Kenwood TS-520SE: High Grid Bias and Low Power Output

Discussion in '"Boat Anchor" & Classic Equipment' started by W1TTL, Feb 19, 2017.

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  1. W1TTL

    W1TTL Ham Member QRZ Page

    Hi everyone,

    Well, my trusty Kenwood TS-520SE decided to start giving me problems. Essentially, receive is fine but when transmitting CW, the power output is ~ 13W. When tuning up the transmitter, the grid bias will not go below 70mA to the specified 60mA. It will only go 70mA and up when adjusting the bias control.

    I checked the HV (on the TS-520SE's meter) when transmitting CW and it's ~ 800V. I checked cathode resistors R2 and R3 (out of circuit) on the final power amplifier board and they were both within spec (10 Ohms) as well as screen resistors R5 and R6 (100 Ohms). I ended up replacing them anyway since I was in there with exact replacements, but it didn't make a difference (of course). I also checked the screen resistor on the rectifier board (R6: 470 Ohms) and it checked OK in-circuit.

    I didn't see anything burnt in the final driver cage.

    Before I pull the final tubes and have them checked, any suggestions to elsewhere I might look? Thanks for any help...

    73
    Tony W1TTL
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Given the checks you've already performed, I would suspect the tubes themselves.
     
  3. WR2E

    WR2E Ham Member QRZ Page

    Happened all of a sudden?

    Any ALC on the meter?
     
  4. W1TTL

    W1TTL Ham Member QRZ Page

    Thanks, K7TRF.

    WR2E - yes, it seems that way. About a month ago, I connected a Heathkit SB-200 amp to the TS-520SE. I was getting about 500-600W out. A few days ago, I noticed that my SB-200's power meter was a little low on transmit. I took the SB-200 out of the loop and hooked up a Dosy power meter to the TS-520SE and saw that it was only putting out 15W or so on 20m and 40m. I thought maybe my power meter was off so I borrowed a friend's Drake wattmeter and sure enough, the TS-520SE is putting out around 13W.

    I am getting ALC on the meter as I can peak it with the Drive control when the TS-520SE's mode is in TUN. I don't really see a dip on the IP meter with the Plate control, but I can peak the RF (on both the TS-520's RF meter and the Dosy wattmeter) when in CW and controlling the Drive and Plate controls.

    73
    Tony W1TTL
     
  5. KI4AX

    KI4AX Ham Member QRZ Page

    Tony:

    Have the HV and Screen Caps ever been changed? Weak HV Caps will cause the HV to sag while transmitting and result in low power output. Take the bottom cover off the rig and look at the bottom of C19 and C20 to see if there is a blister forming. That's a sure sign that the caps are going bad. Test them if possible. Same for the Screen Caps (c15 and c16).

    The TS-520 was manufactured in the late 70's and are pushing 30 years old now. It's getting time to change the HV Caps if they have not been changed yet.

    I have a TS-520 and had to change them due to similar issues.

    Dan KI4AX
     
    Last edited: Feb 20, 2017
  6. W1TTL

    W1TTL Ham Member QRZ Page

    Dan,

    Thanks for the info. I just wanted to follow up to the thread ...

    The reason my grid bias wouldn't go below 70mA was because I was in CW mode while adjusting the bias during transmit! Oops. I forgot to put it in USB or LSB for that adjustment. Once I did, I could adjust the grid bias fine.

    Ultimately, though, it ended up being the tubes. I purchased an NOS pair of Machlett 6146B tubes and that brought my output right back up. I now get around 90W out on almost all bands. My output had probably been slowing going down over time and I never noticed until I got an amp and started measuring power output...

    73
    Tony W1TTL
     
    KD2ACO likes this.

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