I have one of these DTT900 that began failing a few weeks ago - shuts off for no apparent reason and would not come back on unless I unplugged from the wall (power button did not work). It was so bad that it would not stay on for more than 2 minutes before shutting off again.
I have had this for about 4+ years - constantly on.
I managed to repair it by replacing 4 capacitors in the power supply
C121 and C123 1000uF, 10/16 Volt - replaced with 1000uF, 16 volt
C122 and C124 330 uF, 10 volt - replaced with 470uF, 25 volt
So far (2 days constant on) it is working great.
To begin, I took off the cover and inspected the boards - no obvious heat or other damage.
I then measured the power supply voltages at the connector P101.
Pin 1: 5.4 VDC (marked as 5.4VA )
Pin 2: GND
Pin 3: 2.9 VDC (marked as 3.2VA )
Pin 4: 2.2 VDC (marked as 2.4VA )
Pin 5: GND
Pin 6: 2.2 VDC (marked as 2.4VA )
Pin 7: GND
Pin 8: 0.4 VDC (marked as SMART)
So the voltages were lower than marked - suspicious.
I then removed and mapped out the connector on the power supply PCB (printed circuit board) and found:
Pins 4 and 6 are the same on the PCB and are fed thru a diode (HER202)
The input to this diode is the Pin 3 voltage.
Pin 8 goes nowhere on PCB (a wire jumper is missing)
So I then grabbed another bench power supply I had with 5 and 3.3 volt (nominal) outputs.
I adjusted this supply to 5.4 V and 3.2 V to match the DTT900.
I then used alligator clip jumpers into 3 points on the bad power supply:
5.4 volt (after rectifier diode)
3.2 volt (after rectifier diode, which then feeds the 2.4 volt thru the HER202 diode)
Jumpering in after the rectifier diodes prevented any 'back feed' of power to the other power supply circuits.
I powered on my bench supply (the DTT power supply was unplugged) and the converter worked - stayed on for several minutes and the voltages were correct:
Pin 1: 5.4VDC when ON, 5.6 VDC when OFF
Pin 2: 3.2VDC when ON, 3.4 VDC when OFF
Pins 4/6: 2.4VDC when ON, 2.6 VDC when OFF
I also measured current draw:
5.4 volts: 0.38A ON, 0.08A OFF
3.2 Volts: 0.47A ON, 0.03A OFF (this was total current for both 3.2 and 2.4 volt feeds)
I took a guess that the associated electrolytic capacitors on the power supply were bad.
There are 7 of them, but I only targeted the 4 on the output side:
C121 1000uF, 16 V
C123 1000uF, 10 V
C122 330uF, 10V
C124 330uF, 10V
If this repair doesn't hold, I will need to try replacing the other 3 caps:
C103 47uF, 200V
C108 1uF, 50V
C109 33uF, 25V
(I didn't have these caps in my parts stash, so that is why I held off replacing them.)
Hopefully this may help others with bad DTT900 converters in getting them working again.
If anyone is interested, I could post a picture or two pointing out the parts I replaced.