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SB-220 HV and line voltage Concern

Discussion in 'Amateur Radio Amplifiers' started by N8PWM, Aug 19, 2016.

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  1. N8PWM

    N8PWM XML Subscriber QRZ Page

    No as I wanted to test with them in and once tuned it seems that they do very little. Increasing the mesh on each one just seems to reduce the output a small amount. It's almost imperceptible. I assume this is increasing loading.
     
    Last edited: Aug 29, 2016
  2. N8PWM

    N8PWM XML Subscriber QRZ Page

    Hi Bryan. I am going to replace the caps just to make sure there are no issues. Thank you for the information. I had the amp on all day today and when I started out the day the HV was at 2100 in CW/Tune. By the end of the day it was at 1800 with no keydown i.e. Amp not online. I am seeing about a 300v drop on keydown.
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    I asked because it's possible the added variable capacitors don't have enough plate spacing to support the voltage. Most of those size aren't rated very high (air has a dielectric withstand rating of 3 KV/mm or 1KV/0.039"). The capacitors appear to be Johnson Type R/149-series (link) or similar. Increased C = decreased loading.

    The best thing to do would be, before replacing anything, observe where the arcing is occurring. THAT's where the problem is. ;)

    HV should not change over time. I suggest verifying the accuracy of the HV metering (it's possible the INDICATION is changing, not the actual HV). The easiest thing to do (after making sure the electrolytics are discharged) is measure R6, R7, and R8 (4M7/1W resistors). I found one to be barely in tolerance, another to be definitely high in value, and the third to be WAY out of tolerance. After replacing them, HV indication appeared correct.
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    You must lift one leg of the upper resistor to get an accurate reading.
     
    WA7PRC likes this.
  5. N8PWM

    N8PWM XML Subscriber QRZ Page

    Understood. I will check this tonight. Would this also account for the 300v drop on keydown? Interesting fact: throughout the day I am not seeing any change in output level or grid currents. Simply put, I do not see a corresponding change in overall performance with the slowly dropping HV readings.
     
  6. N2EY

    N2EY Premium Subscriber QRZ Page



    The added capacitors are spaced at least as much as the stock loading cap, so it is doubtful they are arcing. However, as they serve no useful purpose for N8PWM, might as well disconnect them.

    Agree 100%.


    Good advice, but there's more: It could be that one or more of the resistors listed above is temperature-sensitive, and is changing value over time if the amp is left on for hours. Then, when the amp is turned off and cools down, the resistors cool off and go back to their original value.
     
    AA7QQ likes this.
  7. AA7QQ

    AA7QQ Ham Member QRZ Page

    Ya beat me to it...

    Any older questionable resistors should just be replaced, as they are cheap. Be sure to get ones that are rated the same or better.

    Ed
     
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    They all should be replaced, there are only a few, and it is best to start fresh.
     
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Sorry!

    If, indeed, they are questionable. I have seen too many examples of folks whose idea of "repair" is to simply replace stuff until the thing works, without really ever figuring out what the problem is.

    "Anything can be fixed if you replace enough parts."
     
    Last edited: Aug 29, 2016
  10. KM3F

    KM3F Ham Member QRZ Page

    That is what happens when owners try to repair their own cars and trucks.
    Just replace parts on a hope.
    Don't read or understand the trouble codes or mis understand them thinking they tell what parts to replace. They do not.
    It's all driven by not wanting to pay for expert or dealer repairs.
    It very often ends up costing them more by the time they are forced into expert service.
    Good luck.
     
    WA7PRC, N2EY and KD2ACO like this.

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