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Old 16th January 2015, 06:34 PM   #1
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Default AN7164

Hi!

Philips AS665C went silent. Everything seems to be working, but no audio. So i thought that the output chip was dead. Removed the old one and checked the voltages on the PCB. For reference http://www.filedropper.com/philipsmi...660c665c765csm

Page 22.

Dont get any of the suggested values. And the output pins 7/8 and 10/11 rise slowly to the same voltage as the supply (24V). Pin 6 is 4.3VDC, pin 1 is about 3VDC.

Nothing on the board seems to be burnt. So I thought that maybe the voltages will be okay when measured with the chip in. A new chip fried the second the amp was turned on. Removed it, voltages are the same.

Input signals are okay. Grounds are okay. Supply on the 12 pin is okay. The output has a strange layout that i dont grasp.

What to check next?

AN7164 pdf http://www.ges.cz/sheets/a/an7164.pdf

Last edited by mekaiver; 16th January 2015 at 06:53 PM. Reason: More info
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Old 17th January 2015, 12:20 PM   #2
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Anyone?
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Old 17th January 2015, 02:06 PM   #3
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Headphones work?
The outputs must reach 50% of rail. Try a new chip.
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Last edited by JonSnell Electronic; 17th January 2015 at 02:08 PM.
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Old 17th January 2015, 02:09 PM   #4
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Pin 3 is ground?

I have just read that you removed the IC and checked the voltages? They are meaningless without the IC, especially the output readings as they will float.
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Old 17th January 2015, 03:22 PM   #5
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Yes, pin 3 is ground. I posted a link to the service manual. Page 22 is the schematic. I cant measure it with the IC on board, because it will start to smoke and gets extremely hot. Afraid it will damage something when I let it work like that.
Headphones output works fine.

So from what I understand, its normal that the output pins have the same voltage as the supply and the DC filtering is performed later by the output capacitors?

Have not seen any schematic where the IC outputs are connected with Vcc.

Last edited by mekaiver; 17th January 2015 at 03:27 PM.
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Old 24th January 2015, 04:41 PM   #6
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So I put the smoked chip back again and did the measuring.
Pin 1 is 4.1 VDC but the schematic says 24VDC. Also pin 6 /standby/ is 3.4VDC but shoud be 4.4VDC (chip datasheet says 5V).

Output pins are still both 23 VDC. Input pins are 1.1VDC and produce audio.

Could someone smarter look at the manual I posted in the first message and confirm the voltages and from where to measure the AMP_ON voltage to see if its okay or not.

Last edited by mekaiver; 24th January 2015 at 04:44 PM.
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Old Yesterday, 03:57 PM   #7
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Try removing capacitor 2293 and check what sort of voltage it will still sustain. If that really only is ~3 V, a diode test may do the job.

This chip has one of these frustrating '80s datasheets with zero detail, but I suppose the ripple filter circuitry may be rather unhappy if that cap is shorted.
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Old Yesterday, 06:31 PM   #8
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Thanks for the replay. Cap is okay. What is the purpose of the 4V7 zener diode (6272) in the pin 1 circuit?

Pin 1 needs 23VDC and the voltage on A+ is 24VDC. Then it basically goes to a diode (a voltage drop 0,7VDC) and we should get about 23VDC. But then there is that 4,7V Zener connected to ground and (as a result?) we also get on pin 1 about 4,1 VDC.
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