SB-200 Fluctuating HV

Discussion in 'Amateur Radio Amplifiers' started by KG7FIU, Mar 21, 2020.

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  1. KG7FIU

    KG7FIU Ham Member QRZ Page

    Have an SB-200. For some reason, just idling the HV voltage is fluctuating rather significantly.
    Within the space of a few seconds, it drops from 2100 V down to 1800 V or so, and then goes back up.
    It seems to be jumping around in the range [1800, 2200]. What would cause this, and is it a sign of problems?

    Note: I just replaced the three 4.7 M resistors and the 15K resistor.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You may have one or more filter capacitors on the verge of failing and may hear a loud pop and blow a circuit breaker if one of the filter caps does actually fail.

    Was this fluctuation on the front panel meter happening before you replaced the HV metering resistor divider chain? Are your other meter functions like idle mode plate current stable or do those readings fluctuate as well? IOW, it could still be a metering problem even if you replaced the HV voltage divider resistors. Some additional testing like inserting a stable known current into the grid or plate current metering shunt resistor should show you if the metering is otherwise stable. It could even be something as simple as dirty contacts on the meter mode switch.

    But if the HV itself really is jumping around I'd probably look into replacing the HV supply filter caps and associated bleeder resistors especially if they're original.
     
    WQ4G likes this.
  3. W1QJ

    W1QJ Ham Member QRZ Page

    A poor connection at the function switch as there is very low current in this loop. Or as above, a filter cap going bad.
     
    WQ4G likes this.
  4. AF7XT

    AF7XT Ham Member QRZ Page

    ...and
    occasionally a bleeder resistor intermittently opening will do this
     
  5. K1LKP

    K1LKP Premium Subscriber QRZ Page

    JOHN,

    GREETINGS FROM NEW HAMPSHIRE.

    I DON'T KNOW HOW MANY IMPROVEMENTS YOU HAVE
    MADE TO THIS SB-200.

    BY NOW YOUR SB-200 IS AT THE AGE WHERE IT IS
    GOING THRU THE " CHANGE OF LIFE ".

    I RECOMMEND THAT YOU DO A SEARCH AND FIND
    ALL THE IMPROVEMENTS THAT HAVE BEEN
    INCORPORATED IN THE THE SB 200.

    YOU COULD HAVE A SAFETY ISSUE AND SHOULD
    NEVER HAVE THIS UNIT POWERED UP UNATTENDED.

    HAVE FUN AND ENJOY THE WONDERFUL WORLD
    OF HEATHKIT AMPLIFIERS.

    KEEP HAPPY AND WORK SAFELY.

    DANGER HIGH VOLTAGE.jpg

    73 - K1LKP

     
  6. KG7FIU

    KG7FIU Ham Member QRZ Page

    K7TRF -- thanks, yes -- the fluctuations were happening before I replaced the resistor divider chain. The HV was way low before.
    It is now a bit higher--but still fluctuating badly. From what I can tell, other meter operations look stable --
    at least plate value, rel power, swr values look constant.

    W1QJ, AF7XT -- thanks. Might have to check out the filter caps and bleeder resistors.

    K1LKP -- yes, you're probably right. It's an old unit and there are probably a number of improvements/upgrades.
     
    K1LKP likes this.
  7. K7JOE

    K7JOE Ham Member QRZ Page

    Yep. New HV caps. One is going bad. It will soon free the trapped smoke inside... along with the nice aroma.
     
    WQ4G likes this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm impressed people are still using SB-200s.

    I built mine 52 years ago and sold it about 48 years ago.

    You don't see so many people driving 52 year-old cars.

    For someone who built one "new" and is still using it, cost of ownership would be eleven cents per week.:)
     
    WQ4G, WB5THT and K1LKP like this.
  9. K1LKP

    K1LKP Premium Subscriber QRZ Page

    ================THANK YOU JOHN
    LIKE THUMBS UP.jpg
     
  10. K6BRN

    K6BRN XML Subscriber QRZ Page

    John (KG7FIU);

    If you are reading voltage from the built-in SB-200 meter, which you likely ARE, be aware that these meters become intermittant after a while. A simple test is to flick the meter with your finger, hard, and see if the displayed voltages changes when it is reading low. The nature of the problem is that the meter will read low, or zero but can come "back to life" with a little mechanical shock.

    I changed out a misbehaving meter on mine - but the old one may be fixable - seems like a loose or diirty connection in the meter. Just need to be a little clever to fix it, I think.

    Brian - K6BRN
     

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