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Old 10th July 2011, 01:52 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default AudioBahn A12001DQ

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This amp appears to be having trouble with the pre-amp section. Just behind the RCAs, the following has happened:

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The amp produces +-60v rail, and +-13.7v regulated voltage There is also ~17.2vDC directly across the speaker terminals. Several of the JRC072D opAmps (IC10x) have ~7.5vDC on their outputs:

IC107:
Pin 1: -7.42
Pin 2: -7.41
Pin 3: -7.37
Pin 4: -13.52
Pin 5: -7.36
Pin 6: -7.38
Pin 7: -7.37
Pin 8: 13.87

IC106:
Pin 1: -7.35
Pin 2: -0.402
Pin 3: 0.221
Pin 4: -13.52
Pin 5: -7.30
Pin 6: -7.30
Pin 7: -7.30
Pin 8: 13.87

IC102:
Pin 1: 0.234
Pin 2: 0.234
Pin 3: 0.234
Pin 4: -13.52
Pin 5: 0.234
Pin 6: 0.234
Pin 7: 0.234
Pin 8: 13.87
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Old 10th July 2011, 03:58 PM   #2
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Replace R101 and recheck the voltages. It's a 220 ohm resistor.

This typically happens when a transformer shorts or the positive speaker wire shorts to ground.
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Old 11th July 2011, 01:05 AM   #3
Dr Zeus is offline Dr Zeus  United States
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I replaced the 220ohm resistor. Opamps are still reading the same as above but the speaker terminals have ~0.005vDC on them.

Im going to put this amp back into the sync and give it some audio tests.

Edit: This amp still cannot produce any audio. I believe there are several problems still with the pre-amp section.

Question: Should R101 be directly connected across the small C10x capacitor next to it? i cannot make out the number. Also, there was a trace underneith R101 which was severed that I had to repair.

Last edited by Dr Zeus; 11th July 2011 at 01:12 AM.
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Old 11th July 2011, 01:12 AM   #4
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Confirm that there is no DC on the RCA shields.
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Old 11th July 2011, 01:27 AM   #5
Dr Zeus is offline Dr Zeus  United States
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I made some progress

In my above readings, I noticed IC106 had bad readings on pins 1-3. I also noticed the voltage reglators liked to run pretty hot. I clipped Pin1 of IC106 and afterwards measured -14vDC on it's lifted pin 1 and only 0.075vDC on the pad under it. All previously mis-reading opAmps (IC107, IC104 mainly) now measure only ~0.030vDC on the input and output pins.

Theres 0.090vDC on RCA shields, but that reading is after I lifted pin 1 of IC106.

There is still no audio coming out of the speaker leads. Should I replace IC106? its a JRC 072D.

Last edited by Dr Zeus; 11th July 2011 at 01:30 AM.
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Old 11th July 2011, 01:42 AM   #6
Dr Zeus is offline Dr Zeus  United States
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I just replaced IC106 with a like-opamp from a parts amp and now this amp is able to produce audio at speaker terminal outputs. Time to order up some JRC072D opamps!
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