 5 years ago, and it was working fine. It has been moved a lot in their house etc, so maybe it got a knock? Unfortunately I can't read schematics, although now might be a good reason to start learning :) The amp doesn't mean anything to my brother in law, so taking it to a tech isn't really an option. If it doesn't work, then he'll just throw it away. Obviously I don't want that to happen, so I'm doing everything I can to resurrect it :)
Ok, start by your brother "throwing it away" into your house.
Since you are here asking things, *maybe* someday you'll want to build something, say, a pedal , a simple battery powered amp, the usual starting blocks.
Then you might want to build some simple tube amp, such as a Champ or one of the AX84 projects, or a Kit (Weber/Ceriatone/etc.) which don't **really** need much Electronics knowledge to be built, usually being able to follow instructions and follow a layout drawing is enough, think "painting by the numbers" or cooking something unknown *faithfully* following a TV or You Tube recipe and instructions.

The Ceriatone layout is actually way better, modern, with coloured wires and very readable labels, but they use some trickery so I can't paste it here :(
https://www.google.com.ar/url?sa=i&...pHg-u2doG9KMNf-k5FMbnNcg&ust=1458602183065327
What's my point?
The usual deal breaker for many such kits is not the purely Electronics stuff, which usually comes neatly packaged in a (heavy) cardboard box

but: chassis + cabinet + speaker.

BIG problem ... until you remember that MG15 stashed in *your* attic ;)

FWIW this is what Triode Electronics (the suppliers of the above mentioned kit) say about it:
Tear the guts out of that solid state combo and build this inside. Great for DIY Builds. All you need is a Chassis, Cabinet, & Speaker!!!
 
Last edited:

J M Fahey

Member
Messages
2,363
OK.
IF some day the amp building bug bites you, now you know that you should start by getting a dead amp from somebody ... for free of curse ;)
 




Trending Topics

Top