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Old 22nd December 2008, 03:57 PM   #1
PQatPIT is offline PQatPIT  Finland
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Default Rockford T20001bd

Hello!

Working on RF T20001bd, had blown power supply FET's and their driver transistors and gate resistors. Now all of them are replaced and in place, though amp doesn't turn on when remote is applied. Voltages at UC3526UAW pins, measured with oscilloscope:

1 4.56Vdc
2 2.56Vdc
3 4.96Vdc
4 4.96Vdc
5 4.96Vdc
6 80mVdc
7 80 mVdc
8 4.80Vdc
9 3.71Vdc
10 50kHz sawtooth, Pk-Pk 1.78V, Rms 1.97V
11 200mVdc
12 50 kHz 95% duty on square pulse, 0V to 730mV
13 20 mVdc
14 10.3Vdc
15 Gnd
16 180mVdc
17 11.1Vdc
18 4.90Vdc

I tried and tried to understand why it would not start up but couldn't- what voltage values might be wrong? I also have the schematics but still. There is two smd thermistors on FET boards, I checked them with DMM and cold spray and at room temp they show about 2k6 ohms, when cold spray is applied resistance goes up. No lights come ON, no drive signals to power supply FET's.

Replacement parts were IRFP054N for HUF75344, BC807 for MMBTA56L and BC817 for MMBTA06L and individually checked 0805-style resistors.

Thank You in advance!
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Old 22nd December 2008, 09:49 PM   #2
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Location: Louisiana
If I'm not mistaken, the sync should swing from 0-5v. If yours is swinging to less than 1v, that could be causing the output to be shutdown.

Can you lift the sync pin to determine if something outside of the IC is dragging the voltage down?

If this doesn't help, send me a copy of the schematic.

babin_perry@yahoo.com
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Old 22nd December 2008, 11:45 PM   #3
PQatPIT is offline PQatPIT  Finland
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Email sent.
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Old 11th January 2009, 05:09 PM   #4
PQatPIT is offline PQatPIT  Finland
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Join Date: Nov 2008
Did lift R15 away, sync pin 12 swings from 0 to 4.6 volts but no output signal on pins 13 or 16. Do you think that this IC is dead?

I happened to have my hands on a working one, and measured voltages:

1 2.7 Vdc
2 2.7 Vdc
3 1.4 Vdc
4 5 Vdc
5 5.2 Vdc
6 0
7 0
8 5 Vdc
9 4 Vdc
10 same sawtooth, but with some noise
11 noise, 2 Vpp
12 52 kHz 95% duty on square pulse, 7.0 Vpp
13 25 kHz 5% duty on square pulse, 12 Vpp
14 10 Vdc
15 0
16 same as pin 13
17 21 Vdc
18 5 Vdc

I have been all over the board for e few times and I just can't figure out what is missing here. I could come up nothing else than this IC U2 pwm controller.

Thanks!
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Old 11th January 2009, 06:59 PM   #5
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Location: Louisiana
It appears that the 3526 is defective.

You may want to order a replacement 74hc74 also. It should not have been causing the sync output to be pulled down so far.
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