Kenwood TS-930s FIRE!

Discussion in 'Radio Circuits, Repair & Performance' started by STEVEEB, Jul 2, 2017.

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    The Driver Transistors are more critical on over voltage than the PAs . . . so they are the ones that normally fail if the supply goes up to 40V.

    The original low-gain MRF422 Drivers are hard to find and VERY expensive . . . so a better option (and MUCH cheaper!) is to fit similar gain CB PA Transistors, powered through a 12V regulator IC. (this is a very simple mod I did on my TS930 and a couple of others . . . I consider it an upgrade, as the regulator then protects the Drivers - I will send you details if you need them)

    Roger G3YRO
    N7EKU likes this.
  2. WD8BWW

    WD8BWW XML Subscriber QRZ Page

    What we have here is the classic 930 Power Supply failure as described by G3YRO a few responses back. I would check the Caps but likely they are ok. One of the pass transistors fail (shorts thru) voltage goes from 28 vdc up to ~40 vdc which cascades through everything. The only thing that seems to survive is the fuse that is designed to blow and protect everything. I would suggest you just rebuild the circuit board. replace all of the active components, transistor, diodes and the charcoal resistors. Most of these parts are easily found. Replace the big pass transistors, no matter what, they are cheap. Unplug the PA (big red 12 awg wire)module and get the rest of the radio working. Check the resistance of the red wire to ground. There is a zener inside the PA module that sometimes shorts to ground and stays shorted. You don't want you rebuilt supply working into a 20 amp short.

    I have been lucky in that I have not come across blown finals. The drivers are 75% likely blown. Good luck finding those. I tried some reported NTE replacements that lasted about a day and went out in their "blaze" of glory. High gain MRF485 even with resistor change sound like fire crackers when they go. Roger "G3" the trouble with the mod ( i have heard and therefore did not try) is the current demand on a 12v regulator is at the edge of capability. I'm looking for a good solution there.

    One of my many back burner project is a 930 that melted down, which I totally gutted all PS parts and installed a 28VDC switcher. The radio receiver sounds great with zero negative impact from the switching (noise). It is very clean and neat install and the radio loses about 8 LBS. Where the project stopped is finding a good solution the the MRF485s. If you find any they are likely counterfeit. Last ones I bought from a very reliable supplier were in a T0-220 package. MRF485 are not in a T0-220.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Now that's funny right there I don't care who you are.
    WB5THT and G4COE like this.
  4. G4COE

    G4COE Ham Member QRZ Page

    A fuse will not protect a over voltage situation unless driven by a crowbar or something, even a fast blow fuse is not fast enough to protect silicon slabs.

    The only downfall of the 930/940 is the poor soldering, the selection of PA transistors for 28V use.... and 85deg. C electrolytic capacitors mounted 'snug fit' around hot running devices on the digital board, we must accept technology changes with time.

    WB5THT and KD2ACO like this.

    STEVEEB QRZ Member

    My parts order so far. If anyone seems something wrong, please let me know. I'm going to replace the 85deg caps with 105degs to start. Plus some flameless resistors and the D5 diode that has gone open.

    Plus a few NTE parts from Newark, including a replacement for the 2SB861 and MRF485s.
  6. G4COE

    G4COE Ham Member QRZ Page

    D5 is a WZ-182 zener diode that supplies the fan control circuitry from the 28V line, a search for WZ-182 indicates a 18V 0.5W zener and the equivalent is a NTE5027A.

    If 'I' was doing the job I would fit 105 deg. C. capacitors, since it isn't a switching power supply I doubt if fitting low ESR types would be more beneficial but would certainly do no harm.

  7. G3YRO

    G3YRO Ham Member QRZ Page

    No, not if you use the 3 amp version of the 7812 . . . (the drivers should only be drawing less than 2 amps on peaks)

    My rig, and a friend's, have been running hundreds of hours with this mod, and it works fine ! The CB transistors have the same gain as the original Driver transistors, so there's no messing about with bias or drive levels . . . just solder them in.

    But as suggested before, I would certainly disconnect the Output of the PSU section, and feed 26V in from an external variable/current limited Power Supply, to assess if the actual rig has suffered any damage BEFORE rebuilding the internal Power Supply.

    Roger G3YRO
  8. KA5IPF

    KA5IPF Ham Member QRZ Page

    Please tell us where you can buy a 12V 3A regulator?
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    Digi-Key shows linear regulators available in a TO-220 package but, they're not the LM78XX/LM340 series... pn LM1085/LT1085, and MIC29300 (link). At full throttle, ya better heatsink that linear regulator REAL good. I'd use one of the many much more efficient switchers, and not worry about heat.
  10. G3YRO

    G3YRO Ham Member QRZ Page

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