Dead Marshall MG100

[cpuslave]

Hi, new to the forums here. I did a quick search and didn't really find too much info, but a couple months ago my Marshall MG100DFX just died on me for unknown reasons. It started crackling for a minute or so, then it just stopped making noise period. Nothing. Headphones work fine. I'm most positive the speaker's fine. I took out the headunit and checked all the soldering joints and they seem fine.

I really don't know what I'm doing here, or what I'm looking for, but I need to fix this soon. Does anybody have any idea? Or even what some common problems are with these?

Help is much appreciated.
Thanks.

[Gtr_tech]

I had one of those in this week. Powered up ok, just no output. Ended up being the connector that goes from the main board to the output IC board....long white multipin job. The main pcb standoffs weren't snapped in all the way through the chassis from the factory, so the board was warped and that connector warped along with it. I seated the standoffs into the chassis properly, then had to use a little force to get the connector to seat back in the way it should. Looking at the chassis with the pots in front of you it was the right end that was popped up. Check that first. If thats ok then I'd say the output IC has gone south. And make damn sure that fan still works ok, that IC won't last long without it.

[Enzo]

MG100DFX? There are more than one MG100 models.

HOW did you determine the speaker was fine?

If the speaker works - and please actually test it - then I'd suspect your power module died. That is the 15-leg IC on the tiny board on the heatsink with the fan.

[cpuslave]

Okay so I saw what previous posts were talking about, this TDA7293 chip. Is that what you were reffering to? I have no idea what a standoff or an ouput IC is. How can I test if it's bad?

It's an MG100DFX, the combo. I determined the speaker worked by plugging it into another amplifier.

[Am Peg]

CPU -
The Stand-off: Imagine looking down into the blank chassis of an amp head. Nothing but a shiny metal plate across the bottom. Now, you want to install a circuit board. Can't place it against the metal chassis, so you set it on plastic stilts - or stand-offs. There are holes in the circuit board that match the locations of the stand-offs allowing the board to snap into place on the top of the stand-off's tips. If the board's not snapped in - it'll potentially warp, like Gtr Tech mentioned.

Second: Output IC. Gtr Tech was merely referring to the Output "Integrated Circuit" (IC) board. I'm not certain if this is in reference to the TDA7293 you mentioned, but the part number is certainly reminiscent of an IC.

An IC by the way, can refer to any number of devices. It can be an 8 pin chip, a 3 pin regulator or a 48 pin processor. Depends on the person using the term. Like a violin vs. a fiddle. Same instrument - different player.

Bottom line - if you heard crackling noises - that's probably from something arcing. If that's the case, something inside is either not all in one piece anymore or its a different color than when it started. (Charred).

Got a DMM? And if you have to ask what a DMM is, well - it might be a good idea to get someone to help you with that amp.

[cpuslave]

1) Standoff: Check, got that concept down.
2) Second: The chip that's behind the heatsink/fan reads TDA7293.
3) Bottom line: Heard crackling noises, music stopped, but still powers on/fan turns on.
4) I actually had to google that term, but yes I have one. What do I do with it? Maybe you can help me to use it, I don't think I'm using this properly. On a side note, what setting should I use to test fuses?
5) I'm a broke college student, so I can't really afford sending it to somebody. I work at a hardware store though, so I have easy access to parts/tools.

I appreciate the help so far though.

[Am Peg]

Think of a fuse as a piece of wire. Electrical current passes through it with ease. There's no resistance whatsoever - zero "ohms" (theoretically).

If you touch the ends of your Digital Multi-Meter (DMM) probes to the metal ends of a good fuse, you would read pretty darn close to zero ohms of resistance.

MAKE SURE YOU HAVE THE AMP UNPLUGGED FIRST!!! ( I just don't want you to get zapped).

On the other hand - an "open" fuse is one where there was too much current trying to pass through it and the fuse did it's job - it sacrificed itself to save the circuitry behind it. The wire in the fuse melted and opened (disconnected). No current can pass through it so it has an infinite resistance to current flow. Your DMM will read something like over-range or overflow.

[Enzo]

The TDA7293 is a whole power amp in one IC chip - 100 watts!!! Yiou can't tell by looking if it is bad, but it is a common failure. It is a solder job to replace one. ABout $6 from Mouser.

Since the headphones jack turns off the main speaker, you may as well spray some contact cleaner into the phones jack, then quick plush a plug in and out of the jack several times to shove the cleaner around in there. You might get lucky.

[Enzo]

Always remove fuses from their clips to measure and test. Otherwise false continuity readings may result from parallel paths.

[cpuslave]

okay so I tested the fuse and every reading on each setting comes out to zero. So does that mean it's good? And on to troubleshooting this amp, what's the next step?

[Enzo]

If the fuse measures the same as touching the meter probes together, then it is OK. I think if the fuse was out, the amp wouldn't power up anyway.

Here, I would be checking that cutout contact on the headphones jack, and looking for signal onthe input pins of the 7293. I suspect the 7293, but it cuold be something else. It would be pretty dificult to steer you through finding a failure in the mute/standby circuit for example.

[cpuslave]

Okay so I sprayed some Electronic cleaner into the headphone jack and slipped one of those headphone adapters in and out a few times.

I noticed this little guy here (see attachment) that was a black color. It didn't look like it was burnt, but then again what does a burnt resistor look like? Is that what it is? A resistor? It says R15 right underneath it. Should I try to replace this?

Also, how do I go about looking for signal on the input pins on the 7293 chip?

Once again, love the help guys, I'm learning alot.

I just looked at the picture again, it looks like I'm pointing to that little blue thing, I'm actually pointing to that big black dude just above and to the right of it.

[teemuk]

Yes, it's a resistor. Judging by the photo it doesn't look burned, but then again your photo is kind of afwul. Burned resistors look, well... burned. They are significantly "darkened", either partially or in the worst cases completely charred black or brown so that you cannot even read the color coding. Often the board underneath it shows brownish coloration as well due to all heat.

However, this resistor just looks to be one with a black body. It's a reasobly high power one (you can tell that by the mere physical size) and those often have some deviation from the normal resistor appearance. The resistance is likely simply written in it instead of using a color code.

[Gtr_tech]

That resistor is supposed to be black.

[cpuslave]

Okay, just thought I'd check. Any other ideas as to what's going wrong?