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Old 31st May 2016, 11:08 AM   #1
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Default Rockford Fosgate T30001BD Preamp Problem!

Hello everyone.
I am in possession of an amplifier Rockford Fosgate t30001bd.
He had all the chips TL072 not working, in fact, U100, 101,102,103,104,105,106,107, the pin outputs 1 and 7, was DC voltage.
I changed them all and the situation is much better, in fact, now, on pin 1 and 7 of U107, there is an excellent audio signal, but as soon as the signal passes through C335 and R300 to get to the pin 2 of the U300, the signal distorts, in fact, comes from pin 1 and 7 of U300 with a horrible distortion.
Then the amplification stage, amplifies this horrible audio signal.
all TL072 are new and for this reason are all perfect.
what do you advise me to check?

Last edited by jazbo8; 31st May 2016 at 11:15 AM. Reason: caps
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Old 31st May 2016, 11:29 AM   #2
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Circuit?
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Old 31st May 2016, 11:34 AM   #3
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Quote:
Originally Posted by JonSnell Electronic View Post
Circuit?
I use the schematic of rockford t20001bd which is almost equal.
Attached Files
File Type: pdf rockford - powerT20001bd.pdf (827.7 KB, 19 views)
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Old 31st May 2016, 11:42 AM   #4
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Have a look at 15 volts regulators R63, R64, R67, R68 etc.
No reason to change the 072s, check supply first.

Last edited by as_audio; 31st May 2016 at 11:49 AM.
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Old 31st May 2016, 11:51 AM   #5
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Quote:
Originally Posted by as_audio View Post
Have a look at 15 volts regulators R63. R64, R67, R68 etc.
No reason to change the 072s, check supply first.
The power supply works perfectly ...
the voltages +/- 15V, +/- 6v, 10v are there and are fully functional.
Each TL072 is properly supplied with the correct voltage.
Apparently there is no plausible explanation why the sound is distorted in that way when it comes to U300.
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Old 31st May 2016, 11:51 AM   #6
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U300 supply OK? Any DC on pin1 U300?
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Old 31st May 2016, 12:29 PM   #7
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Quote:
Originally Posted by JonSnell Electronic View Post
U300 supply OK? Any DC on pin1 U300?
Actually I did not check.
but I think it is ok, otherwise, I think it did not go out even distorted sound from pin 1.
rather, I'm thinking that because the pin 1 of U300 is connected to pin 3 of D311 through R303, the problem might just be in the area D311, Q313 and Q312.
I'll try to remove R303 and see what comes out at that time from pin 1 of U300.
WHAT DO YOU THINK?
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Old 3rd June 2016, 04:36 PM   #8
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good news!
i've replaced C355, c356 and C359.
Their are three electrolytic, C355 and C356 are concerned with filtering the inverting and non-inverting input of U300A, C359 deals with filtering the resulting signal and inject it in the LM6172 comparator that generate the PWM signal and the carrier.
Now the amplifier works fine, but I have a slight problem ...
to the ignition command, there is no delay that enables the driver circuits.
For this reason, there is a violent "bump" in the speaker.
What can I check?
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