sangean: Sangean ATS-909 current consumption and memory backup fault

I got this nice radio in good working order.

Current consumption looks normal to me. 60uA when turned off, 65mA on FM and 68mA on SW at minimum volume, and around 100mA at nearly maximum volume.

However, I realized that it is not able to retain the memory presets nor the clock time as soon as the four dry batteries are removed.

This issue seems to be caused by a common fault of the super capacitors of certain types and age. This one looks kind of dated in terms of packaging when compared to the disc types manufactured these days. 

My radio have date code of 2003, so this 5.5V 0.022F supercap encapsulated in a plastic square box have now about 20 years since it was produced. 

To my capacitor tester, it looks like a open circuit with just parasitic capacitance of few nanofarads. Resistance reading is around 15 MOhm. Trying to charge it with 3V and then reading with a 10MOhm multimeter shows just residual storage volage at around 0.1V. So it lookd dead.

Time to search for a new, encapsulated like the original, supercap, maybe someone still have these as new old stock items.

 

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Hello Mr. Mesquita,

a replacement for this could be a part made be KEMET. FME0H223ZF or FM0H223ZF schould be a good match.

regards
Markus Zluwa

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Thank you for the information, Mr. Markus Zluwa.

Meanwhile I ended up to order some made by KORCHIP from a chinese seller. They had the 5.5V 0.022F DA5R5223 at double price of the 5.5V 0.047F DA5R5473, so I ordered the cheaper with double capacity. I don't think it will be an issue for the radio circuit to charge it, although it may take longer to finish.

Best Regards,

Jose

 

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I have received my order of 0.047F/5.5V supercap replacement parts.

 

Interesting note on the MCU supercap backup supply C329

• Several pictures published by owners, including my own radio unit, shows a 0.022F/5.5V supercap installed at position C329.
• However, the schematic diagram shows a 0.047uF/5.5V in that position. So I went to the larger capacitance, as it was half price of smaller one (eBay seller).

 

Checking the standby current consumption with a good supercap installed

• Test condition
  • Larger sized 0.047F supercap at C329
  • Power supply from 4 x 1.5V batteries

• Initial rush current
  • The 6VDC from the batteries passes thru a RLS4148 series diode D305 and a 220R series resistor R323 to reach C329.
  • A quick calculation for t=0 where C329 impedance is equivalent to zero Ohm, gives me (6V - 0.6V) / 220R = 24.5 mA
    • (​Used 0.6V for forward drop for D305 as seen in the RLS4148 datasheet. ​Being it a nonlinear device, its voltage drop will vary with the current. It is expected to see a lower value, like 0.5V or so, once the supercap is fully charged and the steady standby current is around 65 uA)

• Registered current consumption from the 6V supply

The current consumption of the MCU and additional circuitry while in the standby state is of course part of the reading below, despite being of a much lower magnitude.

The final voltage at the supercap terminals was 5.51V

By turning the radio ON for a minute and then OFF, the standby initial current is around 2.3 mA decreasing at a fast pace to the steady 0.07 mA value registered above under 3 minutes.

 

Testing the MCU supercap backup power supply

I wanted to find out how long would the MCU run on the supercap power supply.

From the 65 uA consumed from the 6V battery while in standby state, at this stage I didn't bother to find out how much is consumed by the MCU circuitry and what the supercap self-discharge current actually is.

So the 6V supply was removed, leaving the MCU clock running on the supercap (Battery indicator blinking).

Amazingly (or maybe not),  the 81230 3V regulator IC305 was able to feed supply on VDD line to keep the MCU running when its input voltage was actually below its nominal 3V output.

 

Partial circuit diagram

(voltage readings taken with the radio in standby state, and the supercap fully charged)

 

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