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Motorola spectra lost audio.

Discussion in 'Radio Circuits, Repair & Performance' started by E75MC, Nov 16, 2019.

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  1. E75MC

    E75MC Ham Member QRZ Page

    Hello from Bosnia and Hercegovina.
    Recently I purchased Motorola spectra A5 (D43KXA model) that worked fine but now it lost audio output.
    The capacitors are changed by previous owner . It was poor job anyway but it worked. After I reprogrammed radio few times it lost audio output. There are no errors or fail messages on display, I see that there is signal present in the receiver ( busy lamp light up) but there is nothing on the speaker. I remember that last time I programed radio, I enabled one of the buttons to control "second speaker" option ( If I remember name of this option correct) .
    After that last programing and turning on the radio, audio was normal but after I switched button for enabling second speaker option, audio was going low and gone totally.
    I tried to change output audio chip TDA 7256 with another one but there is no audio again. (I am not sure is that other good also but owner says that radio is fine)

    So few questions here arise:
    1. How can I check is TDA 7256 audio IC gone? What voltages and where can I expect?
    2. Did I made this issue just by enabling this "Second speaker" option or was it just coincidence?
    3. Maybe the acid from capacitors eventually destroyed some of the connections around the IC?

    Many thanks 73.
     
  2. KD0RCB

    KD0RCB Ham Member QRZ Page

    upload_2019-11-16_17-13-45.jpeg Here is pinout , pins 9 & 11 are the output pins, just google this chip # to see the rest of its data, etc. If you disable the 2nd speaker option in the software, does audio then return to normal? Also, I have found that capacitor acid can certainly destroy copper traces, run new wires over any that appear bad. I guess the easiest and quickest thing to do would be to check with a small external audio amp ( or scope) at pins 11 & 9 to see if you have audio output there when receiving a signal. If not ,check input pins 2& 3 , and B+. Since it "faded" away, it could be the acid damage on board getting worse. Good luck, Randy.
     
  3. E75MC

    E75MC Ham Member QRZ Page

    Hello Randy , many thanks for reply. Well, this TDA7256 that is in the radio is different than datasheet. It has 15 pins but in datasheet it says that it has 11 pins ( like the photo you linked). It seems that TDA 7255 datasheet could be right, at least about pins, so I will hold to this one.
    Yes, after I programed radio with another codeplug problem still remain.
    Unfortunately I dont have the scope so I will have to try another way to see do I have audio at input pins 2 and 3. Like you said, another amplifier could help or maybe headphones, just to verify is there any audio present on the input.
    If there is problem with destroyed traces that will be fun, huh.
    I am a bit short with the time these days so I will write any progress here after I do it.

    Thanks again and hear you soon.
    EDIT: I soldered two wires on pins 2 and 3 of the audio IC and connected it to the headphones. There is no single beep on the input. I dont know is this right way to check is there audio present to the input.
     
    Last edited: Nov 17, 2019
  4. K8AI

    K8AI Ham Member QRZ Page

    upload_2019-11-18_8-52-20.jpeg

    Is this the one?
     
    E75MC likes this.
  5. E75MC

    E75MC Ham Member QRZ Page

    Hi Curt, thanks for reply. Well, it seems like it is. I just saw on the board that it seems right. :)
     
  6. KD0RCB

    KD0RCB Ham Member QRZ Page

    Im assuming there is proper B+ on pin 12? And is that a 7 amp fuse supplying that??? Wow! .. Yes it would be very helpful if you had a small audio tracer to use. I also notice there are a few electrolytic caps in that area, do you have an ESR meter to test them? I use my Sencore LC 61 all the time, to check these types of caps. Pin 11 on your IC must be front- out audio pin. Check any questionable traces with an ohmmeter from point to point, wherever they lead to. Keep pluggin away! Randy.
     
  7. E75MC

    E75MC Ham Member QRZ Page

    Yes, there is 13V on B+ line. That 7A fuse, maybe typo and could be 0.7 A, I don't know. Unfortunately I don't have audio tracer nor ESR meter. Visually I can not see brakes on board. Schematics would be very helpful! Service manual for Spectras that I found on internet, is very poor and there is no schematics and board images are not usable, so if someone have one it would be superb. ( Or at least, part of manual with this area of interest)

    Thanks. :)
     
  8. E75MC

    E75MC Ham Member QRZ Page

    Hello guys. Let me raise this topic again. I really want to repair my Spectra and it is on my work bench. Documentation says that both pins, 11 and 13, of the audio amplifier IC TDA 7256 ,are on the half of the A+ value.
    This means that I should be measure about 6 to 7 V DC on each pin ?
    Well, I don't have those values in my case. I have something like negative 100mV on each.

    Probably dead IC?
    Thanks in advance.
     
  9. W8NGA

    W8NGA Ham Member QRZ Page

    I am not sure on the Spectras, but I know that you cannot ground either of the speaker wires on the GM300 or Maxtrac radios. If you do, it will blow the audio output IC. I speak only from experience.

    Greg
     
  10. E75MC

    E75MC Ham Member QRZ Page

    Hi Greg and thanks for reply. Yes, You are right and I am aware of that. I managed to solve the problem few days ago. It was faulty PNP transistor in the audio enable line, that goes to pin 10 of the TDA 7256.
    Fortunately, it was not audio chip. I had a replacement for that faulty PNP SMD but none for audio IC. :)
     
    KB0MNM likes this.

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