tecsun: Tecsun PL-230: Restoring the sound quality

I got this radio sold as working but with "poor sound".

I found out excessive current consumption, well above 100mA, even with the volume at minimum. The RF sensitivity was good on all bands.

Several electrolytic capacitors were found to have resistive leakage and the main offender had just 61 Ohm. Most of them had high ESR and the measured capaticance was well below the nominal value.

I decided to replace all of them as they were from either from one or another of the two chinese brands used in this production.

After having new caps installed, the current consumption reduced to normal levels around 30mA at minimum volume level. However, the sound distortion remained as before. 

At first I suspected of a defective audio power IC due to the initial high current consumption, but after switching the 12 Ohm speaker with a 8 Ohm spare unit I had at hand, the sound quality improved drastically. 

Pressing the speaker cone, I could hear a typical scratching noise when there is something abnormal in the magnetic coil-inductor assembly. As the speaker and cabinet were clean, with no signs of metal filings, I decided to dismantle the speaker only to find out that indeed the voice coil was rubbing against the armature at one of the sides.

How this happened is a mystery. There are no physical damage marks on the radio case or speaker, and the radio has never been opened before, judging by the foam stickers applied to the PCB.

I can only conclude that somehow, the armature magnet moved over time (unlikely), or it was a factory defect.

I decided to replace it with another Chinese speaker, but as the original uses 12 Ohm impedance (it is connected in bridged mode), replacing it with a common to find 8 Ohm unit would result in poor low end ressponse (the speaker is connected thru two 220uF caps). So initially I installed two 330uF caps to maintain the same low frequency ressponce using the 8 Ohm speaaker at hand. The bass response improved a bit, but as these are small speaaker units we can not expect a goos bass response. 

Then I found a chinese supplier offering 16 Ohm speakers of the same size (57mm diameter, just 12mm weight). Installing this unit resulted in a more relaxed sound, with better bass response. I believe the 8 Ohm unit was overloading the audio IC due to the bridge mode setup.

 

Attachments:

• Tecsun PL-230 electrolytic caps values and location (267 KB)

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