Kenwood TS-940s AVR question

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

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  1. G3YRO

    G3YRO Ham Member QRZ Page

    Well all I can tell you is that the side of the PA heatsink where my 7812 is mounted doesn't get at all warm ! Not even operating CW in a contest for hours on end.

    I'm presuming that the 2SC1969s will be drawing just over an amp . . . but on SSB that is only on peaks of speech . . . and of course NOTHING on receive.

    So the AVERAGE dissipation will be pretty low. And don't forget that you can mount the 7812 direct on the heatsink - you don't need an insulating washer. (but do solder small RF de-coupling caps direct onto the pins)

    Either way, it's not a problem, so I wouldn't worry. This mod has been working in my rig and a friend's for over a year now, with no issues.

    Roger G3YRO
  2. W1GCI

    W1GCI Ham Member QRZ Page

    Here's a bit of an update. 1) There are still problems to be solved on this rig. :) The fun continues. I installed the 2sc1969's. I used a lab supply I have with a variable current limiter and at full output key down CW, I get 430 ma at 12 volts. At first, I was getting no output - but that turned out to be a bad coax jumper to my watt meter/dummy load - my bad. After replacing that, I was getting 50 watts out. This was on 40 meter and I decided to try other bands. On 80, I am getting 110 watts. But yet other bands are showing around 20. I'm more suspicious of the filter board than the final board, but that investigation will come next.
    I was surprised that when I switched the meter to SWR when the coax was shorted / open, I didn't read any reflected power. Also, current on the meter never reads anything. So, that too indicates a problem. However, even after a key down for more than 10 seconds on 80 meters with over 100 watts out, I didn't notice much heat from the 1969 cases or the finals for that matter. The good news is that I am getting power out and look forward to continuing the investigation.

    arnie - W1GCI
  3. CX3CP

    CX3CP Ham Member QRZ Page

    Hello, I suppose that you are talking about Q1 on the PA board, according with the schematics an 2SC2075 are the transistor intended for that app. as pre-driver stage. 2SC1969 are not suitable for 28 Volts PS. That could cause overheating the transistor. You must also check for the right biasing voltages of all next stages. Also check for open (R23 to R26) must be 3R9 ohms all, not 3.9K as show in the schematic ( another case of bad printing from factory).


  4. KM1H

    KM1H Ham Member QRZ Page

    The ELNA electrolytics on the AVR board are notorious for leakage and I rebuilt them all in the 90's when I had about 6 940's bought real cheap. Im down to two nice runners that havent failed since.

  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    When it folds back on power, There is no power to measure.

    I would be suspicious of the filter board also. It may be stuck on 80 meters.

    Good Luck.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Glad you seem to be getting it working Arnie !

    No Albert . . . he's replaced the Drivers, Q2 and Q3 with 2SC1969s, as per my suggestion. (guess you didn't read through the thread properly!)

    As a test, he's just fed them from a 12V bench supply . . . but I suggested feeding them via a 7812 regulator fed from the 29V rail. (I use a 3 amp version)

    Well Arnie, as I told you, 2SC1969s fed via a 7812 have been in use in my rig for over a year, including many hours of contest operating, with no issues.

    Roger G3YRO
    Last edited: Jun 6, 2018
  7. W1GCI

    W1GCI Ham Member QRZ Page

    Thanks everyone for you comments. I'm finding that there is more not correct with this unit than I previously knew. Just more of a challenge. Yes, Roger, I've now connected the driver transistors to the output of a LM340T12, which is a 1.5 amp regulator. With the drivers pulling less than 1/2 amp key down, I feel pretty safe with this regulator. Giving it the finger test, it's not really getting hot - not yet anyway. So, I'm dropping about 16V at 1/2 amp, which would be 8 watts. The Junction to case resistance is about 5 degrees C/watt - about 40 degrees C plus whatever resistance case to sink. So with a max junction temp of 125 degrees C for the device, it should operate without much difficulty.

    As it turns out, the 940 has the switch thrown to allow transmit outside the ham bands, so I progressively incremented the frequency 1 mhz at a time and keyed in CW mode to measure the output. The carrier and drive controls are all the way up. Beginning on 1.5 mhz I get a steady increase in output to about 5.5 mhz at which time it begins to drop from what's now 125 watts to about 75 watts at 7 mhz and down, down, down, to about 20 watts at 29mhz.

    The low pass filter unit is switching as it should be and the AT unit is switched out. I think my next step will be to remove the final unit from the other 940 I'd been working on and see what happens when I plug it into the one I'm currently working on. Oh, I put on a 40 meter antenna and then transmitted on 80 meters to create an SWR situation. The power did fold back as I would expect, so the protection circuitry does seem to be working.

    Besides the power issue, here are a few other problems I've observed:

    I can't read current on the meter.
    I can't read SWR on the meter. However, I can read voltage, forward power, and the S-meter works.
    Even with the carrier control all the way up, I can't product a carrier in AM mode.
    I can't produce a carrier in FM mode.

    Besides all that, receive seems to work just fine. :)
  8. G3YRO

    G3YRO Ham Member QRZ Page

    Glad you're getting there Arnie . . .

    Well it COULD be that your new 2SC1969 drivers are not genuine . . . if so, they may just be audio transistors that have enough gain on the lower bands, but fall off as you go higher.

    When one of my Drivers failed, I hadn't realised exactly what the problem was at first . . . I was confused, as I had full power output on 10m, but power gradually dropped off as you went down through the bands.

    What I discovered, by doing measurements, is that this PA Module - if working correctly - has LOTS of gain on 10m, but very little on the low frequencies. But this excess gain at high frequencies is then flattened out by the RF negative feedback wire that goes through L16. (ie lots of feedback on the higher bands, which gradually decreases as you go lower in frequency)

    So the fact that you are lacking in output as you go HIGHER is rather suspicious.

    Also - it can be very confusing, as once you start generating any ALC voltage, the RF Level fed to the PA Module is gradually turned DOWN. So if you want to measure the RF from stage to stage on the PA Module (using an RF Probe), you need to keep the drive level very low.

    Also note - that when the PA Module is working properly, (ie has a fairly flat frequency response), the actual amount of ALC reading produced for a given Drive level will be pretty constant from band to band.

    Regarding your lack of AM Tx, I seem to remember Clif (the Kenwood Guru) mentioning on here that some TS-940 versions have this inhibited (for some reason) and you have to clip a diode or something to enable it - hopefully he will see this and advise.

    Roger G3YRO
    Last edited: Jun 6, 2018
  9. KA9JLM

    KA9JLM Ham Member QRZ Page

    How did you test this ?
  10. G3YRO

    G3YRO Ham Member QRZ Page

    To rule out any problems with the BPFs or ATU, I would just measure the power straight out of the PA module . . .

    The harmonics won't affect the readings . . . but bear in mind you won't be generating any ALC, so keep the drive LOW as there will be nothing to limit the power!

    (You won't blow up the PA, as this Motorola design used by Kenwood is rated at 300W output . . . but the rig's PSU won't supply enough current and you might damage it!)

    Roger G3YRO

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