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

    I just picked up an SB-220 Linear Amp. After giving it the hairy eyeball, I plugged her in to see what would happen. Finals light up and I have lights at the meters. This amp is plugged into 12oV.

    Here is the issue.
    1. The meter lamps are rather dim. Not nice bright as expected.
    2. HV is showing 1200 in CW/Tune and 1500 in SSB.
    3. Fan is not turning
    This was simply a smoke test. Result: No smoke :)

    Based on my observations, I suspect that the amp is wired for 220V and not 120V. Of course now that I have powered it up, I cannot do anything for a day or so inside the cabinet just to be sure that the caps are all discharged.

    I am looking for advice and opinions as to my suspicion.


    Craig N8PWM
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    The lights are suppose to be dim. 6.3 running on 5.0 VAC..

    As for your high voltage, one of the first things to go if this is a stock SB-220 are the high voltage meter multiplier resistors.
    When I got Oscar (my sb-220) the high voltage in the SSB position was just over 2KV.

    There is a Dayton replacement for that fan that is cheaper than the Hrabash one.
    Don't leave it on very long without air...

    There is lots of SB-220 Goodness in this thread...
    In there you will find replacements for those meter resistors.
    You will find the Dayton fan motor replacement and a link to the Granger page for that fan motor it is cheap and a very good replacement.
    Along with some other good stuff.
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Yup, sounds like it is wired for 240. Take the outer cover off and turn amp upside down. In the back left corner you will see a terminal strip. If terminal 1&2 and 3&4 are jumpered it is set for 120v. If terminal 2&3 are jumpered (only) then it is set for 240. That is simple enough to check. Since you know that the HV meter works, if the meter reads zero then the caps are discharged. You can always ground the HV with an insulated screwdriver to be sure. No need to wait a day. The SB-220 has very good bleeders, I have never seen an open one in over 30 years.
    KD2ACO and AF6LJ like this.
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    Oh and...
    It may not be wired for 220

    Turn the amp over and see if you can see the terminal strip near the back corner where the cord comes into the amp...
    If you see one jumper between two lugs in the middle of that strip it is wired for 220 if you see two jumpers linking the first to the second and another jumper linking the third to the fourth terminal it is wired for 110.
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    Beat me to it. :)
  6. N8PWM

    N8PWM XML Subscriber QRZ Page

    Thank you to both of you. I appreciate the advice. With the amp unplugged and the power switch on I am showing zero on the HV meter setting.
    I had thought of making a chicken stick just to be sure so I think a trip to Home Depot is in order lol.

    I have another question if you want to tackle it. This amp has two additional dials on the front they appear to be variable caps and they are directly above the Tune and Load controls, right below the nameplate. Knowing that I have not taken the amp apart, I have no idea what they could be. I have searched a multitude of mod sites for this amp and find nothing. They are not marked. The sticker in the attached pic is just a note on how to set the Tune and Load controls. No reference to the other two variable caps.

    This amp was owned for its entire life by a ham operator so no CB mods.

    Attached Files:

  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I cannot really comment until I see the insides.
    WG7X likes this.
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    If running it on the correct voltage doesn't fix things,
    • The Dayton replacement fan supplied by H*rbach is pn 4M070. It is available from Grainger (link) at half the cost. For best airflow, you'll want to make certain the fan blade is centered in the rear PA compartment panel, and the top-rear panel is in place.
    • The three 4M7Ω/1W carbon composition HV meter multiplier resistors are notorious for soaring in value. If that's the case, replace them with resistors rated for the voltage (about 1KV each), such as the Vishay/BC Components VR68-series (datasheet). They're stable and rated up to 10KVDC. Digi-Key has them as pn VR68J4.7MCT-ND for $0.78 each.
    No clue what the extra knobs are for on the front panel. Since this unit was in the hands of a CB-er, it would be a very good idea to go over it with a fine-toothed comb to see what else (if anything) has been changed.

    Bryan WA7PRC
    AF6LJ likes this.
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    They're 30KΩ/7W wirewound and needlessly waste power while heating the electrolytic capacitors. The capacitors don't like heat. Replacing the resistors with up to 100KΩ/3W types is a good idea. Assuming HV = 3KV, resistor dissipation drops from about 38W to 11W. Upon shutdown, it takes only a little longer for the resistors to bleed HV down to zero.
    W8TJM and AF6LJ like this.
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    There ya go very good advice.

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