Kenwood TM-732A few problems.

Discussion in 'Radio Circuits, Repair & Performance' started by E75MC, Aug 3, 2019.

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  1. E75MC

    E75MC Ham Member QRZ Page

    Hello.

    Local friend gave me his TM 732A radio with few problems and after lot of work I repaired display unit, after acid from C8 destroyed many traces.
    Two other problems I still have are these:

    1. Sudden drop of power on 145 MHz!
    On 2m RF power goes to null after few seconds of transmitting, regardless of power level. HI, MID or LOW.
    I was almost sure that it was another S-AV 17 broken traces problem but after several resoldering and
    checking with OHM meter I am not sure that it is hybrid problem. Everything seems fine. When that power
    drop occurs, voltage on PIN 2 of S AV power module goes to negative 0,6V!
    Off course, there was none of thermal paste so I apllied some on the power module and heat sink.

    2. Low audio.
    Audio level on RX is somehow low. It is not distorted, just low. I have to turn volume pot to max, to be able to hear sound normally.

    Many thanks in advance for any help with these.
    Best regards... Nermin ,E74D
     
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    1. Reflow solder connections on Q232(2SD1902), it's on the UHF board. It provides power to pin 2 which controls power out of module. You may have to add some jumpers.

    2. Don't know, try an external speaker first.
     
  3. E75MC

    E75MC Ham Member QRZ Page

    Many thanks Clif, for your precious help, as always. :) What would we do without you!
    I was looking on schematic in service manual and UHF board it self, and could not find that Q232 on board yet. I see it on the schematic and I assume it is on the lower side of the UHF board. Anyway, I will do it and will write progress.

    Hear You again.
     
  4. VE1IC

    VE1IC Ham Member QRZ Page

    as for number 2: probably a capacitor going bad after the audio IC. Look at C82.
     
  5. KA5IPF

    KA5IPF Ham Member QRZ Page

    It's the big honker standing vertically at one end of the UHF power module. Almost thumbnail size. Should have a metal clip holding it to an ear for cooling.
     
  6. E75MC

    E75MC Ham Member QRZ Page

    OK, I have found Q232. I see that it was already changed. I removed both boards from casing and I did detailed reflow and resoldering all the path and connections in that circuitry and nothing changed. If I find another transistor I will install it definitely. For now I will put to a rest this radio until I find that part. Looking at schematics I cant see any reason why this is happening. I mean, volt-meter shows O voltage when that RF power drop occurs.
    Thank You very much Clif and Max. I will write further progress in future.
     
  7. KA5IPF

    KA5IPF Ham Member QRZ Page

    Check that the base of Q232 is switching correctly. Don't depend on the thru hole connections.
     
  8. E75MC

    E75MC Ham Member QRZ Page

    Ok, I soldered wire directly on the base of the Q232, in case that is needed for jumper purpose. Returned UHF board back in place and measured voltage on that piece of wire, When PTT was activated, for a few seconds as usual TX was fine. Since power setting was on MID, voltage on DB was about 6V. After that few seconds, voltage on DB was back to zero again BUT I noticed that voltage on the base of Q232 increase to 12, 75 V !

    I hope that this is not problem of IC 210. As I can see on the schematics, pin 7 of the IC 210 is driving base of the Q232, so how the heck did it sending 12,7 V to the base? :confused:

    Edit. That 12,75 V is the same voltage like on the FB pin ( which is same like voltage for the radio from the power supply). So, could it be that the Q232 is broken and sending full voltage from the collector to the base?
     
    Last edited: Aug 4, 2019
  9. E75MC

    E75MC Ham Member QRZ Page

    Hi there. Here is the update of this work. It was definitely transistor fail. It is Q232, just like @KA5IPF Cliff said. I found in my junk box some similar device so it is working right now. :)
    But, there is still low audio level. I will try to see the caps. It is not critical low, it can be used in normal environment.

    Also, strange thing happened here. Sponge that was placed on front panel, behind those 6 buttons, have been destroyed in time. It turned into very small pieces and part of it got to between two layers of glass on display! As I can see there is no way of separate those layers of display. I really cant believe my own eyes how those parts of sponge got there anyway! :confused:
     

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