TDA2822M Oscillatiing in Grundig Receiver

[jff6791]

I"m troubleshooting a grundig SW receiver with a low frequency output oscillation that's using a TDA2822M chip in bridge mode. The problem goes away with headphones plugged in and also with a 100 ohm resistor in series with speaker. Have subbed out the chip and bypass/filter caps in circuit as well as 8 ohm speaker. No luck. Any thoughts appreciated.

John

 

[sgrossklass]

Grundig, TDA2822M? Probably a Chinese-built "Grundig" model then (rebadged Tecsun or something)?

Anyway, low-frequency oscillation sounds like you should turn your attention to power supply decoupling. I guess you'll find a few bad electrolytics with sky-high ESR. (Got something resembling a makeshift ESR meter?) Also check whether the voltage regulator still does its job.

 

[nec3]

Is the receiver operated on batteries or AC mains ?
It oscillates also in low volume levels or only when turned up max ?
What is DC voltage at the input and output of 2822M ?
When you short inputs to the ground, the oscillation stops?

 

[jff6791]

Thanks for those suggestions. The radio is branded Grundig/Eton S350. I had never run it on batteries but when I tried that - problem gone. So I opened up the wall wart and there is no regulator in there or inside radio either - just some large electrolytics which I had already replaced. I was getting 8-10 volts out the supply even tho it's nameplate ssys 6 - have never seen that before. Swapped it out for a switch mode adjustable - set it for 6 volts and its back in business. Go figure.