sb-220.something added not sure why

Discussion in 'Amateur Radio Amplifiers' started by KD5JRV, Mar 14, 2011.

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  1. KD5JRV

    KD5JRV Ham Member QRZ Page

    Just bought this from a ham around dallas. it was a local pick up i have not plugged it in but the operater said it was running good.this is my question. i have included a couple pics of a ceramic cap and 3 resistors connected to one leg of tube socket and one to resistors. i am presuming it was just a replacement? my planes are harbach mods all of them since one bleedor resistor was burnt and capacitor bank looks a little chared a complete cleaning and maybe new paint and face plate. this will be my first big project.
    Travis KD5JRV

    Attached Files:

  2. KM1H

    KM1H Ham Member QRZ Page

    Those resistors are directly from filament to ground and look like somones attempt to reduce filament voltage or absorb some RF drive. Id remove them and get the amp back to original and then see what it needs.

    The melted plastic blocks are normal when the equalizer resistors are placed too close. You can simply reverse them by loosening the 4 cover screws, unsolder 3 wires from the caps, and temporarily remove the short buss wire connecting the 2 capacitor banks together. Pull up on the cover until you can remove the upper 4 caps and then the lowers. You can get new and better caps at Mouser and elsewhere by using the 105 degree C Snap-In style such as the CDE 381LX series, I wouldnt recommend their lesser temperature series. The 330uF/450V will be a perfect diameter fit into the reversed blocks and get 8 100K 3W MOX resistors to replace the heat generators. There is no need to buy any Harbach stuff except the Soft Key to mate with a SS rig and the Step Start when using the higher value caps. Their PS board has problems and the parasitic suppressors are pure garbage.

    Lift one coil end from each parasitic suppressor and if the resistors measure no more than 60 Ohms they are still OK to use. If not find four 100 Ohm 2W carbon composition resistors (measure before using and discard any that are more than 120 Ohms), place the original coils over 2 of them (or wind new ones exactly the same as he originals) and reconnect as 2 in parallel for each tube. There is another method to construct new suppressors that also work as well as the originals if need be.

  3. W0BTU

    W0BTU Ham Member QRZ Page

    Pay attention to what Carl said! If you plug the keying line from that SB-220 into your solid state rig, you could damage the rig. The Harbach Soft Key mod will prevent that.
  4. KD5JRV

    KD5JRV Ham Member QRZ Page

    thanks so much for your help, i will take your advice. the resistors are connected to the lug on the tube socket the other end is connected to the disc cap that is connected to the coax leading to the wafer on the band selector. so i should remove the resistors and leave the correct value disc there? and after looking at the other resistors on the tube sockets i think i will change them as well.should i stick with the same color bands are change value. any more help would be apreciated.
  5. KD5JRV

    KD5JRV Ham Member QRZ Page

    thanks mike i will heed the warning.
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd put it back to original configuration. Maybe someone was trying to "pad" the input because their transmitter had too much output power and they needed to reduce it a bit. It's hard to read the resistor color bands but kinda looks like brown-green-black, which would only be 15 Ohms each, if that's what they are. Not much (three in parallel would be 5 Ohms). Maybe I can't read the colors well, they do look a bit faded.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Actually, reading from the other end, maybe they're really 200 Ohms each (red-black-brown). That would make more sense.
  8. K9STH

    K9STH Ham Member QRZ Page

    While you are at it, I would change the rectifier diodes to 1N5408. They are cheap and will give you a better safety margin.

    Glen, K9STH
  9. KD5JRV

    KD5JRV Ham Member QRZ Page

    Thanks for all the help, I will post some pictures as I go, so I can be steered in the right direction
    73s. And tnks for the help
  10. ae7lv

    ae7lv Ham Member QRZ Page

    I'm sorry, but I read those resistors as brown-brown-black-red, which would be 110 ohms 2%. In tech school I was taught to begin the count with the color band closest to one end. With the disagreement I'd suggest decoupling and doing a measurement.
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