453
Item nr.


HengXing HX6B Pocket radio

Educational radio.


Data for HengXing HX6B
ProductionThe Netherlands, 2017.
Price was 5.87€.
BandsMW (530-1600kHz).
Semi-
conductors
9018 (ocs/mix), 9018 (1st IF), 9018 (2nd IF, det), 9014 (AF), 2x 9013 (output), LED.
CabinetPlastic. Size 12.5x7.5x2.5cm. Weight 175gr (inc batt).
PowerBatt 2xAA, 15-20mA.
DocumentsSchema and building instructions.

The Design

It still exists in 2017, a simple Medium Wave portable radio. It does not use any IC, but just discrete parts, it is a superhet with six transistors and one LED. The LED takes 5mA, about one third of the power consumption. And it not very easy to see from the outside, so to extend battery life it would be wise to cut out the LED. The inside of the radio would look very familiar to somebody constructing radio's fifty years ago.

Probably the same or a similar cabinet is used for FM radio's, because I see a possibility to mount a rod antenna on the back. HengXing is a Chines company based in the city of Zhengzhou.


Obtained3/2017 from Aliexpress.com.
Condition8.
DisposedScrapped 3/2021.

This Object

The small packet arrived from China about a week after I ordered it, and construction could start. The radio was good for about 21 hours of hobby fun, that is one hour to assemble the radio and three days to get it working.

As usual, assembly wasn't hard and posed no special problems. Except that the instructions were in Chinese, and I had to dig out Google Translate to get the correct colors of the three HF transformers (T2 is red, T3 is white, T4 is black, in case you want one too). And I measured the phase inverter transformer to find out the correct leads.

But when all the parts were on the board, and power and speaker were connected, the radio didn't work. So I started checking and measuring everything, and I found out that in the output stge, biased by R7 through R10, I mixed some of the 100R and 120R resistors. I unsoldered these and put them back in the right place.

Still, nothing! I dug out the signal generator and found out that some scratching noise could be heard with signal on the tap of the volume pot. But only with the voluma half way, not in the zero or max position. Then I found out that the PCB contained an error: there was an extra copper strip, connecting the top of the potmeter to ground. I had to scrape away this strip. Then the sound was still extremely distorted. I unsoldered V3, checked it but it was OK. I measured many voltages in the HF part, resoldered all joints, but without succes.

I concentrated my measurements in the AF part and found that one of the 9013 transistors wasn't conducting at all, the center connection between then was at 3V from ground! I unsoldered and measured both transistors, replaced them by two BC337's, but all without effect. Seeking a cause for the unbalance in the output stage, I followed all the connections and measured the voltages on all coils of T5. Finally it turned out that, when swapping the resistors in the output stage, I had not soldered one of the leads well. Correcting this was done quickly, and finally the radio worked!


Part of Gerard's Radio Corner.
Generated by SiteBuilder on 26/2/2024 by Gerard (g.tel@uu.nl)