MG100hdfx blowing fuses STILL!! - diyAudio
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Old 3rd October 2010, 05:19 PM   #1
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Default MG100hdfx blowing fuses STILL!!

So this amp started with the issue that there was no sound, but thanks to llwhtt, I have solved that issue. I changed a zener diode (6.8V) and now there is 6.8V making it to the output IC(TDA7293) standby circuit, so that is awesome. Now the output IC is not stuck in standby.

New problem, now there is .3V AC making it to the output IC, 6.8V making it to standby, 44V DC to + power, -44V DC making it to -power, so that seems to be all good. When the output IC is NOT plugged in, I do not blow fuses. When the IC board (power module) is plugged in I blow a 3.15 slow blow fuse, and the IC chip burns out. I am lost at this point, and the worst part is I am afraid to turn the amp on if I "think" that I have found the problem because I will fry a $30 power module if I didn't actually find the issue.

Here are the schem's.
http://www.ampix.org/albums/userpics/10003/MArshall_MG100DFX.pdf
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Old 3rd October 2010, 06:00 PM   #2
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Is the output IC mounted on a heatsink? Isolation checked? If possible with the used connector you can try to take out the pins and see when the fuse blows. Start with the output pins then power GND.
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Old 3rd October 2010, 06:05 PM   #3
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The output IC is in fact mounted onto the heatsink, but there is a plastic grommet blocking continuity. The problem is, when the used connector is in place, there is continuity that shows a "false" reading, since there is a ton of shorts in the actual IC. But, when the power module is in place, without the IC on the board, the fuse does not blow. I am wondering if there is a short before the power module, if the fuse would blow without the IC in place??
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Old 3rd October 2010, 06:12 PM   #4
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I was thinking, does the amp itself draw more power when the IC is in place? If it does, does that mean that one of the "gates" on a transistor or an output IC may be allowing more current to flow than is supposed to, only when the IC is in place? How can I check an output IC or transistor to see if it is bad?
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Old 3rd October 2010, 06:13 PM   #5
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Quote:
Originally Posted by Rotus623 View Post
But, when the power module is in place, without the IC on the board, the fuse does not blow. I am wondering if there is a short before the power module, if the fuse would blow without the IC in place??
If the fuse doesnt blow with the IC removed there can be a fault in the circuitry at the output of the IC (therefore you can disconnect the outputwires from the connector) or the IC is faulty. I do not suspect a fault before the power module since you said the IC is getting hot.
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Old 3rd October 2010, 06:33 PM   #6
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Quote:
Originally Posted by boyofwax View Post
If the fuse doesnt blow with the IC removed there can be a fault in the circuitry at the output of the IC (therefore you can disconnect the outputwires from the connector) or the IC is faulty. I do not suspect a fault before the power module since you said the IC is getting hot.
Okay buddy, so you are thinking that I may have had a bad IC? That is what I am thinking. I know that my output signal wire should not be running continuity with the chassis when the power is on. It is in fact running continuity with the chassis when the power is off. Is this normal?

If it is it sounds to me like I am just going to have to take a chance and try another IC. If if blows yet again, I will rule out that I did not have a bad IC the last time it blew. Would you agree?
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Old 3rd October 2010, 06:42 PM   #7
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Disconnect these outputwire's from the PCB and see what happens first.
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Old 3rd October 2010, 06:59 PM   #8
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Gotcha, and if I do not blow the IC, then we know the problem is in the output jack area somewhere. And if I still can't find an issue, then Ill plug my main outputs back into the main PCB and see what happens. Is that what you are suggesting?
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Old 3rd October 2010, 07:00 PM   #9
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Yup
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Old 3rd October 2010, 07:12 PM   #10
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Cool. Cross your fingers, Ill hit you back after I check it out.
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