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Marshall MG30DFX Transformer Upgrade

Started by substatica, October 22, 2016, 09:35:21 AM

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DrGonz78

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
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

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.

phatt

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.

substatica

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.

J M Fahey

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.

substatica

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.

Enzo

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.

substatica

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.

gbono

#23
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.......

J M Fahey

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.  :)