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TS 520 S Problem- on CW Key down - Power Drops from 100 to 75 W

Discussion in '"Boat Anchor" & Classic Equipment' started by WB1FDW, Oct 11, 2018.

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  1. WB1FDW

    WB1FDW Ham Member QRZ Page

    Greetings !
    I restored 3 used old TS 520 S to the best of my ability and have a pending issue.
    I got 2 repaired but mine is still with a small issue.

    Power Output drops on CW Keydown into a 50 Ohm Dummy Load on 7.150 Mhz.
    Power starts at 100 W (on my Bird Watt meter) and within 10 seconds settles at 75 Watts
    On SSB most power out is about 120 W across all the bands...

    Any pointers?
    I believe I saw somewhere the remedy, but my time is running tight, so I thought I'd ask the Ham community for assistance.

    I have NOT replaced any other components
    (ie) HV Caps/Resistors/Cage etc
    I DID clean all the wafer switches however.

    Thank you all for your assistance - it's a great hobby
    I'm finally enjoying this hybrid that we got on the air back in 1976 !

    73's and God Bless you,
  2. G3YRO

    G3YRO Ham Member QRZ Page

    I presume this is happening on ALL bands, not just 40m?

    Most likely fault is a resistor going high resistance with heat - probably in the Screen Grid circuit . . . but could even be the Cathode resistor (that acts as a Meter shunt)

    Roger G3YRO
  3. WR2E

    WR2E Ham Member QRZ Page

    I believe the 520 uses a 12BY7 driver? I had a similar problem with my 830 once upon a time that changing the driver tube cured. Swap out one from one of the other rigs and see what happens. Also what Roger said...
  4. KE4OH

    KE4OH Ham Member QRZ Page

    Top two causes for this on a TS-520 are weak 12BY7A driver tube or plate and screen supply electrolytics need replacing.
  5. WB1FDW

    WB1FDW Ham Member QRZ Page

    Okay - thank you all for the great response - will try these suggestions and reply
    I'll start w/the easy one first....the famous 12BY7A driver tube..

    Many thanks ! Enjoy the weekend !
    By the way, today we survived some bad wind/rain in the NC area - my QTH just missed the heavy storms
  6. WB1FDW

    WB1FDW Ham Member QRZ Page

    Gent's !
    I changed out the 12BY7A for a used 12BY7 that I had in the shack and it works a lot better.
    Not sure what the difference is between a -7 and a -7A?
    I''ll get a known new tube and go from there.

    Power out is better on CW Key down =100 W
    and only drops about 10 W compared to 30 W drop w/other tube.
    Thanks again!
  7. WR2E

    WR2E Ham Member QRZ Page

    Perhaps someone with a better memory than mine can chime in, but I recall that certain brands of the by7 are better than others, and as I recall I found that out myself when I was working on my 830.
  8. W5RKL

    W5RKL Ham Member QRZ Page

    To determine if the driver tube is at fault, power up the 520 for about 15 to 20 minutes.
    Next, perform the TUNE mode DRIVE control adjustments for a peak ALC meter indication.
    If the meter pegs fully to the right simply reduce the drive by rotating the CAR CCW for a lower
    meter indication.

    Once the peak ALC meter indication has been achieved, leave the transmitter keyed in TUNE
    mode for an additional 3 to 4 seconds. Watch the ALC meter. If the meter starts to drop back towards
    zero, whether slowly or rapidly, then the driver tube is at fault not the final tubes.

    TUNE mode reduces the final tube screen voltage by approximately 40% which reduces the DC input
    power in the final amplifier. This allows the transmitter to safely remain keyed for a few seconds longer
    without harming the final tubes.

    A watt meter is not required to determine the DC input power in the TS-520S. All that's needed
    is note, after completing the transmitter tuning into a dummy load, the HV and IP meter indications.
    Multiply the HV indication by the IP indication which indicates the actual DC input power in the
    final amp. Multiply the DC input power by 0.6 to get the actual RF output power at the antenna
    SO-239. The final amp in the 520 operates in Class AB1 which has an approx input to output efficiency
    percentage of 60%.

    Mike W5RKL
    K7TRF, N2EY and KA9JLM like this.
  9. WB1FDW

    WB1FDW Ham Member QRZ Page

    Okay. Thank you for the detailed info. I'll try that next.
    I love these old radios - beauties !
    David, WB1FDW

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