Help fix my Dad's Sony ICF-SW7600

Thread Starter

Clapos

Joined Apr 15, 2019
6
Hello,

Kinda terrifying asking for help with limited knowledge. Kindly bear with me, for I am not an engineer. There's no service available to help me fix the radio either..

Uploaded is the uncolored service manual (since the colored one is over 20 megabytes) and the specifications of a component I replaced.

I found the radio among Dad's belongings, and after digging I found out I need to change all electrolytic capacitors (smd) that have been leaking for ages inside the radio. The capacitors caused some local damage to the pads/traces but I managed to solve the lifted pads and replaced all of the capacitors.
There are no shorts and all of them have been tested with continuity tests

Please don't judge the brands of the caps. It's what I could find at the moment. All checked for good capacitance!


However, the device worked properly for an hour before it lost the FM signal lock (only interested in FM). I could only hear static of the FM no-lock sound.
I started using a basic multimeter to check for more lifted pads but instead I found a dead chip inductor in the DC/DC converter. Some of the leakage from the adjacent capacitor might have migrated inside.

From my understanding the DC/DC converter, converts the 3 volt supply to 14 volts.


After installing the relatively bigger chip inductor (Marked 221k = 220 uH), the device started working again, however there was this "squeal/squeak" noise in the background. An annoying sound that says something is wrong. Suddenly the device started to fail again, this time losing signal, FM normal sound, all sound and only the squeak is left..

Every time I leave it for a day and power it up again, I hear normal FM (no lock) sound that stays for a few minutes before it's gone with the squeaks coming back instead..

The current measurements:

-The DC/DC is now getting 2.6-2.7 volts (instead of a solid 3), which is the input to the chip inductor that I replaced
-The output on the chip inductor is 1.4 - 1.6 volts (Losing so much voltage over a few ohms..)
-Resistance across the chip inductor (the replacement) = 8 ohms
-Resistance across the chip inductor still on board (upper left corner of the pcb)= 17 ohms
-14 volts supply = 0 v. on FM (0.66 volts and 1.6 volts for xxx0 and xxx5 on shortwave)


Something is consuming so much current. I couldn't trace the problem sadly. I understand the 2 most important capacitors for FM to work are C69 and C127.
Q41 transistor on the back of the pcb supplies C69 (measured voltages as the pic)

The pad/trace repair I've done and tested with continuity tests (and for any shorts)






Which component could be causing the problem? Anything i should measure (Voltages, resistance etc..)?
 

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Dodgydave

Joined Jun 22, 2012
10,325
Q31 and Q32 are probably faulty. I would remove the dc/dc converter and replace it with a better one.
 
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Thread Starter

Clapos

Joined Apr 15, 2019
6
Q31 and Q32 are probably faulty. I would remove tge dcd converyer and replace it with a better one.
Alright.. I ordered a few transistors of the same specs (L6) and I am waiting on delivery. If I may ask, the DC/DC converter is a part of the pcb, do u mean replace everything that's on it?
I thought I should change the transformer (T13) too but I don't really know its specifications. It just has a "Sony part number" and I'd need to order that from abroad I guess (which would be veryy expensive..)

The 3 circled pins are grounded, don't know if that's normal or not..


I'll replace these 2 transistors once I get them and I'll update it here to let you know. Hoping it's just these..
Thank you for taking the time to help!
 

Dodgydave

Joined Jun 22, 2012
10,325
There is another thread with this radio, having the same problem with the DC converter getting hot and taking too much current, they have replaced it with a ready made converter.
 

Thread Starter

Clapos

Joined Apr 15, 2019
6
There is another thread with this radio, having the same problem with the DC converter getting hot and taking too much current, they have replaced it with a ready made converter.
Realllyy?? Do you mind sharing the link? I searched but couldn't find it..
Unless it's my post where I am asking if I can use a module for testing purposes ^^'
 
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