troubleshooting help needed for radio!

Discussion in 'General Electronics Chat' started by BillRush5, Feb 14, 2013.

  1. BillRush5

    Thread Starter New Member

    Jan 13, 2013
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    0
    I have an older solid state radio receiver that is powered by 4 D cells. It has a built in battery check meter, so I replaced the batteries as they were showing low. Same result, battery check still showing low despite having a brand new battery set. I do have schematic and service info on the device. It looked to me like the DC ground from the batteries was a separate ground from the metal chassis so, taking a chance, I coupled both grounds together. A zener immediately burned out in the radio and so I replaced it. This time, I put a voltage and current limited power supply in place of the batteries (again using chassis+electrical ground combined as the power supply still showed low batteries if I used only electrical ground). Now if I set the voltage at the normal 6 V input to the radio, but increase the current just enough to get "full-ok" on the battery check meter, the radio is operating normally! Any idea what's going on? I thought maybe shorted or open bypass caps from the electrical ground to chassis ground, but I don't see any such caps within the radio and there aren't any on the schematic. I guess the question is: why won't this radio work when left on the battery ground and will on a greatly current reduced radio+chassis combination ground?

    Thanks in advance for any troubleshooting help!

    Bill
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    What kind of batteries are you using? The radio was probably designed for dry cells and if you are using rechargeable cells it will read low. Dry cell =1.5V/cell while rechargeable = 1.2V/cell so instead of supply being 6.0V it is only 4.8V
     
  3. BillRush5

    Thread Starter New Member

    Jan 13, 2013
    24
    0
    I'm using normal, alkaline D cells. Also, I forgot to mention this previously, I bypassed the original battery holder in the radio with a brand new one with brand new alkaline batteries, but same result when trying to use electrical ground.
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
    348
    Have you measured the voltage of the batteries when trying to operate the radio? If you can operate with an external supply set at 6V and the battery life meter reads good, it should run on batteries IF they are holding a 6V output. If new batteries are loading down, you may be on the right track of looking at capacitors. Any chance you could post the schematic? If so, mark the zener that lost its magic smoke.
     
  5. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    give me the make and model of the radio and ill dig out the service manual for it and we can have a look at how things are connected up .....
     
  6. BillRush5

    Thread Starter New Member

    Jan 13, 2013
    24
    0
    Ok, after a lot of poking around tonight, I thought I had the problem solved but definitely not the case. Attached is the schematic. The service manual seems to point to the power supply as the source of the trouble, so I tried to check out the components. I replaced Q7 and Q8 first, but really didn't effect output. Once I replaced the pass transistor, Q6, the unit seemed to behave normally on batteries, full voltage, but only for a while then 0 voltage. I believe the pass transistor may be burned out again. Not sure what could be causing this. Hope someone can help as I'm about at my wits end.
     
  7. BillRush5

    Thread Starter New Member

    Jan 13, 2013
    24
    0
    After I put everything back together, everything went dead. I had replaced pass transistor Q6 and everything came alive again on battery power, but once back together everything died. It's possible there may be a short to chassis ground that immediately burns out Q6. Don't know, but going to investigate today. I don't even know if this is the right direction to go with this circuit. A power supply with pass transistor and zener regulator should be easy, but I am sure having a lot of difficulties. I wish I could isolate just the supply from the rest of the circuit. How would I do this? Maybe cut the traces after C17, but I'm not sure.
     
  8. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    can you give make and model so i can get a better copy of the schematics used in your radio....with this info a repair will be pretty easy.....
     
  9. BillRush5

    Thread Starter New Member

    Jan 13, 2013
    24
    0
    Thanks, sheldons, for the offer, but I think I've solved the problem. Since the unit works on a positive ground, which took me a while to realize, it was not the right thing to do when trying to bond the electrical and chassis grounds. That simple fact, plus the fact that there was a short somewhere within the battery section of the unit that kept arcing to chassis ground meant completely removing and changing to new insulated battery holders. Since that has been done, I've had no more problems with burnouts.

    Bill
     
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