TM-331A Power On Reboot

Discussion in 'Radio Circuits, Repair & Performance' started by KR6J, Jul 12, 2020.

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  1. KR6J

    KR6J Ham Member QRZ Page

    I need help! I have a Kenwood TM-331A that is rebooting itself when I power it on. It acts like it has no back-up battery installed. Here's what's happening:

    • Power on the radio; display intermittently comes up scrambled
    • Power radio off, then back on; radio will come on normally and I can dial a frequency into the VFO
    • Power radio back OFF and back ON, it will reboot to 220.000
    ... and on and on it goes.

    I've replaced the lithium battery with a new one from Pacific Coast Parts. That didn't fix it. Connectors 4, 202, 3 and 201 have had their pins re-soldered, and that had no effect.
    I have checked Q201, Q202 and D203 and all seem to check good. I have noticed something interesting though: I measured 5 volts on the cathode pin of D203 when the radio is switched ON and I would assume that's normal, but when I power off the radio, I should expect to see the roughly 2.5 volts (3.2-0.7) from the B/U battery, but it intermittently drops to around 1.2 volts. I replaced D203 anyway (as much as I dislike shotgunning parts) and it had no effect.

    So I removed the backup battery and ran 3.2 volts from my lab supply directly to the + and - connection points on the X57-3320-10 B/2 board where the B/U battery would normally mount, and the results were the same. But here's where it gets weird: When I increase the voltage from the supply from 3.2 to 5.0 volts, the radio powers on normally time and time again. The display comes up good and there's no loss of memory whatsoever (hence, why I do not suspect the four connectors mentioned earlier). So my question is why does Vdd terminal (pin 58) on the CPU happy to see 5 volts on power OFF and not the 3.2/2.5 it is suppose to see from the lithium battery? In other words, when I decrease the supply voltage back to around 3.8 that's when the trouble occurs.

    By the way; when the radio is on and everything is functioning normally - from a memory and display perspective - the receive and transmitter sections do function perfectly.

    I'm not a technician - well, not yet anyway, but I truly love learning to do this. Sure, it can be frustrating at times, but that's part of the journey that makes the payoff that much sweeter.

    I hope there's someone out there that can help me.

    Best 73,
    de Kenan, KR6J
  2. KI4ZNV

    KI4ZNV Ham Member QRZ Page

    Something is likely pulling more current from the battery than it should be. Get the schematic and start checking the things that are powered by the battery and make sure the voltages are where they should be. If they aren't, look for series resistors that have increased in value or (MOST LIKELY) leaky electrolytic capacitors.
  3. KR6J

    KR6J Ham Member QRZ Page

    Thanks, Scott.

    I'm expecting two Kenwood TM-231A's, which have the exact same control board, so that I can start making those exact checks. Unfortunately the service manual does not go that far into the weeds when it comes to current drain info on the b/u battery. Nonetheless, it's good information to have on hand. I will also take Q201 and Q202 out of the circuit and check them again.
    As for the resistors, all of the ones I have checked look good and are with tolerance specs. I have already re-cap'd the entire radio so I'm not too suspicious of any bad electrolytic caps.

    Thanks for taking the time to reply.

    Kenan, KR6J

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