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Old 6th November 2011, 02:44 PM   #1
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Default Boston Acoustics GTA 800m

Hi,

I own a gta 800m that recently died. It's out of warantee and I have experience as an audio tech, so I opened it up. I replaced the output and power transistors, along with the driver transistors for the TL494. I also replaced a 22uF cap near the output relay that appeared to be bulging. The amp is still in protection mode and I only get 6v at Vcc of the TL494.
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Old 6th November 2011, 04:11 PM   #2
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Can you post a photo of the inside of the amp?
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Old 6th November 2011, 07:12 PM   #3
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I've also included a closeup of the TL494. The traces around the IRF3205's were really burnt up. The hot glue was me insulating the traces I had to fix, along with the 47 ohm resistors I had to replace.
Attached Images
File Type: jpg GTA800M.jpg (229.1 KB, 37 views)
File Type: jpg TL494.jpg (243.0 KB, 36 views)
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Old 6th November 2011, 07:33 PM   #4
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Does Q1 supply 12v to pin 12 of the 494 via the 10 ohm resistor?

If so, post the DC voltage on all 3 pins of Q1.
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Old 6th November 2011, 08:51 PM   #5
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No, the 10 ohm resistor feeds the IRF3205 drains. I did some tracing on the bottom of the board and pin 12 of the 494 comes up between Q14 and R17, and again between R30 and Q9.
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Old 6th November 2011, 08:54 PM   #6
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Are you referring to the 10 ohm resistor next to Q1?

Did you find that connection with your meter or did you follow a trace?

Did you find any low resistance connection between any pin on Q1 and pin 12 of the 494?
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Old 6th November 2011, 09:09 PM   #7
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The center pin of Q1 has continuity with pin 12 of the 494.
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Old 6th November 2011, 09:22 PM   #8
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Please answer all of the questions.
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Old 6th November 2011, 09:35 PM   #9
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Sorry.

I am referring to the 10 ohm resistor next to Q1.

I found that connection with my meter.

There is a low resistance connection between the center pin of Q1 and pin 12 of the 494.
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Old 6th November 2011, 09:44 PM   #10
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If you look at the drains of the power supply FETs (with a scope) after you get the amp working, you'll see that the resistor is not connected to the drains. The connection that you found between the resistor and the drain is through the transformer's primary winding.

Does the other end of the 10 ohm resistor connect to the firs leg of Q1?

What do you mean by 'low resistance'?
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