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Restoring Sony STR-AV780 AV receiver

Sony STR-AV780 is a combined receiver and audio/video control center. This receiver is manufactured by Sony in around 1986 and it is capable to deliver 2 × 80W audio output power with 8Ω speaker load.

The STR-AV780 which I got had two problems. The first problem is in the power amplifier stage and it produces distortion in high frequencies. The second problem is in FM radio receiver and it is completely dead.

The power amplifier of STR-AV780 is based on two Sanyo STK4042 XI AF power amplifier ICs. To fix the distortion issue we replace all the electrolytic capacitors in both STK4042 power amplifier boards. After replacing capacitors distortion issue got fixed and the power amplifier starts to produce a clear output waveform.

The FM receiver of this AVR is mainly based on 2 ICs. For IF stage it uses LA1235, and to demodulate FM stereo signal it uses LA3401. Apart from that Sony CX7925 is used as PLL frequency synthesizer. The front end of FM tuner is based on a couple of transistors and FETs.

To fix FM receiver we inject FM test signal into the receiver and check each stage of the FM receiver. Based on VCO output (of CX7925) and FM front end output we identify LA1235 as a faulty component. After replacing LA1235, FM tuner starts to work again.

Fixed AV780 mainboard - tuner stage.

Due to the weak nature of PCB copper tracks, we attach LA1235 into the main PCB through an IC base.

After replacing all above-described components we clean and repaint the entire AV receiver.

During test stages, we check this receiver with several speaker setups. Out of all the speakers, this receiver performs extremely well with our Pioneer CS-707 speaker system.

Due to lack of service manual / schematic Sony STR-AV780 is a quite difficult receiver to repair. We trace and fix components of this receiver based on PCB layout and IC data-sheets.
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Comments

Anonymous said…
Any ideas on what to do if the display doesn't always seem to work.
There are days that when it is first turned on it will work from a few minutes to maybe an hour. But then it goes dark.

Thanks.
August 24, 2019 at 10:17 AM
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