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Old 20th March 2012, 12:23 AM   #1
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Default Want to fix my sub and amp

I was given an MTX Sub8 powered sub with 100w amp. The reason my friend gave it to me was because it started blowing fuses and would never stay powered on for more than 3 seconds.
I pulled the amp out and looked over everything but nothing is obviously blown or burned up. Where do I start?
I checked the continuity of the transformer and it checks good so there aren't any broken windings as far as I can tell. The 2 larger caps (35v each IIRC) seem to be charging up evenly with the multimeter test, I checked the NPN transistor and it showed 0 on the hFE. I then checked it later and it showed 1, so I'm not sure what that means. What else can I check? Could it be as simple as the on/off switch shorting out? Yes I am using the 1 amp slow burn fuses it is supposed to use.
I'd like to fix this amp instead of buy one, I don't know of any reason this thing is not repairable.

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Old 20th March 2012, 01:40 AM   #2
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Disconnect the power supply from the audio circuitry. Then plug it in and turn it on. If the fuse blows, there's a problem in the power supply. If the fuse doesn't blow, the problem is in the audio circuit.
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Old 20th March 2012, 01:52 AM   #3
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Ok, I'm getting ready to do that right now. But I may have an issue because the power supply is 17.8v/ 3A output and I CANNOT find anything like it.
If it isn't in the power supply then where is a good start to look for the audio circuitry problem?
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Old 20th March 2012, 02:16 AM   #4
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If it's a bad power supply, it can either be repaired or replaced. diyAudio is the perfect place for these "I CANNOT find anything like it" situations.
As for the question, I was going to include in my first post... Remember 3 words when troubleshooting - safety, divide, and conquer. In that order.
Having said that, heat is the enemy of electronics so it can often be assumed that a failure is in that part of the circuit that handles a lot of heat/power, eg the discrete output transistors or chipamp.
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Old 20th March 2012, 02:32 AM   #5
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Ok, I can order an output transistor but can't seem to find a TD73708 IC/Amp chip. Is there a replacement I can use, updated design or am I screwed? Not even sure it is bad but I figured if it were inexpensive enough I'd replace both of those and give it a shot
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Old 20th March 2012, 02:49 AM   #6
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I'm a little confused because it's very unlikely there will be an output transistor and a chipamp. What is this transistor number?
For the IC, I think the correct part number is TDA7370B. That's a quad chipamp that is available. Googling "TD73708" got only a few unrelated hits.
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Old 20th March 2012, 02:52 AM   #7
jaycee is offline jaycee  United Kingdom
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Some pictures would help.

At this point, it's worth checking the sub's voice coil. Push the cone slightly with your fingers, it should move smoothly. If it grates, the voice coil is shot.
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Old 20th March 2012, 03:10 AM   #8
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Well, to say the least, I am a little new at this. So my amp is a chipamp according to where the mods put this post. The transistor I was referring to is a TIP122 100v that is attached to the heatsink next to the TDA7370B chipamp.
eBay has been my saving grace as I can pretty much replace everything on the circuit boards for about $35. I don't want to do that, but I have a couple of capacitors that have a stained brown tinge to them like they leaked or something leaked on to them. I am going to replace these as they are cheap. I am also going to replace the chipamp and the TIP122 transistor I'm pretty sure.
How can I check the chipamp to see if it is good or not? This is the most expensive piece to replace so I want to make sure it is or isn't the problem.
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Old 20th March 2012, 03:23 AM   #9
jaycee is offline jaycee  United Kingdom
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Is there only one TIP122 transistor? I would say it's probably used as a regulator or something like it, rather than any part of the power amp.

Checking the chipamp would basically mean taking it out of circuit, and building the reference circuit in the datasheet to check it.

Like i said, check the subwoofers voice coil. It might well be fried if the chip is dead.
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Old 20th March 2012, 03:29 AM   #10
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Yes there is only one TIP 122 transistor. The voice coil is fine, plenty of movement with no noise. Did I mention that one is the times it fired up there was a hum or buzz from the speaker for about 5 secs before the fuse finally burned out?
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