Fixing noisy car head unit - Page 1
Forum section: electronics / beginners

author: Fritzoid26
author_rank: Newbie
timestamp: June 15, 2025, 10:02:06 pm
content: Hi,I'm a beginner to general electronics repair and have been chasing what I thought was a ground loop for a very long time on my car. I have now realised it is a fault with the head unit itself as all grounds are at the same point and the noise disappears when using a different newer head unit, the faulty unit is an old clarion from the mid 90s. The noise consists of two parts - an engine speed dependant click, and a whine. The click disappears over time as the unit heats up. Interestingly, the click disappears from cold if I supply the amp with a separate supply direct from the battery.The noise occurs in all sources, CD player, AM/FM etc, so I suspect the issue is with power filtering or an issue on the sound processor daughter board or the pre-amp board. The head unit outputs line level only. I have already re-flowed the board twice, so I suspect it is a capacitor. What is the best way to trace this issue? I have an oscilloscope, can I just ground the probe on the radio chassis and then probe the signal path and watch for noise? Why does the clicking sound disappear when I supply the amp separately? And lastly, I have attached the schematics. The first circuitry that takes the +12v in (ACC) can be seen on the bottom left of page 5 connector E 2/3, link and screenshot also attached, but I can't work out where the filtering section of the circuit is and what the likely culprits are? I assume capacitors - there are two big 330uf 16v capacitors near what I assume is the "choke block pwb" - they look okay, but could it be these at fault, or the choke itself?Schematic:https://www.kevinchant.com/uploads/7/1/0/8/7108231/pu-9836a.pdfThanks in advance.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 16, 2025, 09:51:35 am
content: have you checked it does not come from the alternator circuit, a dying car battery .... in the motor sectionyou have a ground braid from the frame to the motor, or some cars have 2 or 3 ... the battery ground, is it okay ? normally one of this wire goes to the body/framefor the Radio Eq Amp are you taking the ACC and power from the right places, with good awg / size wires ? all added items outside the radio take power directly from the battery

author: Fritzoid26
author_rank: Newbie
timestamp: June 16, 2025, 10:34:16 am
content: All of those are good yes - the thing is I'm pretty certain its the head unit as it only occurs with this one. Not with a spare I have.

author: Sorama
author_rank: Frequent Contributor
timestamp: June 16, 2025, 10:54:53 am
content: have you tried a (fused) dedicated and new power (+) wire from battery tot head unit and other audio devices ?You should be able to use a scope (grounded to chassis indeed) and visualize the whining when probing + or - wire.It might as well come through via the ground wire.

author: Fritzoid26
author_rank: Newbie
timestamp: June 16, 2025, 11:35:31 am
content: Yes, tried running a new separate +ve wire direct from battery to head unit and amp respectively. Both are grounded at the same point using a separate wire, and have good grounds

author: Sorama
author_rank: Frequent Contributor
timestamp: June 16, 2025, 11:41:41 am
content: Is the negative terminal of the head unit in the connector, or does it have a separate ground on the metal chassis of the head unit?If metal head unit, you can try with a dedicated ground between unit and chassis and cut or remove the inner ground cable in the head unit’s connector.But again, it would be good to know first how it is coming in (ground or positive terminal) and at what device (probably head unit).You can use a scope or fra for that.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: June 17, 2025, 12:45:58 am
content: Your description of the unit and what you are trying to do was not complete.Clarion OEM from what? year/Make? Interfacing to what?OEM units do not have RCA outputs and most can not be treated as such and require either capacitive or transformer coupling to interface with aftermarket units without designing circuitry.Jeff

author: Fritzoid26
author_rank: Newbie
timestamp: October 01, 2025, 11:05:11 pm
content: It turned out the noise was in fact being introduced via a faulty volume pot on the equaliser. The noise I saw in the ft view was a red herring.