MG100HDFX No sound to cab

[phudgee]

Hi,

New here. Did a quick search but the only 100HDFX problem I could find posted was for the fuses continually blowing.

I too had this problem, and I replaced the amplification chip labeled: TDA7293

This fixed the problem of the fuse blowing, but now I only get sound from the Emulated Headphone jack. No sound out of either output to the cabinet.

The issue with the amplification chip was a pretty easy one to figure out, because it apparently is a common issue, and there were many threads on the net about it.

Im having difficulty with finding any threads about the issue of sound only coming out of the headphone jack, and not the cab, and the ones I do find usually say take it to a tech.

Im looking to find out if any other DIY electronic hobbyists have had a similar issue, and/or some direction as to what may be causing the issue, and where I should focus my attention.

Thanks in advance for any help and/or advice.

[JohnH]

Hi phudgee - welcome to GN2. I know you came here to look for input on your amp problem, but do take a look around. This is by far the best site on the net for guitar technology.

Someone, or Sumgai, with extensive experience of amp repair may have come across your problem. But heres my thoughts to start off:

The MG series seems to be somewhat mysterious in terms of non-availability of schematics that could focus the search for problems. I have a little Mg box and had no luck finding out info.

But from the descriptions of yours, and reading the manual, some things can be figured. The headphone out doubles as a line out, and has filters to emulate a cab. It is also aparently taking an output before the master volume control, since the manual states that it is controlled by the channel volumes, and that turining down the master volume will allow for silent practice with headphones. This suggests to me that the headphone has its own output circuit, different to the main power amp that drives the speakers.

Although that is not necessarily bad news, it is not good either, because it would mean that the problem could be anywhere in the power amp. The alternative, where the headphone is just on a switched jack at the power amp output, would have been simpler, with the problem localised to just around the jack.

In your case, it could be as simple as an output jack, or a fuse in that section, but it could also be something more major such as the power transistors.

Beyond minor (and very careful) poking around, it sounds like a job for a tech.

EDIT - in referring to other 'DIY electronic hobbysts', does that mean that you are one too? If so, maybe you have the knowledge to set up some testing of the power amp section.

cheers

John

[sumgai]

phudgee,

Hi, and welcome to the NutzHouse!

As John intimated above, I am one of those crazed amplifier technicians (and more, when the occasion demands). I've seen your complaint so many times in the last two decades, I'd be richer than Bill Gates if I had a dime for every one of 'em.

John hit the nail on the head, although he didn't know it. (I'll bet he remembers it from now on! ) The Headphones/Line Out jack is indeed the culprit. The reason? It is Cheap, with a capital C. This poorly designed and built component has been plaguing the professional musician's industry for a long time, at least 20 years. The tip of the plug normally opens a switch (mechanically), and when the plug is removed, the switch is supposed to close back up. If that doesn't happen, then you get...... (wait for it)...... no sound beyond the jack - everything past that is dead in the water.

How do you fix it? You could try cleaning the contact points, but that can be cumbersome, and on some jacks, they are enclosed (sealed from dirt, etc.). Plus, it's only temporary - the materials on those contact points are sub-par at best, which is why we have the problem in the first place.

IMHO, the only one way to do this correctly is to give it the old "R&R" treatment - remove and replace the little bastid. Install either a new one of the same kind (how long does it have to last..... until you sell it?), or a better grade of jack, such as those made by Switchcraft. Once that's done, your problems are moot (for the time being, if you opted for El Cheapo again).

HTH


sumgai


p.s. For all concerned here, in general, if you have a Line In, look at it first. (Sometimes also called the Effects Return jack, it's the same thing.) Normally, Line out doesn't have the switching capability, so you can probably ignore it. However, Headphones jacks often have switching. So it depends on whether or not you ever use either of these jacks - once a jack is used a few times, it's only a matter of time before the stinky stuff is forcibly ejected from the whirly thing.

[JohnH]

Sumgai - are you sure about that? The description in the manual seems to indicate that the headphone line/out is independent of the power amp, and draws its signal before the power amp and has its own output circuit, hence to a jack which is independednt of the spaeker jacks and does not switch them out

Heres the manual:
www.marshallamps.com/downloads/files/MG100%20hbk%20Eng.pdf

which says this:

23. EMULATED LINE OUT &
HEADPHONES jack
This jack can be used as either an Emulated
Line Out or as a Headphone output. This means
you can use this socket to connect your amp
directly to your multi-track recording machine and
you’ll get a great, speaker emulated guitar sound
direct to disk or tape. Alternatively you can plug
your headphones in here for a killer, speakeremulated
sound!
TIP: How to practise or record in ‘silence’: The
Clean Channel GAIN control (2) and the
Overdrive Channel VOLUME control (15) will
control the level of the headphone and line out.
The speaker output can be adjusted
independently using the MASTER VOLUME
control (20). Setting the MASTER VOLUME
control (20) to zero will allow silent recording or
practice.

The speaker volume is independent of the headphone. Thats why I think the headphone jack is unlikley to be affecting the speaker jack, and its different to the more common arrangement of headphone cutting out the speakers when inserted.

John

[sumgai]

John,

I'm sure about what I said. I'm not sure that absolutely all headphone jacks (or even line in/out jacks) are wired exactly the same way, but I did relate what my experiences have shown me.

In point of fact, if what you say is true for phudgee's amp, then indeed, the matter is simply that he muffed the replacement procedure, possibly by bad soldering practices, or by blowing the TDA chip itself during the operation. It's possible the replacement part was bad before it ever got to him, but we won't know that until further testing shows one way or the other.

~!~!~!~!~!~!~

phudgee,

You'll have to double-check your work carefully. If the fuse no longer blows, that's good, but you could still have a poor solder joint, or a solder bridge that's grounding the signal, there are a number of possibilities here. Most likely, it'll be visible under close scrutiny.

But just in case, still double check that headphone jack, it can't hurt to be sure about it.


sumgai

[JohnH]

phudgee - you've got our attention now! So be sure to let us know what you find out about it. Or if there are any other symptoms that you'd like a view on, post them here!

good luck

John