I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.
This thread folks recommend replacing the power caps and the transformer.
http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279 (http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279)
The caps are no problem, but I can't find any specs on the transformer. I don't have much experience with transformers specifically, the markings are sparse and I can't find a part number. Obviously it's not the best amp, but would be nice to have to take places where folks might abuse it. Any idea what would be a good upgrade for the transformer in this amp?
http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG30DFX.pdf (http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG30DFX.pdf)
Transformer? Please have someone explain to me how the transformer goes "weak", and then comes back again too. I have a hard time imagining this.
This is called guessing. Change this, see if it helps, change that, see if it helps, change another thing, see if it helps. You need to determine what the problem is, then address it.
If your filter caps are failing, the amp gets hummy. Does your amp hum when the sound drops?
Isolate the problem. When this happens out your speaker, does it also happen in the headphones, or do they remain strong and toneful?
When this happens, give the FDD button a wiggle, or even push it in and out a time or two, does that bring it back? That switch affects the loudness and tone, and dirty contacts could do this.
Does it make no difference which channel you use?
Run a strong signal into the CD input, is it also affected?
My prime suspect? The headphones jack itself. The speaker return line runs through that jack, so it can cut off the speaker when phones are used. But if it gets dirty, you lose sound.
Caps fail, but have pretty predictable results. Transformers fail only rarely, and again, not like your symptom. Replacing those items from hope, is like replacing the motor in your car because now and then it loses power... When all it really needed was a clean fuel filter.
The amp is not pricey, and not that important too me, numerous folks have reported upgrading the caps and transformer solves the type of issues I'm experiencing with it, ie, sporadic volume drops and cut outs.
I cleaned the jacks, visually checked components, and the amp works, just here and there cuts out, volume drops, which seems like it could easily be the amp stressing the power circuit and IC's taking a powder because they're under powered. If upgrading these components doesn't fix the issue I may not bother with more because extensive troubleshooting is probably not worth it for me, for this amp.
I'm reading 22.3 VAC across the output of the transformer, so I guess it's a 110VAC to 24VAC, not sure what amp rating I should be looking for.
Fair enough, but the simple tests I suggested take very little time and might tell you something. Why replace a $30 transformer when it needs a $2 jack?
Quote from: Enzo on October 22, 2016, 12:45:49 PM
Fair enough, but the simple tests I suggested take very little time and might tell you something. Why replace a $30 transformer when it needs a $2 jack?
It's a cost/benefit call, from what I've read upgrading the power supply fixes this amp for a number of people. There's a great many other things it could be, but that takes time to test, and some of those tests are not reliable, meaning I may end up replacing other parts which test intermittent and not fix the problem and start sinking time into it. Upgrading the power supply components will, without doubt, make this amp more reliable. If it turns out not to solve the issue I'm experiencing then the risk I took will have bitten me.
Most testing and diagnosis result in a calculated guess as to the cause of the problem, that's all I'm doing. For an amp that routinely goes for $50, I'm hesitant to pull anything off the board unless it has a fair chance of fixing the issue -- which without much testing, upgrading the power supply would seem to be the best chance at success with the least effort.
Sure, cleaning some jacks is certainly in order, but from what I've experienced with the amp I don't believe it's them.
Tell me how it's a calculated guess when your troubleshooting an amp based on other threads. You for get some of the people here have been in the business for a while and a cost benefit thing to them is to replace only the bad part. If the test are not reliable may it is the person performing the tests. These guys will not steer you wrong when giving you a procedure.
If you want the best chance at success with the least effort maybe you should take the amp to a qualified tech rather than bucking good advice.
my 2cents and then some
nosaj
Quote from: nosaj on October 22, 2016, 01:42:50 PM
Tell me how it's a calculated guess when your troubleshooting an amp based on other threads. You for get some of the people here have been in the business for a while and a cost benefit thing to them is to replace only the bad part. If the test are not reliable may it is the person performing the tests. These guys will not steer you wrong when giving you a procedure.
If you want the best chance at success with the least effort maybe you should take the amp to a qualified tech rather than bucking good advice.
my 2cents and then some
nosaj
I don't understand your first sentence, are you suggesting that I follow advice on this thread but discount advice regarding similar issues with the same amp on other threads?
I know how to troubleshoot the amp. That's not what I want to do with this amp. Time is a cost, troubleshooting is time consuming, it will probably cost a lot more in time to chase down an intermittent issue than the cost and time to replace two caps and a transformer.
A qualified tech would tell me unless it's quick and easy, it's probably going to cost more than the amp is worth to find and fix the problem -- which is what I'm trying in vain to explain on this thread.
I appreciate the expertise on these forums, on this particular occasion I was asking for some advice about a transformer upgrade.
"quote"I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.
This thread folks recommend replacing the power caps and the transformer.
http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279"quote"
This is how your guessing. Goober over at this other site said doing this fixed his amp what can I buy to fix my amp ?
Get voltages at your rails to really see if you have a power supply issue. The transformer and caps are only a part of the power supply, just as oil and water and gas are only parts of a car engine.
People come here to learn and to hopefully pass it on. Maybe their cost benefit isn't worth your time. Ask for help be ready to supply some answers, if its not worth your time why be here?
It's one of those days so excuse me
nosaj
Quote from: nosaj on October 22, 2016, 03:00:06 PM
"quote"I picked up an MG30DFX, for free actually, because it had intermittent issues for the previous owner and they got fed up. I've read it's most likely an weak power supply.
This thread folks recommend replacing the power caps and the transformer.
http://www.marshallforum.com/index.php?threads/marshall-mg30dfx-problem-volume-fade-instability.36882/#post-1346279"quote"
This is how your guessing. Goober over at this other site said doing this fixed his amp what can I buy to fix my amp ?
Get voltages at your rails to really see if you have a power supply issue. The transformer and caps are only a part of the power supply, just as oil and water and gas are only parts of a car engine.
People come here to learn and to hopefully pass it on. Maybe their cost benefit isn't worth your time. Ask for help be ready to supply some answers, if its not worth your time why be here?
It's one of those days so excuse me
nosaj
Folks at other forums aren't by default Goobers. It's not that any component in the power circuit is bad, the theory is that they're just not providing enough power, a bunch of stuff get's overheated and things start going wonky. Testing the power circuit won't tell me that.
Anyhow, I didn't post to get lectured on how to troubleshoot an amp, I asked a simple question where my experience is lacking and it's not looking like I'm going to get an answer.
Browbeating me for looking for the quickest solution based on other's experiences instead of using the scientific method isn't going to change the fact that I don't have the time to troubleshoot this amp component by component. I've already replaced the filter caps, I'll look elsewhere to answer my transformer inquiries.
Hope you read this before leaving:
you do not have a power transformer problem.
Neither did anybody at the Marshall Forum.
Quote from: J M Fahey on October 22, 2016, 03:45:11 PM
Hope you read this before leaving:
you do not have a power transformer problem.
Neither did anybody at the Marshall Forum.
Not sure what the motivation is for all this hostility, if anyone wanted to help they could've offered advice on how to spec and select a transformer. Everyone in this forum has tried a fix that they weren't 100% sure would work because it had a fair chance and was quicker and easier than the next troubleshooting step -- that's all I'm trying to do. None of the pontificating in this thread has been useful, the quoted reply included.
How does the amp sound when you plug in headphones and play it that way? Enzo is right about it might only be a $2 part or cleaning the headphones jack. Actually all the advice given so far is correct and nobody here will agree with misleading advice. So you ask for help locating a PT? Well measure some voltages to test your theory of a bad transformer. PT's don't go weak and only just go bad, there is really no in between here. To offer advice to spec and select another PT we would want to know what B+ is on the amp. Hostility?? I don't see anywhere here that someone was being hostile with their advice. Straight to the point honest, but not hostile.
Quote from: DrGonz78 on October 22, 2016, 04:44:23 PM
How does the amp sound when you plug in headphones and play it that way? Enzo is right about it might only be a $2 part or cleaning the headphones jack. Actually all the advice given so far is correct and nobody here will agree with misleading advice. So you ask for help locating a PT? Well measure some voltages to test your theory of a bad transformer. PT's don't go weak and only just go bad, there is really no in between here. To offer advice to spec and select another PT we would want to know what B+ is on the amp. Hostility?? I don't see anywhere here that someone was being hostile with their advice. Straight to the point honest, but not hostile.
I never suggested it was a bad transformer, or a bad power circuit, just that it was under spec'ed for this amp, as a result the amp is prone to overheating and that causes a variety of components to flip out, the IC's specifically, and eventually fry.
I don't believe anything is fried on my amp (yet) since it works perfectly fine, just every so often the volume drops to almost nothing and odd things start to happen.
In that referenced thread, not just one, but a number of folks detail similar issues and also relate that upgrading the filter caps and transformer alleviate the issue. Not sure why everyone here is so against trying that first if it's worked for others with the same amp, experiencing the same issues.
I swapped the 2200uf 25v filter caps with 4700uf 50v, ran the amp for the last 30 minutes and no sign of the issue, that's not to say it's fixed, or fixed in the best way it could be, but it may be fixed enough for me not to bother any further with an amp I could buy fully functional right now for under $100. I appreciate these forums, and got a lot of help recently trouble shooting and fixing an '81 JC-120, but that took a lot of time and effort, which this particular amp isn't worth.
OK, I didn't intend to start a back and forth. The few tests I described take just a minute, like finding out if the headphones do the same thing. DO disassembly required. Plugging into the CD in, takes just a moment. But those simple tests isolate the possible fault areas to a limited part of the amp. These are pretty much the tests I do to a customers amp at my counter while he stands there. Note that none of my basic tests involved opening the amp.
An under spec'd transformer might put out too low a voltage, which will ONLY make the amp have a lower maximum output. If it cannot provide enough current, it will drop voltage on its own rather than harm the circuits. The power amp IC will work less hard. Many of these transformers have thermal fuses inside them, but those are open or not, they don;t reset themselves.
And I apologize if you feel the directness is hostility. I for one haven;t any hostile feelings. Just as you don;t want to spend hours troubleshooting, when I offer advice, I don;t want to spend a lot of time beating around the bush, and otherwise "softening the blow" so to speak. I just say what I think. The reason Juan and I react to transformers as diagnosis is that for some reason that is the first thing the novice jumps to when it ought to be the last thing on the list. Not calling you a novice, just that the transformer hopped to the top of this repair too quickly, from our experiential point of view.
The JC120 is a complex - overly so - stereo amp, and is really not remotely like working on this little amp.
Quote from: Enzo on October 22, 2016, 05:43:56 PM
If it cannot provide enough current, it will drop voltage on its own rather than harm the circuits. The power amp IC will work less hard. Many of these transformers have thermal fuses inside them, but those are open or not, they don;t reset themselves.
And I apologize if you feel the directness is hostility. I for one haven;t any hostile feelings. Just as you don;t want to spend hours troubleshooting, when I offer advice, I don;t want to spend a lot of time beating around the bush, and otherwise "softening the blow" so to speak. I just say what I think. The reason Juan and I react to transformers as diagnosis is that for some reason that is the first thing the novice jumps to when it ought to be the last thing on the list. Not calling you a novice, just that the transformer hopped to the top of this repair too quickly, from our experiential point of view.
The JC120 is a complex - overly so - stereo amp, and is really not remotely like working on this little amp.
I don't mind directness, there have been overtly inflammatory statements made that I consider hostile, not by you. The largest barrier to troubleshooting this amp is that the problem is intermittent, and I'm not talking about wiggle a plug intermittent, I'm talking about unable to ascertain the circumstance intermittent. Using the amp for 20 minutes straight the issue may not arise, other times, right away. I'm sure you can understand that makes things especially difficult from the get go.
If you read the thread that I referenced it's a very similar problem, a number of owners on the thread with the same issue, some talk of jacks and others talking about the power circuit. I'm familiar with the switching jacks, have some spares around, but the jacks would be a pain to swap out and support the circuit board in this amp so swapping for anything other than the exact same board mounted jacks (which I don't have) may cause complications, I cleaned them, maybe that will fix it, maybe not, I will monitor the jacks if the problem arises again. While I had it open I swapped the filter caps for double the uf and double the voltage rating, maybe that will fix it, maybe it won't -- if I'd gotten information regarding a suitable transformer upgrade I probably would have swapped that out too and had a fair chance of fixing the issue, without having to delve too deep.
As it is, I've done all that save the transformer and the issue may be gone, I'll have to continue to use and monitor.
I've got very limited time to dedicate to this sort of thing, so frankly it's either risk the quick fix or do nothing, especially on this amp which many consider disposable -- due to issues such as this.
Does the volume signal cut in and out only when it is being played loud or all the time?
A while back there was this thread on SSguitar.com. Unfortunately the OP disappeared from the thread and nothing came to light there...
http://www.ssguitar.com/index.php?topic=2829.0
The problem is expectations. Even if that thread came to a conclusion, it assumes there is a "right" answer. The complaint is very common across amps of all types: loss of sound or volume, change of tone, etc, and intermittent. Everything in the amp goes into making sound, and so most of the amp can be involved. Ther is no one cause for this symptom. What caused it in one amp can be a very different thing in another, yet they act the same. What works for one guy may or may not work for another, simply because different things caused the symptom. That is one problem with the internet, the expectation there is some sort of look-up list of THE cause for THAT problem. This is exacerbated by people taking the amp apart, changing some parts, and the symptom goes away. They think the new part cured it, but what really happened was the act of disassembly and reassembly and general soldering was the real cure. A common example of that is an amp that "eats preamp tubes". A guy has an issue, changes a preamp tube, symptom goes away. A week later, problem returns, another new tube, symptom goes away. After four or five new tubes, we go online to find out if anyone else has "seen this". The one thing that guy never tried was reinstalling the old tube. The old tube would have worked too, because the issue was the socket, and just pulling a tube in and out deferred the symptom.
I'd not even bother fixing this.
These amps and many like them are common in land fill second hand stores, just wait until you find another bit of land fill amp trash with a larger transformer.
Or go read the data sheets, the chip draws 5Amps at full tilt so any transformer larger that what you have will likely improve it a tad.
Purchasing a new 4~5Amp 24VAC transformer is false economy for such a small amp.
If you suspect the amp suffers from excess power draw then simply probe the supply rails while running at full volume and note how far down the supply rails fade/sag.
Also you mentioned the preamp might be suffering,, well check those supplies under full volume as well.
The way some of these chip amps derive the chip supply is very basic, often just a resistor and a cap on each rail.
Without pictures or even better a schematic there is little to go on
-------
Oh just noticed that the good *Doctor Gonz* has linked to an earlier post and has the schematic.
Arrh huh,, this amp has relays and a dedicated 5volt rail for the digi FX setup. :'(
If there is some sag they might not work right,, so in that case a larger Xformer might inadvertently fix it.
So just maybe the bigger Xformer kinda fixes things but if one had time to dig deep you may find it has more to do with the 5volt rail which runs the FX setup.
OR,,, the preamp rails are not able to regulate under load,, Just some quick calc on the voltage you gave earlier,,
24 VAC x 1.4 = 33.6 VDC /2 = 16.8VDC...
So ZD1 and ZD2 are 15 Volt zener and they need at least 2~3 volts ABOVE their rating to work.
even a small sag in supply will make the preamp supply fall out of regulation.
simply by replacing those two Zener's with 12 Volt units would fix the issue,,,Far cheaper than a new transformer me thinks.
of course it's all up to you. 8|
Phil.
Yes I agree with Enzo who just posted. I could add many similar stories like his.
Quote from: phatt on October 23, 2016, 07:29:41 AM
Or go read the data sheets, the chip draws 5Amps at full tilt so any transformer larger that what you have will likely improve it a tad.
Purchasing a new 4~5Amp 24VAC transformer is false economy for such a small amp.
(...)
Arrh huh,, this amp has relays and a dedicated 5volt rail for the digi FX setup. :'(
If there is some sag they might not work right,, so in that case a larger Xformer might inadvertently fix it.
So just maybe the bigger Xformer kinda fixes things but if one had time to dig deep you may find it has more to do with the 5volt rail which runs the FX setup.
OR,,, the preamp rails are not able to regulate under load,, Just some quick calc on the voltage you gave earlier,,
24 VAC x 1.4 = 33.6 VDC /2 = 16.8VDC...
So ZD1 and ZD2 are 15 Volt zener and they need at least 2~3 volts ABOVE their rating to work.
even a small sag in supply will make the preamp supply fall out of regulation.
simply by replacing those two Zener's with 12 Volt units would fix the issue,,,Far cheaper than a new transformer me thinks.
of course it's all up to you. 8|
Phil.
Yes I agree with Enzo who just posted. I could add many similar stories like his.
Thanks, I hadn't thought of checking the chip sheets. I'm okay with a little bit of false economy since I got the amp for free. If it keeps exhibiting the problem I'll have a look at ZD1/ZD2.
QuoteI'm reading 22.3 VAC across the output of the transformer, so I guess it's a 110VAC to 24VAC, not sure what amp rating I should be looking for.
Mmmmhhh, I find a problem with your measurement.
To begin with,there is not "
the" output and quoting a single number (22.3VAC) or
Quoteso I guess it's a 110VAC to 24VAC
, all of which imply a single winding, because that transformer has two secondary windings, sop'froper answer would be quoting two voltage values (even if identical).
Look at the transformer schematic,you will see that secondary has two windings, one connected from W3 to W4 and the other from W4 to W5.
Do we agree so far?
Now,
where did you read that 22.3VAC value?
a) from W3 to W4
b) from W4 to W5
c) from W3 to W5
In any case, please remeasure VAC from W3 to W4 and from W4 to W5.
Also measure and post DC voltage available at the positive end of C49 and the negative end of C50 ; such voltages are also called +V and -V and directly feed the chip amp TDA2050.
Now measure DC voltage available at the positive end of ZD1 and the negative end of ZD2 , where I expect to find +15V and -15V respectively but ... who knows? .... that´s what measuring is needed for, to separate reality from speculation.
Notice I only asked for power supply measurements, the area which is worrying you, not said a word about jacks, cleanliness or anything else.
You do not need to trust
me, just your own measurements.
Thanks.
Quote from: J M Fahey on October 23, 2016, 12:21:09 PM
QuoteI'm reading 22.3 VAC across the output of the transformer, so I guess it's a 110VAC to 24VAC, not sure what amp rating I should be looking for.
Mmmmhhh, I find a problem with your measurement.
To begin with,there is not "the" output and quoting a single number (22.3VAC) or Quoteso I guess it's a 110VAC to 24VAC
, all of which imply a single winding, because that transformer has two secondary windings, sop'froper answer would be quoting two voltage values (even if identical).
Look at the transformer schematic,you will see that secondary has two windings, one connected from W3 to W4 and the other from W4 to W5.
I haven't done a lot of work with transformers, so I'll describe how I got that measurement. There are three leads coming off the secondary side of the transformer, two red and one black. I had thought the black was some sort of ground -- is that not the case? The amp is off the bench for now, but I would think the black was W4. Measuring VAC between the two red leads was 23.3VAC, measurement between either of the reds and the black was something in the neighbourhood of 17VAC.
When next I open the amp up I'll check the voltages you describe, after upgrading the caps and cleaning the jacks, I'm going to see if the problem is still there -- so far it hasn't occurred but I'll need some time to verify. Thanks, the sentiment on this thread seems to have changed for the better and I'm grateful for that.
The black one is connected to ground, but in terms of the transformer, it is the center tap. Whatever you get red to red, black to either red should be half of it. In other words your 24v red to red ought to net you 12 from black to either red. Or 17 there should make 34v red to red. Those readings don't really work.
Or you have some strange situation. Pull the red wires off the board and measure them unconected to circuit. Oh, only if you feel like pursuing it. I don't know about under spec, but it could just be defective.
Quote from: Enzo on October 23, 2016, 02:58:00 PM
The black one is connected to ground, but in terms of the transformer, it is the center tap. Whatever you get red to red, black to either red should be half of it. In other words your 24v red to red ought to net you 12 from black to either red. Or 17 there should make 34v red to red. Those readings don't really work.
Or you have some strange situation. Pull the red wires off the board and measure them unconected to circuit. Oh, only if you feel like pursuing it. I don't know about under spec, but it could just be defective.
Thanks, good to know. I'll take those measurements again when I have it open.
QuoteBrowbeating me for looking for the quickest solution based on other's experiences instead of using the scientific method isn't going to change the fact that I don't have the time to troubleshoot this amp component by component. I've already replaced the filter caps, I'll look elsewhere to answer my transformer inquiries.
better to move on and find the magic somewhere else.......
Hi gbono, I felt about the same when I read that post but it was long ago , we´re well past it now, and substatica understood what we asked about measuring and posting values, hopefully we can solve this problem now. :)