Above is your service and repair manual, below is the Grundig, service Information, it details may causes and fixes for you to try.
Service information at these two sites below, to assist you to troubleshoot.
Power Supply repair is NOT easy, but here goes. First Without Power do as"Dry" Multimeter Ohms, check of ALL discrete components looking for "Shorts" Check, Diodes, Caps, Resistors. Coils etc.., Check all Transistors, B.C.E. with your meter set on "Diode test" look for "Bad" junctions. Then you check to see if you have power to and on the "other side" of the Bridge Rectifiers/Diodes. if not, replace that. if yes, proceed. Check for the correct Voltages around the Controller Chip. and Check the output of the Chopper Transformer, Check to see if the Waveform around the Controller IC is right. Check ALL voltages are correct & within range.. Now during this you should have found the faulty Bit(s). Replace as necessary. BE CAREFUL. I always work on ANYTHING connected to mains through an Isolation Transformer
Before we can attempt to repair anything at all, there is one thing that one MUST do before anything else is done. Now that is, the making sure that the Power Supply,of the unit, no matter what it is, has the correct Voltages, coming into it, and more importantly, coming OUT of it. This is absolutely imperative, as, IF these Voltages are NOT correct then NOTHING will work, properly at all. So we must measure these voltages with a Volt Meter. All Voltage measurements are made with reference to ground. Meaning we clip, our Black, Minus, Lead onto the Earth, or Chassis, or Negative (-) "Rail" of the Power Supply. Now with the Red lead, we can now, measure these Voltages. Now sometimes, the Voltage of each Output is annotated,or written, onto the PCB, that will indicate the Voltage of that output. However most usually they are NOT, so in this case we MUST have a manual to tell us. Now this manual is called a "Service Manual" or sometimes a "Repair manual" This manual is absolutely vital so much so, that it can be near impossible to do without it. As apart from having "Troubleshooting" step through's, it also has our Voltages. When we measure these Voltages they are 100% EXACT, so IF the Voltage you measure, except for a few rare exceptions, the "Rail" that is NOT at the correct voltage is the one, that the "Fault/Cause" is "ON". Therefore we can now work outwards from the Power Supply, looking for the cause of that problem. Now if the Voltage is Lower than specified, we are invariably looking for a "Short", something has "Faulted" and is "Dragging" the Power Rail down to earth. Now if the "Rail" is "Higher" than the specified Voltage then the fault we are looking for is usually some sort of "Open Circuit" possibly a Diode, or resistor that has changed it's "Value". Now knowing this and knowing that the Power Supply is the FIRST place to start you troubleshooting and repair from. You now have a MUCH better chance of a "Fix". One thing more, DO NOT forget to get your SERVICE MANUAL. for you exact make & model. even if you pay for it, you will be glad you did.
Below is a pay for service manual link..