sony: Repairing SONY ICF-7601 band switching erratic operation

Got this radio yesterday. It do nor power off, erratic band switching, SW2-10 LED allways ON.

I was afraid that the IC2 UPD-65005C-198  (D65005C198 ) Sony Part # 8-759-141-09  was defective, as reported elsewhere in the internet. This is customized NEC gate array with a somewhat complex true table using output latches with power buffers to handle the required current for the different radio sections.

Checking the IC2 pin 20 Vdd shoed 5.3V, about right if we add 0.7V dropping from the D16 diode that is connected to the +6V battery. Also all other IC2 outputs looked to respond as expected, only the voltage levels were incorrect.  Strangely the IC pin 19 was going to high voltage when pressing the power off button, wich looks correct, despite the Q19 transistor not cutting off the power to the IC1 chip.

Q19 was good, also it do not explain the band switching wrong voltage levels.

Then I decided to use the good old technique to sistematically test all components around IC2, and yes, I found diode D16 to be open! So, despite being open, IC2 still manage to get the "correct" power supply voltage. Stange. Then I realize this IC2 is a CMOS chip, so D16 being opened still manage to pass some current acting as a high resitor to the CMOS chip.

After replacing D16 (1SS148) with a more modern 1N4148, all operation was restored.

My guess to the cause of this failure was the use of an external power adapter that applied over-voltage causing the protection Zenner diode D22 to conduct forcing excessive current on the series diode D16 that blew up as a fuse.

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