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Roberts Unologic DAB/FM rechargeable radio power problems

andypandyplod

New Member
Hi,
I am trying to repair a Roberts Unologic radio,which when portable uses 4 X D cells,either alkaline or rechargeable which can be recharged by the roberts 7.5 volt 0.8 amp power supply which was supplied with the radio when new. The radio has stopped working correctly. I have checked battery connections and the power supply which were ok.When I measured at the 7.5v input I measured 56 ohms at all times,which to me would indicate a partial short. When the radio is connected to a 7.5v the display is overbright with all digits boxed out.When I supply the radio with a separate 6v supply it works fine including the lcd display.I assume regulation on the PCB has a problem.When I measure the current taken when working it shows about 350mA.This I believe is 200mA being used by the radio(normal) ,and 150 mA in the shorted component or components.
I cannot find any information of this fault,or circuit diagram (or schematic) on the Internet.The fault is not made easier by the PCB being double sided,and Roberts for some reason have put small tin screening cans over some components! I have never seen this on any other make of DAB radio. Any Help would be gratefully received please?
 

Diver300

Well-Known Member
Most Helpful Member
Photos would help.

Is the power supply a switch-mode one? Switchmode ones are light and small. Linear ones are heavy. What voltage does the power supply put out.

The metal screening cans would be quite common for a DAB radio. They may cover the power supply to stop it interfering with the receiver.

What current does the radio take when running on batteries, and does it work more normally?

The resistance measured at the input isn't a useful measurement, because the input won't follow Ohms law, and the current taken at the low voltage the the resistance meter produces won't relate at all well to the current taken at the normal voltage.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The resistance measured at the input isn't a useful measurement, because the input won't follow Ohms law, and the current taken at the low voltage the the resistance meter produces won't relate at all well to the current taken at the normal voltage.
Exactly - it's not something a service engineer would ever do, it's a completely useless 'test' - as it tells you nothing.

However, without a circuit, it makes life fairly tricky - and close to impossible to help remotely.

When I left the TV/Radio trade a few years ago it had already got to the stage that it was next to impossible to get Roberts service data any more.

Previously Roberts Radio official spares and service was done by a relatively small TV dealership - they were excellent, and extremely helpful - any problems and you could ring them and speak to someone who actually knew something. They could often advice solutions where spares were no longer available.

Then it changed to someone else, and went completely useless :D

I've just checked for the Unilogic, there are only SEVEN spares listed as available - and basically 'cosmetic' items - such as an aerial, knob, or rubber foot.
 

andypandyplod

New Member
Thank you for replying.The Power supply came with the radio.It is a Roberts 7.5v 0.8A switched mode,and is supplying the correct voltage. The radio works fine on 4 Alkaline D cells,until the total voltage drops below 6 volts where the radio stops working,and the display is just blank yellow.If 4 NIMH cells are fitted,and the Alkaline/REchargeable switch is switched to rechargeable, the display is still blank,the batteriescharge about 300mA..This tells me the circuit which converts the 4.8 volts (4 NIMH cells) to 6 volts is not working. I have attached some photos.The battery charge ic is a SMD MC33340.I have removed the battery charging regulator 2SB1143 in these photos.THis reduces the current taken by the radio.
 

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