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Help to diagnose SX-880 dead FM (AM works)

Discussion in 'Tuners' started by elnaldo, May 12, 2017.

  1. elnaldo

    elnaldo Addicted Member

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    Buenos Aires GMT-3
  2. elnaldo

    elnaldo Addicted Member

    Messages:
    6,376
    Location:
    Buenos Aires GMT-3
    Well, after 2 months with this unit, I'm giving up.

    Restored the power supply, what was causing a loud HUM, but the FM stays silent. Replaced Q2, Q3 and Q4, not Q1, and replaced the PA3001 without changes in the results. I didn't replace the 1st dual gate FET.

    I can hear at very low volume the AM when I turn the tuning capacitor. I don't know if this mean something.

    I'm starting to assemble the unit and do some last tests while assembling, but I think it's a lost cause, I can't fix it with the tools I have.
     
  3. rcs16

    rcs16 Super Member

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    2,766
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    Baldwin, Ontario, Canada
    Yes it can be difficult to repair a tuner without the proper test equipment.
    I have suggested in the past that you have to isolate if it is the FM front end or the IF and detector that has the problem. You can use another tuners front end to supply another tuners IF and vise versa to determine where the problem lies. The PA3001 is the IF system, you should be able to center the center tuning meter with no IF input.
     
  4. elnaldo

    elnaldo Addicted Member

    Messages:
    6,376
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    Buenos Aires GMT-3
    Well, while assembling to give it back, did some more testing, and attaching the oscilloscope probe to the 1st Fet gate, after the antenna, the FM works! Scope ON or OFF, the FM tunes in both cases. I can hear and tune different stations. So the circuit works, it's something before the 1st FET (antenna amplifier) or at the tuning capacitor itself...

    (oscilloscope probe connected to the Fet gate, ground clip to chassis. If I remove the ground clip, I hear just HUM)

    I'm replacing the 1st 22pF cap just in case.
     

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  5. elnaldo

    elnaldo Addicted Member

    Messages:
    6,376
    Location:
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    Another good new: I'm listening to FM ! with the help of a good antenna ... Without a good antenna, nothing, just silence. It seems my test with the scope probe, the probe was doing the antenna job. So the circuit seems OK.

    The problem: The FM sounds like the "FM muting" is always ON, so I don't hear any hiss between stations. Also, the FM does only MONO, no stereo at all.

    Does this point to a fault at the MPX decoder? It's the PA1001, hard to find new, some "used" on sale at a local auction site.
     
  6. Punker X

    Punker X AK Subscriber Subscriber

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    Panic in Detroit
    Until is it properly aligned it's hard to recommend replacing anything.
     
  7. elnaldo

    elnaldo Addicted Member

    Messages:
    6,376
    Location:
    Buenos Aires GMT-3
    Well, replaced the Ic (it was $15, cheap insurance to trust the IC), and no changes.

    I've managed to check-tweak the local oscillator with a very strong 105.7MHz station, but I hear the AM between stations, actually AM sounds louder than some weak FM stations. I think the AM tuner circuit should be powered OFF when the function selector is set to FM.

    Also, the monitor-source switch doesn't cut the FM sound completely, but it works OK in AUX mode. I've cleaned the switches once. I see a diode at the function selector , connected to the AM power rail, (pin 13 on the tuner board) . I suspect some leak on that diode could do weird things. Pin 13 is powered with 13 V even when the selector is set to FM. The selector seems to work OK on that pin (passive continuity test).

    Of course no Stereo by now.
     

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