Marshall MG30DFX Transformer Upgrade

[substatica]

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

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

[Enzo]

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.

[substatica]

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.

[Enzo]

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?

[substatica]

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.

[nosaj]

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

[substatica]

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.

[nosaj]

"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

[substatica]

"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.

[J M Fahey]

Hope you read this before leaving:
you do not have a power transformer problem.

Neither did anybody at the Marshall Forum.

[substatica]

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.

[DrGonz78]

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.

[substatica]

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.

[Enzo]

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.

[substatica]

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.