| fishman94 |
Recently my brother acquired a 2700W Ultimate amp from a friend. We installed it and it worked fine for a few weeks, but then it did't want to turn on one day so he reached back and touched it while the car was running. He said he got a shock from it. We hadn't tried much with it since the shock.
Then, a couple weeks ago, the friend who gave it to him came to test it and it supposedly powered on. That friday, my dad and brother tried to install it back in the car and when they did it started to emit a lot of smoke. Before (after the shock) it would consistenly blow out the fuse, so I guess my dad installed it without a fuse in the line.
I was at school working when this happened, finishing up my circuits and electronics lab reports. When I came home late at night I smelt burnt parts, and found out the next day what had happened, so I took it apart to see what was wrong.
I tested the resistance between the ground and hot terminal leads and that registered ZERO!!! I there were several smoked transistors. I know the transistors do the amplifiying, and was wondering if bad transistors would cause the short between hot and ground. Also, if the system had a bad ground to start with, would that cause the trasnsitors to weaken, or something else to weaken?, I know a static shock can destroy a transitor because I accidentally detroyed one in lab when my partner had left the power supply turned on the night before.
Any help would be appreciated, and I have tools from my grandfathers shop to use. He used to be a television repair technitian for the locals before he passed away.
ArthurA, |
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| Perry Babin |
To help prevent having to repair it more than once, you need to check for continuity between each of the transistors' metal tabs and the heatsink. Do this BEFORE you disassemble it. BEOFRE you remove the clamps. The center terminals of the heatsink mounted components are connected to the tab so if you can't get to the tab, touch the meter lead to the center terminal.
On some of this type amp, there is a metal 'cup' that grounds the sink to the chassis ground. If this amp has one, remove the end panel, remove the screw from the cup and insert a piece of cardboard between the cup and the sink. Then check for continuity.
The reason I suggest this is because the problem may go away when the clamps are removed. The problem may not show up when the amp is reassembled. If there is a tiny piece of metal between the sink and the transistor, it could take a while before it shorts again. If you find it before you disassemble the amp, it will save a lot of time. |
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| fishman94 |
what does this metal 'cup' look like?
Also, was it the bad trasistors that cause continuity between the hot and ground? And what exactly is the "sink"? |
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| Perry Babin |
See attached. The cup is labeled gndport on this amp.
The sink is the heatsink. I should know better than to use shorthand.
The shorted transistors can easily cause a direct short between power and ground. Generally when an amplifier's power supply fails when there is no inline fuse (or the wrong size fuse), the FETs will smoke and eventually burn open (so there is no longer a short circuit). If it was pulled from the power source before they could burn open, then you may well see a direct short. If it's properly fused and the supply fails, there is often a short across the B+ and ground terminals. |
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| fishman94 |
| I've been doing a little more research on mosfet's and seem to be finding that some replace the drivers when they replace the transistors. What exactly are the drivers for the mosfets? |
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| Perry Babin |
The drivers are a buffer between the output of the PWM driver IC and the FETs (MOSFETs). They will be connected between pins9/10 of the TL494 (kia494) and the FETs. They can vary in configuration. In the most basic configuration, they have at least one transistor, one diode and one resistor. These rely on the PWM IC to drive the voltage high and rely on the transistor to drive the voltage low. Other amps will use two transistors in an emitter-follower pair. The second transistor is used to drive the voltage high (instead of the IC). These are used in larger/more expensive amplifiers which often have more FETs and require more gate drive current.
The attached photo shows a typical driver for a small amp. One transistor/resistor/diode set drives one bank of
FETs. The other set drives the other bank of FETs. |
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