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Old 3rd March 2013, 12:01 PM   #1
ViNzZz is offline ViNzZz  Italy
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Default Autotek 7600 bts problems

Hi to everyone. I'm italian, so excuse me for my bad english
The amp produces +/- 56 volt but I have problems in audio stage: I desoldered all tip35c/tip36c and trying with new ones, they get hot.

PS: there are no transistor driver shorted, so I haven't replaced them. I must replace them? Here's an old pic of the amp.

PPS: on the board I see written that original fets were irfz40. Now I get irfz44, so what gate resistor I've to mount? 220ohm or 75ohm?

Thanks to everyone!
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Old 3rd March 2013, 01:12 PM   #2
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Set the bias pots fully counter-clockwise. Does that eliminate the heating of the output transistors?

The 220 ohm resistors will likely cause the power supply FETs to overheat. Use the 75 ohm if the choice is 75 or 220.
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Old 3rd March 2013, 01:19 PM   #3
ViNzZz is offline ViNzZz  Italy
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No, it doesn't eliminate the heating. If I connect rca and try to get music, I have no audio in output, but only noise.

What voltage I should read on tip35c/tip36c legs when they're connected?
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Old 3rd March 2013, 01:47 PM   #4
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Disconnect the RCA cables from the amp and measure the resistance from the RCA shields to the speaker wires that are NOT used when the amp is connected in bridged configuration. You should read 0 ohms. Is that what you read?

What is the DC voltage measured directly across the speaker wires for each channel?

How much current does the amp draw when the output transistors are heating up?
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Old 3rd March 2013, 02:21 PM   #5
ViNzZz is offline ViNzZz  Italy
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I read open circuit. So, quattro can be the probblem?

Across each speaker wire I read 1,5 volt DC.

I don't know how much current the amp draws, but I'm using 2,5ampere 12.5 volt transformer. I think it's drawing 2,5amp, because the green amp led without tip 35/36 do more light

Thank you so much Perry! You're fantastic!

Last edited by ViNzZz; 3rd March 2013 at 02:29 PM.
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Old 3rd March 2013, 02:29 PM   #6
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The trace in these amps sometimes burns near the RCA jacks. If there is no open trace, check the RCA ground connection to see if it's broken where it goes into the board.
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Old 3rd March 2013, 02:53 PM   #7
ViNzZz is offline ViNzZz  Italy
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There Was an open trace!!! Now I have no DC voltage in speakers outputs and no excessive current drawing.
Now, what I have to do?
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Old 3rd March 2013, 03:07 PM   #8
ViNzZz is offline ViNzZz  Italy
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Works cert good! Clean audio! Thank you very much! I mount all the transistor and after we can do bias regulation
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Old 3rd March 2013, 04:12 PM   #9
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If you don't have an amp meter on your power supply, you'll need to adjust the bias by the voltage across the emitter resistors. Set the bias so that you read 0.001v DC across the resistors (no input signal and no speakers connected).
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Old 5th March 2013, 07:22 AM   #10
ViNzZz is offline ViNzZz  Italy
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Yesterday I tried to connect it in my car for any test. 2 minutes running at low volume and it comes in protection. Now I see no rail voltage: 11 irfz44 measure low resistance from gate to drain or source. What is the cause? Can be that I haven't installed Q201?

Now, what transistor I buy?
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