Kenwood TS-940s AVR question

Discussion in 'Radio Circuits, Repair & Performance' started by W1GCI, May 6, 2018.

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  1. W1GCI

    W1GCI Ham Member QRZ Page

    I bought two broken TS-940s recently. I don't want to do board swaps to get just one working so I began working though the issues that I can identify one at a time. In one unit, the AVR doesn't seem to be sending the right voltages to the pass transistors (Q101 and Q102) to get them to regulate down to 28V. Q1 on the AVR board should set the voltage that these pass transistor regulate to, but it's not 29.8 volts at the collector of Q1, which is the voltage applied to the bases of Q101, Q102, it's 40 volts on one base and 34 volts on the other. Now for the actual question. On my schematic, there is a 15K resistor (R3) going from emitter to collector of Q1. The board has a 1K resistor plus a series diode where R3 would go and it looks like it came that way from the factory - the solder is not disturbed. Is this a factory mod or something I should restore to as it appears on the schematic?

    arnie - W1GCI
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    Shorted pass transistors. Also replace Q1.
    KA9JLM likes this.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    Clif, You are the best. :)

  4. WQ4G

    WQ4G Ham Member QRZ Page

    And, build and install a crowbar circuit in the power supply so that this doesn't happen again....
  5. W1GCI

    W1GCI Ham Member QRZ Page

    Yes, one of the pass transistors is shorted. I replaced both. I has some on hand. Also, it appears that the 2.2 ohm base resistors for the two pass transistors are open. I moved the AVR board from my other broken 940 and I now get all the right voltages. I had to order the 2.2 ohm 1 watt resistors, so, I'm in a wait till I can check out that board. In the mean time, I opened up the final unit and found a burned mess around one of the driver transistors - MRF485. I'll need to do a bit more assessment of that board. It's a work in progress, but now it's time to plant potatoes :).. Thank you Clif for your always sound advice.

    arnie - W1GCI
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, the Drivers can't handle 40V, which is why they pop !

    Low Gain MRF485s are no longer available, of course . . . I replaced my Drivers with 2SC1969s fed by a 3 amp 7812 regulator (did the same on another rig too) . . . works well, as they have similar gain and drop straight in, so no other mods required.

    I'm hopeful that my mod of providing a constant load on the PSU will prevent failure of the Pass Transistors. (apparently there is an issue of the On/Off switch disconnecting the output of the regulator from the rig causing a spike)

    Roger G3YRO
    WQ4G likes this.
  7. W1GCI

    W1GCI Ham Member QRZ Page

    I had already ordered 2SC1969 transistors but I'm not clear on the 12 volt regulator. You knocked down the 28V to 12 for the 1969? Sounds easy enough to do. Any bias issues? I'd like to see a write up if you produced one.

    Thanks Roger
  8. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, just using a 3 amp version of the 7812 regulator. (you don't want more than 12V on them, or they will try to deliver too much drive)

    No bias issues, as the gain is much the same as the original drivers. I set the bias pot so they were drawing about 100mA total standing current.

    I mounted the 7812 on the inside of the PA heatsink, right where the 28V cables enter the PA board. There's a tapped hole in the middle of that edge, but the bolt from the outside doesn't go right into it . . . so you can screw a short bolt into the same hole from the other side ! (so no need to drill any extra holes).

    I glued the wire from the output of the 7812 along the underside of the edge of the PA pcb to the top of L7, so it stays very neat. (You just lift the top of L7 from the 28V rail - I added a .047uF decoupling capacitor to deck, which also anchors it)

    I've done this on 2 rigs . . . but I'm always in a hurry, so never took any photos !

    My rig's a TS930 . . . but the PA is exactly the same module, as is most of the PSU.

    As you may have seen me post elsewhere, I have also replaced BOTH the original fans with 24V ones running all the time, straight off the 28V rail. This has TWO advantages - one is that the heatsinks don't get hot in the first place, and more importantly, there is still a load on the 28V regulated line when you turn the rig off. (the spike caused by the removal of the load is what one guy reckons causes the Pass Transitors to sometimes fail)

    Roger G3YRO
    Last edited: May 8, 2018
  9. W1GCI

    W1GCI Ham Member QRZ Page

    While waiting for parts for the amp and AVR, I moved the good amp and AVR from my second broken TS-940 into the one I've been working on, and now have a mostly working radio. I believe the unit took a lightening hit or something similar. The protection arrester across the antenna terminal was blown apart and discolored. Also, the two .047 caps in the antenna output circuit were unsoldered - yes, they both fell out of the radio when I turned it over. Now the only known problem with the radio is that the auto tuner will now provide a tuning match on 80 and 40, but will on 20, 15, and 10. I didn't try any of the other bands. I had a nice QSO on 40 - auto tuner off. Now I'll fiddle with the auto tuner while waiting for parts for 940 number 2.

    KA9JLM likes this.
  10. W1GCI

    W1GCI Ham Member QRZ Page

    Okay, I've got everything working on both radios except for the final amp on one of them. I just received the 2sc1969 transistor and am about to do the 12 volt adaptation. However, I'm a bit concerned about using a TO220 regulator to get to 12V. That means dropping 16 to 17 volts from the 28-29 volt (28.5v) supply. At one amp, that's 16 watts to dissipate at just 1 amp draw. From what I can see, the Junction to case thermal rise is 5 degrees C per watt. That little sucker is gonna get really hot. But, BUT!!!!, I don't know how much current the 2sc1969s are going to draw. I'm just assuming that since everyone is advising 1.5, 2 or more amp regulators, that it's going to be at least 1 amp. Does anyone know for sure? I guess I can just use a 12V lab supply initially to find out. Thoughts?

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