Debug faulty radio :)

Discussion in 'Wireless & RF Design' started by DenisM, Apr 23, 2012.

  1. DenisM

    Thread Starter New Member

    Apr 23, 2012
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    Hi,

    I'm retired now and have an interest in electronics. Lately I've turned to radio as an area to explore. It's always been a mystery to me. Digital I've been doing since the seventies :)

    I've think I've now got a good understanding of superhets.

    I've got a panasonic portable radio AM/FM as a project to explore. Picked up for less than $10.00. AM does not work, FM is perfect.

    I would like to get it going on all cylinders :)

    I have the service sheet and it shows the radio is built around an IC TA8100N. However since most of the action takes place in the IC, I'm having a hard job trying to figure out how to disgnose the problem. The IC has 24 pins at pitch 1.78mm....not socketted, tricky to remove. It may not be the IC that's faulty of course !

    Attached schematic..

    L1 is the antenna coil. L2 is the oscillator, T3, T4 the IF tfs. I suspect that some of the internal gain on the AM side is damaged within the IC.

    All the (AM) voltage readings at the IC pins agree with the service sheet. I have a dual trace scope, but to use it intelligently for this project ??

    Is anybody familiar with this IC ? And would care to suggest a strategy to locate the problem - in the interests of my continuing education :)
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    there is not much to go on since everything is pretty much integrated. if the voltage agree with datasheet, probably only thing left is to try to tune it.

    normally one would need signal generator set to 455kHz and modulated with audio tone like 1kHz. then injecting this signal (through small capacitor) into pin16 could be used to adjust T4 for maximum output. then go stage back and inject into pin22 and adjust T3. this completes IF stage adjustment.

    if suitable generator is not available, one could use signal from another working AM radio that is tuned to some radio station.

    note, for adjustment it is best to use plastic tool and be gentle as IF coils easily snap (very fragile inside, be careful when bringing the cap down, getting cap out against the top of the can is not as delicate but inappropriate force can cause cap to split in two along slot for adjustment tool). metal tool affects tuning of the coils so when you move tool away coil is out of adjustment.

    next step is to adjust L2 so that local oscillator produces about
    1650kHz+455kHz=2105kHz when dial is at maximum (or at 1650kHz).

    check if the frequency changes with turning of dial driving variable capacitor (selecting stations). it should get to lower end of the range or about 560kHz + 455kHz = 1015kHz.

    If the L1 coil is still in place (should be fixed by drop of paint or wax so it does not slide out of it's original position on ferrite rod), all it is left is to try it out and check if any stations can be picked up.

    one may need to adjust trimmers on the back side of variable capacitor to better match up range with frequency scale. be careful though and make sure that you know exactly which one is for what. mark any position before attempting to move it.
     
  3. DenisM

    Thread Starter New Member

    Apr 23, 2012
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    Thanks for your help :).

    I will try as you suggest.

    do you mena by placing it next to the radio or patching a signal physically ?

    pin 22 ? Is that not just the Vcc supply line ? Can you inject through it ?

    D.
     
  4. DenisM

    Thread Starter New Member

    Apr 23, 2012
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    0
    Hi,

    I did build an AM transmitter (small scale thing :) to check if it could be picked up by a good radio set to AM.

    http://www.youtube.com/watch?feature=player_embedded&v=3I_e7gIyfQg#!

    It did - but at a very low volume and was tunable across the band. It appeared at a few places due to harmonics. approx 600 , 900, 1200 on a working AM radio.

    But on the faulty one, while detected at 600 only, it was very low displaying no gain. Thus I'm afraid it's a loss of gain within the IC :(.

    We do not have a local MW station, thus the need for a Tx ti test. However at night I can usually get MW stations from Uk and France on a working radio.

    It seems to me unlikely that any external component could be at fault.....
    What if an external bypass capacitor had gone short - would it ground the AM signal ? But surely it would interfere with the voltage readings on the IC ?

    D.
     
    Last edited: Apr 23, 2012
  5. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    i guess you don't have the generator...

    no, not placing next to it, it would need to be physically connected. for example tie commons of both devices together, then use another wire and small cap as a "probe". one side of the probe can be attached to low impedance of the healthy radio (maybe right before detector). this way healthy tradio will not be loaded much by the "probe" and whatever else probe connects to. actually you may need to attenuate it too (RF output from healthy radio to cap, potentiometer, and another cap to a piece of wire acting as touch probe).

    you are right, that was the typo, should be pin24 (AM input).
    also the other it can be 19 too (in general input of IF stage is more sensitive to injected signal).

    at any rate, if you construct little test probe as described, you are free to probe anywhere on the test subject without fear, there is no risk of damaging anything (actually i should take this back, while any damage is unlikely, i have no experience with this particular IC so theoretically, risk is always there). can you post circuit of your transmitter? if you used something that relies on spark gap then yes, damage is possible. if you did some small battery operated circuit, that would be very unlikely even if you made sort of connections as mentioned here as test probe.

    it need to be point before the tuned LC circuit or there will be no way to tell the difference while trying to make any adjustment. general idea is to work backwards and setup one stage at a time, then move to previous one.

    note that AGC will work against you, as you tune it better, signal is stronger and AGC will try to compensate by reducing gain. this is meant to preserve signal shape or too strong stations would be amplified too much (clipping destroys the envelope information).

    good luck
     
    Last edited: Apr 23, 2012
  6. DenisM

    Thread Starter New Member

    Apr 23, 2012
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    0
    Thanks again.

    It's in the youtube video link on last post = time point 5.41 should give it to you.


    can you spot any obvious way to disable AGC ? Could pin 14 be a control pin ? Neg =disable. Pos = enable ? The data sheet does give the internal circuitry, but is not yet intelligible to me :(

    D.
     
  7. DenisM

    Thread Starter New Member

    Apr 23, 2012
    20
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    This is the tx. It's battery - 9v.

    S.
     
  8. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    naaah,

    forget it man, this thing is not going to hurt a fly. it certainly didn't do anything to put your radio out of commission. in fact this looks like a nice base for a signal injector, tune it to 455kHz, bring audio (preferably sine with fixed amplitude like this
    http://www.freesound.org/people/klangfabrik/sounds/28636/
    ) and this should just do it.
    instead of antenna connect potentiometer and one more cap before probe (just to prevent any DC biasing to test circuit). the pot will allow you to attenuate signal.
     
  9. DenisM

    Thread Starter New Member

    Apr 23, 2012
    20
    0
    Thanks again.

    I will try as you suggest over the next few days. Good advice :)

    I have a feeling I'll be getting a different - all transistor - radio to continue my explorations :)


    D
     
  10. panic mode

    Senior Member

    Oct 10, 2011
    1,320
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  11. DenisM

    Thread Starter New Member

    Apr 23, 2012
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    Hi,

    I set up a signal injection as suggested. (In on pin 16) Adjusting L4 did give a slight increase in o/p. In on pin 22 and adjusting L3 gave no alteration at all. Signal was very low and adjusting had no effect on the output.

    As this stage I'm accepting I've started too far up the ladder and missed a few rungs :(

    I will get the elenco kit and start with basic steps. Maybe I'll come back to this later.

    Thanks for al you input. You've been very helpful.

    Regards

    D.
     
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