Author Topic: Roberts DAB radio ECOLOGIC4 / RD21 / RD-21 no sound / will not switch on  (Read 225 times)

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Offline rh100605

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I have bought one of these radios as faulty. The symptoms are that the display works correctly but there is no sound. A google search suggested two faults. Firstly the earphone socket switch and secondly the final audio amplifier. As there was a smell of overheated pcb and repeated insertion of a 3.5mm plug into the earphone socket made no difference, I opened the radio up.
The final audio amplifier is the CD8227/ TA8227p with a heatsink soldered over it. The series pass transistor (2s)d882 was very warm and had no heat sink, hence the smell.

The 8227 amplifier is specified at 12V DC but has a maximum of 15 to 20 Volts depending on manufacturer.
The supply was before the pass transistor was 20.8V and 19.8 after. I replaced the audio chip but noticed that the 19.8v  was applied even in standby and was also used for the on/off logic Flip Flop CD4013 which is also known to fail and sometimes blow the 10 ohm 1206 resistor in series with pin 14.. Later models increased this resistor to 100 ohm

I could have used 6 or 7 silicon diodes, in series, after the bridge rectifiers to "loose" 4 to 5 volts but these are bulky and would have to dissipate upto 5 watts.

I therefore looked for a high speed switching regulator to place between the power supply board and the radio. The small mp1584 buck 1.5MHz regulator boards on Ebay seemed small enough to place on edge next to the main smoothing capacitor on the ps board, They can source 3 amps, will tolerate upto 28 volt input and the output is fully adjustable.

I drilled two holes in the radio power supply board next to the smoothing capacitor and cut the Positive track between the bridge rectifier output and the two Schottky  blocking diodes used to allow battery operation.
I then installed a wire from the new regulator + output to the blocking diode input and connected the supply ground and positive through the board to the regulator inputs.

This all worked brilliantly (no interference  on the radio) when set at  12v and was very low cost....

I have since installed these regulators on two more radios that had "On/Off switch failure" which is in fact normally the 4013 flip flop failing and/or the 10 Ohm 1206 resistor next to it,  feeding pin 14, going open.  I used the Chip Quik SMD Removal Kit (Farnell 1850218) to remove them cleanly and solder wick to clean up.
The 4013 from NXP will tolerate up to 15V (Not 19.8v)  but 12v is safer. Measuring the preset pot on the MP1584 regulator after setting would allow you to replace it with a smd resistor to ensure longer life.
« Last Edit: June 27, 2019, 05:39:54 am by rh100605 »
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