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ICOM IC-756 ACC1 Audio Output Increase Mod [message #3066] Tue, 03 March 2015 11:58
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Registered: December 2004
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Author: Adam MAURER, VK4CP - vk4cp@bigpond.com

Access to the Main Unit is required.

The "Line Level" audio output on pin 5 of Rear panel connector ACC1 is very low, with the manual stating 300mV. Normal Line Level is regarded as about 700mV. Although my MiniDisc Recorder had a gain control for its Analogue I/P it still couldn't make 0 VU. Here, by adding two resistors we can increase the gain of the audio appearing on pin 5 of ACC1.

1. Remove the bottom cover.
2. Remove 6 screws holding down Main Unit.
3. Remove screw on rear panel in between ALC & Send RCA sockets.
4. Remove heatsinking clip holding transistor to chassis.
5. Remove flat ribbon cable to J3 in far right corner.
6. Carefully pry PBA loose and fold over - see at left. (I actually had to remove RF Unit shield and loosen a shielded cable to allow the Main Unit PBA to fold out).
7. Locate IC-503, a NJM2058 OPAMP in an SMT SOIC14 package.
8. Remove W3 0603 SMT link.
9. Fit an 10k 0603 SMT resistor in W3's place.
10. Fit a 1/8w 5k6 through hole resistor from IC-503 pin 9 to ground.

We have effectively changed the OPAMP IC-503c from being a unity-gain follower into a non-inverting amplifier with a gain of approx 2.5.

Thanks to http://www.qsl.net/vk4cp/ for the modification.
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