Magnavox 42MF531D/37 has high pitched whine coming from speakers after repair

[SurrealMustard]

With the help of the community, I recently repaired my no-power Magnavox 42MF531D/37 LCD television (replaced capacitors and diodes on the power supply and then replaced the mainboard twice). It now powers up and runs how it's supposed to, but I've noticed a high pitched whine that comes out of the speakers. It happens on every input and at every level of volume (except mute). The best way to describe it would be to imagine a room with 5-10 standard definition CRT televisions all running with their sound turned off.

The noise usually comes on right away when the television is turned on, but if it doesn't, it begins after the set has been on for an hour or two. It has the same pitch and intensity regardless of what is playing. Relief only occurs in the brief period of switching inputs, when the sound is muted, or when the volume is turned all the way down.

I hooked up a pair of headphones to the headphone jack to see if the problem happened in there too, but it does not. Only out of the main speakers.

Anyone have any troubleshooting tips or know of what might be causing this?

[Agent24]

Re: Magnavox 42MF531D/37 has high pitched whine coming from speakers after repair

Sounds like something wrong in the amplifier section, picking up interference or just gone bad.

If the headphone jack doesn't have the problem, trace the audio signal from there to the speakers and find where the noise appears, then you should be able to narrow down what its causing it.

[SteveNielsen]

Re: Magnavox 42MF531D/37 has high pitched whine coming from speakers after repair

May be a bad solder joint to a shield ground.

[tom66]

Re: Magnavox 42MF531D/37 has high pitched whine coming from speakers after repair

I've had a similar issue (whining after some time running) caused by a bad cap on the CPU voltage rail. Wasn't bulging but 16 ohm ESR and something like 300mVp-p on an 1.8V rail, way too high. Since your fault is intermittent, I'd get out a can of freezer spray and a hot air gun/hairdryer, and see if you can narrow down the source of the fault.

[SurrealMustard]

idea

I've had a similar issue (whining after some time running) caused by a bad cap on the CPU voltage rail. Wasn't bulging but 16 ohm ESR and something like 300mVp-p on an 1.8V rail, way too high. Since your fault is intermittent, I'd get out a can of freezer spray and a hot air gun/hairdryer, and see if you can narrow down the source of the fault.

Can that be done with just a dryer and hot air gun if I don't have any freezer spray?

[Agent24]

Re: Magnavox 42MF531D/37 has high pitched whine coming from speakers after repair

You can use the hot air to heat things but then you have to wait a long time for them to cool down again. The problem is if you heat more than one thing at once you don't know which one fixed the problem. It is easier to selectively cool things than it is to heat them.

[SurrealMustard]

Happy ending

This one ended up being a stumper for me, but I'm calling it a partial win because the core functionality was restored. I ended up selling it as-is for a (small) profit to someone who was going to be using external speakers and a wall mount with it.

In the future, I'll be better equipped to handle these types of repairs with my new capacitor tester that I picked up while working on the RCA DLP. Looking back, each of these repairs has made me a better technician. Apparently, there's no replacement for practice .