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Audio problems - Serious distortion above 30% volume 700 1990

posted by  Will740turbo on  Fri Dec 15 02:32 CST 2023
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last visit: Wed Oct 22 16:09 CST 2014
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Hey, folks.

I like the factory stereo and this is the first time I've had this much distortion from one. I just took the Amplifier out and repaired it - replaced 3 fast switching diodes (2 were blown) and the primary OP-AMP that feeds the obsolete amplifiers (can't find a new set). That didn't help this:

Hum with headlights or fuel pump on (worse with headlights, always on with kp2).
Loud static pop when antenna lower button is pressed.
Badly distorted sound if volume knob set above 30%.
Happens with tapes and FM radio.

I have a feeling this is a power supply issue like low voltage to one or both the head or amplifier.

Yeah, I guess I can get a new stereo but I prefer to keep the familiar factory unit. It's well-integrated and works well... usually. It doesn't have obnoxiously bright lights and I have a factory 5 or 6 band equalizer that works with it.

Tha FAQ is pretty light on stereo info, so I'm hoping that someone here can guide me in the right direction.

Any ideas? I know I need to clean the battery terminals that feed the interior, but I'm not sure that's related.

Cheers,
Will








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    Audio problems - Serious distortion above 30% volume 700 1990

    uid 46171's pic
    posted by  kittysgreyvolvo on  Sat Dec 30 13:44 CST 2023
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    last visit: Sat Oct 18 15:23 CST 2014
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    Hi Will,

    What model Volvo radio head unit do you have? (I guess factory original?).

    The head unit is the version that powers the dash mount speakers for imaging, and through a five- or six-pin DIN cable connect the radio head unit with the amplifier. There is a few different models of each and am uncertain what 1990 740 Turbo comes equipped with. I'll guess the premium model system made by Alpine (head unit, amp) and now in need of replacement Dynaudio dash, door(s), and rear hat shelf or dunno on wagon.

    Hum - do you have LED headlights? Audio hum may mean spurious RF emission usually caused by, that also causes hums, back conductivity at ground connections directly or related to the circuit.

    Faulty connection with or in the aerial. Grounds? Anything loose?

    Badly distorted sound? Your switching diode replacement worked and you tested the amp outside your 740? You've tested the circuits and no low voltage beyond normal voltage drop? The capacitors in the Alpine and older Mitsubishi amps can bulge, leak, cause problems.

    Do you and your Volvo 740 reside in a humid place like maritime or riparian zone? Salt with snow with moar salt? You've tested your charging system? The DC output is correct? The alternator diode rectifier pack is okay?

    Else, I dunno for now.
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