Roberts DAB radio ECOLOGIC4 / RD21 / RD-21 no sound / will not switch on

[rh100605]

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 when set at  12v and was very low cost....  However be careful, the weaker stations were affected. Screening the MP1584 module maybe necessary in weak signal areas. The other 3.3v switching regulator on the main board is inside a soldered can.

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.

Note that some later models increase the 10 ohm resistor to 100ohm which reduces the voltage on the 4013. However the audio chip is still at maximum.

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.

[nfmax]

I have one of these radios on my bench, with the same fault. My problem is, I can't see how to get the top panel off and/or the receiver PWA off the top panel. Removing the screws that hold the PWA to the top panel is possible, but then there isn't enough space to withdraw the volume & tone controls out of the top panel to remove the PWA - there is a mounting pillar in the way. I think I have found one of the clips that hole the top panel to the case front (marked with the screwdriver in the picture), but that doesn't seem to free up that end of the top panel - could it be glued? And anyway, I can barely see the corresponding clip at the other end, let alone reach it.

Is there a magic trick I should know about, or do I resort to Brute Force & Ignorance?

[coromonadalix]

maybe the small pcb at 90 degree soldered with 2 big blobs  is holding it ?

[rh100605]

NFMAX
I happen to have an Ecologic4 here and was bothered by the way I remembered replacing the 4013 chips.
So I tried again.
I removed the back by removing the 5 long screws then the 3 screws across the top ( centre one is a bolt) and the screw in the battery bay
I then removed all the knobs (4) and took out the six small screws holding the volume and tuning knob surrounds. Then I removed both the surrounds.

Inside the radio, above the speakers, are two plastic cylinders with a screw in the centre. Undo the screw and turn the cylinder thru 90 degrees. This will release two capture tabs on the front panel.
To separate the top panel,  push back the 4 toothed plastic tabs along the inside top of  the radio. Then work a plastic spacer along the gap holding each tab down as you approach it.

The top panel fascia separates from the radio
Then three screws are exposed which, when removed, allow the whole PCB assembly to be moved back !

[nfmax]

I cannot identify the 'two plastic cylinders with a screw in the centre' that you mention. Can you point them out on the picture, or confirm if what I have is a different moulding? What type of screw head & finish do they have? Do you have the row of 3 BZP Pozi's at the bottom - they look as if they might release the front grille

[rh100605]

Max
I your picture you can see a threaded post above the left hand speaker, this should have a locking part over it that is held on by a screw into the post.
I cannot see the right side as wires obscure it. Someone has removed the cylindrical plastic locks I think. IMG_2028.JPG
The last picture shows a lock turned through 90 degrees

I do not think there is a need to undo the 3 screws at the bottom.

Take off all four knobs by pulling, remove the 6 screws holding the surrounds of the tuning and volume controls. Then remove the chrome plastic surrounds
Your photo clearly shows 4 push thru tabs along the top, push on a tab and the front fascia will lift. Using a thin blade inserted in the groove around the top/control panel outside ( I started near the volume control) lift the plastic, put in a spacer to hold it up and work towards one of the 4 tabs. Push each tab down in turn.
The top fascia will come off as shown in my picture.

[nfmax]

That did the trick - well, mostly. The tab by the volume control just would not bend enough to release & ended up breaking. But the top is off, the receiver PWA is out, and the scene of the crime is accessible!

R4 is marked '100', i.e. 10Ω, but is clearly burnt and measures 54.5Ω. The HEF4013B measures about 1.5Ω Vdd (+ve) to Vss. I will replace the IC, and change R4 for a series R-LED combination to drop the supply voltage a bit, there's plenty of room. I need to trace out the circuit that drives the power switching first, to check for issues.

The 2SD882 power transistor on the amplifier PWA doesn't appear to be the switch, it looks like a capacity multiplier (to reduce supply ripple) but quite a crude one (no base stopper?). If so, a suitable resistor from base to ground would reduce the peak voltage to the audio amplifier.

I might possibly stick a resistor across the 18V smoothing capacitor to help limit the peak volts a bit, but I can experiment with that once it's all back together, using the Variac (actually mine is a Regavolt). 'Power Off' draw from the supply I measured at 0.87W, so we could sacrifice perhaps another 0.5W in a good cause, if it's effective.

Enough for tonight though, it's been a long, hard day!