Icom 756 Pro2 stops receiving when warmed up

Discussion in 'Radio Circuits, Repair & Performance' started by WA4MNT, Apr 28, 2019.

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  1. KL7SG

    KL7SG XML Subscriber QRZ Page

    Yes, be sure that you are not inadvertently heating parts beyond Q25, D22 and D23.
    The voltages you show would lead me to believe Q25 might be the problem. The 0.03 volts at the collector of Q25 is very low causing me to think the switching transistor Q25 is shorted collector to emitter or Q25 is not getting the proper voltage from the emitter connection at Q13. Normally, one would expect a saturated collector voltage of greater than 1.0 volts (Q25 transistor switched on) above ground (emitter). I am assuming that Q25 provides the ground connection for Q13 to switch the relay to the "on" state. But, the schematic doesn't really show what is happening with Q13 (Q13 is drawn as an op-amp). You should look at the voltage at R153 (Q13 side) to verify the voltage from Q13 is there. If Q25 has shorted, the voltage drop across R153 will be on the order of several volts. If there is no voltage on the Q13 side of R153, then look to Q13 as the problem. Also, there are multiple connections to the emitter of Q13 ... those also could be causing your problem.
     
  2. KL7SG

    KL7SG XML Subscriber QRZ Page

    Also, if you have time, can you provide the voltages at Q13 and Q25 when the radio is functioning normally and then when it is not?
     
  3. WA4MNT

    WA4MNT Ham Member QRZ Page

    The relay does fire when the receiver goes dead. Checking the "red" voltages from the schematic, on the board, Q25-C , 0.03V is actually 1.28V, Q25-B, 0.65V is actually 1.28V, and Q12-B 4.6V is actually 1.07V
     
  4. WA4MNT

    WA4MNT Ham Member QRZ Page

    Also, Q12 only needs to warm up 20-30 deg. F to cause the receiver to mute, and quickly recovers when cools. I replaced Q12 ans Q13. Same voltages and muting. No improvement.
     
  5. KL7SG

    KL7SG XML Subscriber QRZ Page

    The base and collector voltages on Q25 should not be equal. This suggests a base to collector short when Q25 is hot. The Red 4.6 volts is a control voltage, quick circuit analysis suggests that the Q25 base voltage should be around 0.69 volts when the relay is "on". You should check the Red 4.6 volts area (left side of R154) when the relay is "on" during the fault condition. For a normal relay "off" state, it should be around zero. So, if the left side R154 voltage is less than 1.28 volts, during a fault, you should look at replacing Q25. If it is much greater than 1.28 volts (expect 4.6 volts), it is being commanded to the "on" state and the problem is likely else where.
     
  6. WA4MNT

    WA4MNT Ham Member QRZ Page

    The voltage at Q12-B does not change when it goes mute.
     
  7. KL7SG

    KL7SG XML Subscriber QRZ Page

    How about the left side of R154? I am pretty confident that your original thoughts on Q25 and its related components was correct. If the voltage on the left side of R154 is low ( less than 1.5 "ish" volts then it probably Q25. It is hard to trouble shoot by texting.
     
  8. WA4MNT

    WA4MNT Ham Member QRZ Page

    The left side of R154 is the same as Q12-B, 1.07V, and does not change from good, to mute.
     
  9. WA4MNT

    WA4MNT Ham Member QRZ Page

    It's just a switch, correct? I don't suppose a 2n3904 replace it?
     
  10. WA4MNT

    WA4MNT Ham Member QRZ Page

    I guess I could pull it out and check it with my transistor checker.
     

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