SOLVED!! Old radio repair, cant find any obvious fault.

Discussion in 'Technical Repair' started by smyth602, Jan 8, 2017.

  1. MrChips

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    Oct 2, 2009
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    No. If you left the 10μF cap on pin-9 I can see what you think is pin-9.
    I am also looking for shorts or bad solder joints around the IC - as well as trying to reverse engineer the circuitry.

    The way the orange, black and red wires were connected seemed odd to me.
    Pin-8 should have been connected to one end of the pot (clock-wise for max volume)
    Pin-9 should have been connected to the wiper (centre connection) of the pot.
    The other end of the pot should have been connected to GND.
     
  2. smyth602

    Thread Starter Member

    Jan 8, 2017
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    Do you mean by overheating the board?
     
  3. AlbertHall

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    Jun 4, 2014
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    Overheating and pulling/pushing when not all joints have solder completely removed thus applying stress to the tracks, risking pulling tracks off the board.
     
  4. MrChips

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    Older single-sided PCB designs such as yours with large holes are easier to unsolder than newer microcontroller boards.
    I would go with a solder sucker and solder wick.
    Once you can see all IC pins clear from the solder pads you will be able to remove the IC without damage.

    Make sure you do not spend too much time heating the solder pad otherwise you will lift the pad and track off the board.
     
  5. smyth602

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  6. AlbertHall

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    Consider which it is more important to avoid damaging: the board or the IC.
    Solder suckers, because of the mechanical 'thunk', can themselves remove pads.
    Solder wick is good if you are experienced at using it and can keep the heating to a minimum.
    So, the lowest risk is to snip out the IC and bin it and preserve the PCB pads.
     
  7. MrChips

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    You will need to get some solder wick and clean up those stray bits of solder between pins 6 & 7
    and pins 7 & 8.
    Also at the AM-FM switch.
    You might as well wait to do this when you are ready to install the new IC.
     
  8. smyth602

    Thread Starter Member

    Jan 8, 2017
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    The red wire was not connected directly to pin 9. It was connected higher up the board. The black wire was connected to ground, but also to the middle terminal of the pot. The orange went from pin 8 to one end of the pot.

    Interestingly, I noticed while having a poke about with the capacitor probe that if I touch it to pin 4, the output from the speaker becomes significantly more loud and clear, albeit with a loud overlaid 50Hz hum from my ancient wall wart. Faint but clear sound can be found on pins 1, 2, 6 and 7. Loud(er) and hum-less can be found on pins 9 and 12.

    This is with the red wire not connected and the orange and black now in the correct positions. Unfortunately I can't do any more tonight as my soldering iron has packed in.......
     
  9. smyth602

    Thread Starter Member

    Jan 8, 2017
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    I noticed them and sorted them. The tip of the soldering iron cleaned them away easily with just a touch.
     
  10. smyth602

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    Jan 8, 2017
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    I should also add that all the sound I was finding in various pins was found with my fingers on the antenna core. This really does seem to drastically increase reception.
     
  11. MrChips

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    That is because you (your body) is acting as an aerial and helping to pick up the RF signal.

    DC power from wall warts are not well smoothed and supply a lot of 50Hz line frequency ripple, hence the hum.

    Don't worry too much about testing pins 1-7. These are associated with the radio section. We know this section is working.
    What is amiss is the audio amp, input at pin-9, output at pin-12.
     
  12. smyth602

    Thread Starter Member

    Jan 8, 2017
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    Good news everybody! The replacement chip arrived today so I will have a go at replacing it tonight. My soldering experience has never covered something like this before so wish me luck.
     
  13. smyth602

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    Ugh....... :( IMG_20170117_192421.jpg
     
  14. Alec_t

    AAC Fanatic!

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    Was that fallen-off pad just linking 3 wires?
     
  15. smyth602

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    Pin 9 of the IC, one leg of a capacitor and the orange wire from the volume pot.
     
  16. AlbertHall

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    That's not too bad to fix. Get some thin wire, perhaps one strand of a multi-strand wire, and wrap it around the three component pins and solder. If you've got something to hold it in place so much the better - hot melt glue is favourite.
     
    smyth602 likes this.
  17. smyth602

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    Jan 8, 2017
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    Success!! With the IC replaced and the pot wires where you suggested, it is working again :) I reattached the speaker and fm aerial along with a new PP3 connector too.

    The FM/AM switch seemed to be the source of a lot of crackle if touched, and if knocked even slightly then it lost the station altogether, but a blast with PCB cleaner into the body of the switch seems to have cleaned up the contacts and sorted that :)

    I'd be keen to maybe upgrade the speaker as the one on it currently is very tinny, but at least it works!
     
  18. MrChips

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    Good work and congratulations!

    Don't forget to get out the voltmeter and record the voltages on all the pins of the IC for future reference.
     
  19. smyth602

    Thread Starter Member

    Jan 8, 2017
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    I will do. Thanks ks for all the help folks :)
     
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