The Sangean HDR-1 is a HD tabletop radio which supports digital radio & hybrid signals and RDS functions on FM stations. Other features include dual alarm, IR remote control, multicast capability and Program Associated Data service. Overall, a pretty nice unit. But certain productions runs of this radio had a problem. The radio would quit powering up for no apparent reason. Lets see why.
Disassembly is rather straight forward. Unplug all wires from the back of the radio. Remove the FM antenna then remove the mounting nut from the f-connector. The radio is held together by 6 deeply recessed phillips screws accessible from the back. The shaft of the screwdriver should be 3 inches long and thin. After removing the 6 screws, gently pull forward on the front of the radio. There will be multiple wires that need to be unplugged. Gently unplug all the wires from the front of the radio. All the plugs on the front control board are different sizes so matching up the connectors on reassembly will be easy. With all the wires unplugged and the front removed you should be seeing this:
The AC power from the power jack connects directly to the power transformer, no inline fuse. Checking the continuity of the 120 vac input of the power transformer reveals an open circuit. It would appear that the primary of the power transformer has burned up. Now lets skip ahead a bit. We will come back to the transformer.
An overload or short circuit on the secondary of a power transformer is a common cause of transformer failure. From the above picture we can see that there are 3 center-tapped secondary windings on the power transformer. Lets flip this board over and check the rectifier diodes.
The HDR-1 uses a full wave bridge rectifier module to power the audio output stage and 2 diodes each (circled in yellow) to power the 3.3 vdc and 12 vdc regulators. A quick check with an ohmmeter will show one or both of the diodes on the 12 vdc supply shorted. Here is where we find the factory error. All 4 of the circled diodes are part number 1N5817. The 1N5817 is rated at 14 volts rms and 20 volts peak. The ac voltage for the 12 vdc circuit from the transformer is ~15 vac. Whoops! These diodes are operating right at their voltage limit. A small voltage spike, such as switching on an appliance or light, is all that is needed to overload these diodes.
Later production runs used part number 1N4003 diodes (200 volt @ 1 amp) so lets replace all four 1N5817 diodes with the correct 1N4003 part number. Now back to the transformer.
Looking at the primary side of the power transformer we can see the two power input terminals (red dots). There is also a terminal that does not go through the circuit board (yellow dot). This transformer has a internal thermal fuse between In1 and the yellow dot terminal. The primary winding is between the yellow dot terminal and In2. To restore power to the transformer just add an external 1 amp 120 volt fuse between the yellow terminal and In1. The new external 1 amp fuse will actually provide better protection. In case of a short, the old thermal fuse required the transformer to heat up to trip it, the new 1 amp fuse will blow nearly instantly. Of the 19 Sangean HDR-1 radios I have repaired, every one had this problem.
For added protection, I added a 130 vac MOV across the transformer primary. This is optional, but provides cheap overvoltage protection.
While we have this radio open, there are a few inexpensive upgrades we can perform to increase the sound quality.
The 8 pin IC circled in the above picture is a JRC4580D opamp which is used as the audio preamp. The JRC4580D is a rather noisy low end opamp with limited bandwidth. For an upgrade go with a OPA2134 or a NE5532 opamp.
The four capacitors circled above feed audio into the opamp. Ideally they should be bipolar capacitors, but the factory has used polarized capacitors. For replacement use a bipolar capacitor such as the Nichicon Muse ES series with the same rating of 1 uf @ 50 volts. At this point you will notice a greatly improved sound quality as compared to a stock Sangean HDR-1.
The two capacitors in the above picture filter the power to the audio amplifier. If you like your music loud or with a lot of bass then these capacitors could use an upgrade. The factory parts are rated at 3300 uf @ 25 vdc. For an upgrade a pair of 5600 uf @ 25 vdc capacitors works great.
Total cost of parts for repair and upgrade is about $10 US.