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Puzzling IR remote control problem

Discussion in 'Electronic Repair' started by NeverExpectPowerAlways, Mar 3, 2007.

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  1. I hope I've come to the right place to get some helpful hints.

    I am working on a Mitsubishi TV, model CS-2056R, made in 1988,
    specifically, the IR remote receiver circuitry.

    The remote function for this unit began failing some time ago. At times,
    it would just up and quit working and then just as suddenly, it would
    start working again. At first, there did not seem to be a pattern, but
    after some months, one appeared. The remote would never turn the machine
    on or control it for the first 10 minutes or so, sometimes for much
    longer but mostly only for the first ten minutes. After this "warmup"
    period the remote control function would kick in, but it would die and
    come back to life without hint or warning.

    I finally decided to see if I could fix it. Without going into detail, I
    am 100% sure that the handheld unit is not the problem. After searching
    the web and the newsgroups and spending several hours tracing the traces
    on the tuner board of the TV and taking readings with a multimeter, I
    decided to try changing some of the larger electrolytic caps used to
    filter the AC and anything over 100 microfarads or 50 volts (BTW, I
    don't do this stuff for a living, just a part-time hobby). I also
    changed a resistor that looked like it had failed.

    I also took apart the IR receiver module and changed the three tiny
    electrolytic caps within. About this time the remote function ceased
    entirely. No matter how long I let the unit warm up, the remote would
    not function.

    Today, after more web and newsgroup searching, I decided to buy a four
    dollar Radio Shack IR receiver module and guess what, it worked! I
    mounted the Radio Shack module on the tiny stock IR receiver circuit
    board and soldered the three pins to the appropriate spots, held my
    breath and hit the "power on" button on the remote and the TV turned on.
    I watched an old episode of Perry Mason and then turned it off.

    Now, some four or five hours later, the TV will not respond to the
    remote. The Vcc voltage to the IR receiver module is 5 volts and the
    Radio Shack unit is rated at between 2.4 and 5.5 volts.

    At this point, my next idea is to change all the little electrolytic
    caps and maybe any small transistors that I find in the IR receiver
    circuit. The last step will be to change the 42 pin IC that receives the
    signals from the remote and activates the appropriate circuits. I don't
    think that is the problem, though as everything worked okay when I put
    in the new IR receiver module.

    I have a sneaking suspicion that there may be some AC leaking into the
    IR receiver circuitry, but I really don't even know if that is even a
    valid concept.

    If you have read this far and think you may have some insight, please
    feel free to post it as a reply to the news group.

  2. PeterD

    PeterD Guest

    Suggestions like: if you don't know what you are doing, don't try to
    fix it?
  3. Yeah, let's all never learn anything new.

    Would that have been your advise to Tesla or Edison?

    Let's all just get warm around the lightning fire until it burns out.
  4. ian field

    ian field Guest

    The symptom does have some similarities to an electrolytic with ESR issues,
    if you are going to get into fault finding you really do need to invest in
    an ESR meter such as the Dick Smith ESR meter, changing caps in a random
    manner is more likely to result in damage to PCB tracks or solder bridges
    which would certainly make the fault become permanent.
  5. Thanks, ian

    There WAS a damaged track on the board. I say this because the signal
    voltage between ground and the track that connected the IR receiver to
    the 42 pin IC fluctuated erratically between 2 and 4 volts when the unit
    was plugged in but not powered on. I thought that I had located the
    problem on the track and made a careful repair, but the erratic voltage
    remained. Just this morning I severed the track at both ends and used an
    insulated wire to reconnect. The voltage is now rock solid. The track
    was not being used for any other purpose but to connect the signal from
    the IR receiving module to the IC that apparently controls the switches
    for the various functions that the remote is supposed to control.

    I just measured the signal voltage and it measures +3.7 volts to ground
    with an analog meter and +4.75 to ground with a digital meter. I don't
    understand that. In addition to the signal line, the IR receiver module
    has a +5 volt line and a ground line running to it. The +5 volt line
    measures +5 volts with both meters.

    The unit is still not responding to the remote, however, and I am off to
    radio Shack to buy another $4 IR receiver module to see if that revives
    it again. Perhaps the first one was damaged by the faulty track or it
    was just a faulty unit. The fact that it worked when first installed is
    heartening. It would seem to indicate that the 42 pin IC is not at fault.

    Thanks again,
  6. ian field

    ian field Guest

    The two different meters respond differently to a train of pulses, the
    analogue meter probably gives a reasonable indication of mark-space-ratio if
    you know the value of both ground and Vcc, depending on individual design
    the ADC in some DMMs is confused by pulse trains and can give completely
    nonsensical readings.
  7. I don't know if it makes a difference, but the measurements were taken
    without any signal being sent from the handheld unit.

    Is it common for the signal out line on an IR receiver to have standing
    voltage on it?

  8. ian field

    ian field Guest

    The IR sensor might be picking up the 50/60Hz (actually 100/120 half
    cycles/second) from domestic lighting, but they normally have a 38 or 40kHz
    bandpass filter in addition to an optical filter.
  9. ian,

    I fixed it.

    You hit right on the problem.

    The original IR receiver unit was mounted on a small PCB, 1 1/2" square.
    When I removed the old module and soldered on the new one from Radio
    Shack I may have fried one of the tracks where the pin from the IR
    module poked through. When I went to remove the Radio Shack module to
    try a new one, I noticed that the copper track around the little hole
    was gone. The solder around the pin must have made contact with what was
    left of the track, but it came lose when the unit warmed up.

    I replaced everything with wire and it works again.

    Thanks a ton for the feedback.

    If I ever decide to get serious about repair work, I will look into an
    ESR meter. That and a bit more education.

  10. PeterD

    PeterD Guest

    just shotgun replacing parts is not a learning experience. What it is,
    is a waste of time and money.

    Instead useing proper debugging techniques to determine where the
    fault lies is learning.

    That said, IR remote receivers can be tricky things to work on...
  11. Yukio YANO

    Yukio YANO Guest

    My Flaky Remote Control Experience !!
    In my Motor Home, I was having severe Intermittent problems using my
    Remote Controller driving my Bell Express-Vu Satellite Receiver. It
    turns out that my 12 Volt DC Fluorescent Lighting System would Block the
    Remote Control System. By Shading the RX sensor from the direct
    illumination of the fluorescent fixture,I cleared up the problem .

    I think the 15 KHz 12Volt DC Ballast was modulating the fluorescent Tube !
    Although the Light output from the fluorescent lamp is far from the ~
    900uM IR spectrum I suspect it was still strong enough to overwhelm the
    IR output from the Remote Controller. I have NEVER had any problems
    using this system with my Domestic setup using 60 Cycle ballasted
    fluorescent lighting !!!

    I no longer have access to the Mobile Setup so I cannot use my Scope to
    confirm the frequency of the Fluorescent Fixture or the output of the
    Remote Controller.

    INTERMITTENTS are the bane of troubleshooters !! If you don't understand
    the intermittent, you don't understand the problem !

    Yukio YANO
  12. Replacing $10 worth of 20 year old electrolytic caps is hardly throwing
    money away.

    A $4 Radio Shack IR receiver module solved it the problem.

    Now how did I know to swap that out, clueless as you take me to be.

    Try going to help someone you can impress with your superior knowledge
    and condescension.
  13. It doesn't block it, it over drives it.
  14. Franc Zabkar

    Franc Zabkar Guest

    To be fair, he does go on to say that the IR interference "overwhelms"
    the sensor.

    - Franc Zabkar
  15. Franc Zabkar

    Franc Zabkar Guest

    I have (had?) an intermittent IR fault with a Mitsubishi set of
    slightly earlier vintage. The problem has not recurred since I
    replaced the caps in the IR module.

    Anyway, I just wanted to ask whether your picture tube is still good
    after all these years. The contrast in mine seems sharper than many
    new sets.

    - Franc Zabkar
  16. Why don't you try powering the receiver from a battery and isolating it
    from the sets PS circuitry. I had a Sony that acted similarly and I
    suspect the power supply filters were getting weak and the AC ripple was
    disturbing the receiver circuit. Thankfully I gave the set away and so
    did not get into repairing it. Also be aware when electrolytics fail
    they can dump invisible electrolyte on the board causing nearby,
    electrically unrelated circuitry to act up. You might need to wash the
    PC board.

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
  17. That thought had crossed my mind once or twice. But as it turned out the
    problem was not with the power supply but with the original IR receiver
    module and my poor job of soldering in the replacement unit from Radio
    Shack. The solder made the connection at first but when the set warmed
    up and then cooled down, the expansion and contraction of the solder,
    the trace and one of the pins from the new unit caused the connection
    for that pin to be open. Once I saw that and resoldered all the
    connections for the new unit, the remote control function began working
    like new.
  18. The picture is still fantastic. It's seems as good as new. That is why I
    tried to resurrect the remote control function rather than junk the set.

    I, too, thought that it may be the electrolytic caps and I replaced the
    three that I saw in the IR module plus about 10 or 12 on the main board.
    Past experience and the post linked below led me to try that first:

    In my case, though, a $4 Radio Shack IR receiver module finally did the
  19. Franc Zabkar

    Franc Zabkar Guest

    That's been my experience with the majority of AWA/Mitsubishi sets.
    IMO CRT technology seems to have taken a retrograde step.
    My set uses a "discrete" sensor based on an M51014L IR
    receiver/demodulator. To use a modern sensor, I would have to redesign
    the PCB. Fortunately the IC is still available for around AU$4.

    - Franc Zabkar
  20. I cannot comment on that, except that it seems CRTs are on the way out
    for television reception. I just installed a digital TV tuner in my
    computer and I am amazed at the number and clarity of free digital
    broadcast stations in the San Francisco Bay Area. From what I have seen,
    all new television sets being sold that are capable of receiving digital
    broadcasts are either LCD or plasma or some other non-CRT technology.
    Analog CRT sets are dirt cheap and few people seem to be aware that
    analog broadcasting is scheduled to end in the United States in early 2009.

    The tuner card I bought is more prevalent in Oz then in the US. It's a
    TwinhanDTV ATSC D+A, bought on eBay for $58us delivered. The OZ model
    would be TwinhanDTV DVB-T (D+A), I believe.

    The hardware works fine, but the user interface software for controlling
    the hardware is poorly designed.
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