WHAT??? ANOTHER SB-220????!!!!!!!!!!!!!

Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jul 31, 2017.

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  1. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    The great dollar OUTLAY has commenced, which at this point in my life I certainly can't afford!!! Up to $73 just for 4 PCBs! got 3 of the W7RY series and 1 FAR CIRCUITS for the Cap board. W7RY didn't have a Cap board available. I found the caps at MOUSER for $6.60 each, and for 8 of those we are $52.80! Then there is all the resistors and other small parts. VERY PAINFUL!!! I should have gotten a down payment.

    As discussed with Sue, I will probably try taking apart the Tune cap and file down the melted aluminum on the plates. Those caps seem to unobtanium, I know none are found on Ebay now. I do have the coil and rotary switch assembly if that is neccessary to replace, though I am not fond of donating it to the project.

    When I can't afford dinner over $4, you know how painful it is to spend hundreds of bucks!! (and it's not even MY AMPLIFIER!!!)
     
    AF6LJ likes this.
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    I'll be doing more of the same this next week....
     
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    If you use diode(s) at the meters and there's a HV fault, -B will jump up, and the wiring between there and the meters will smoke. Better to clamp -B as Tom W8JI shows here (link) and here (link). How many rectifiers? He says to use one (or more) rectifiers in to start clamping -B at some point just above normal operation. The SB-220 uses R1 = 1Ω/5W. At IANODE = 500mA, you'll see 500mV and a single diode will start to conduct. Two may be needed, to prevent false indication at maximum IANODE.

    How big should the rectifier(s) be? The greater the C in the HV filter, the greater the current PULSE in a HV fault. This can easily exceed 200A. A 1N54xx rectifier (3A continuous) is rated 200A pulse. A 6A10 (6A continuous) is rated 400A pulse. The cost is only a few pennies more. The rectifier(s) should be added at/near the negative terminal of C17.

    W8JI also talks about using a "glitch" resistor in the +B path to limit fault current 'til the AC fuse/breaker can trip. This should save the transformer from smoking. I used a 10Ω/20W wirewound (Ohmite pn B20J10RE) in place of RFC2 (8.5μH):
    [​IMG]
    (click for yuuge image)
    My AADE L/C Meter II sez it has L = 1uH. It wouldn't be a bad idea to add more C in parallel with C7, to ensure RF is kept out of the HV source. Another 1000pF would be good.
     
    AF6LJ likes this.
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    The H*rbach stuff costs TWICE that. The CDE pn 381LR271M450A032 (datasheet) "Lowest ESR" capacitors are about as good as you'll find. Mouser doesn't stock them. Digi-Key has them for $6.27 each in single lots.
    You want to go further, and smooth any sharp edges to minimize the possibility of an arc. Lou @W1QJ is likely the best source of replacements
    Treat it as yours, and you'll receive respect from the owner.
     
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    The diodes are back-back in SERIES (not parallel). IOW, they will never conduct.
    And as I mentioned previously, they're in the wrong place. They should be at/near the negative terminal of C17 (-B) and chassis.
     
  6. WA6MHZ

    WA6MHZ Subscriber QRZ Page

    I have a whole bunch of plates left over from the SB-226 removed from the Load cap. Will see if they are the correct size. Maybe not, but maybe!
     
    WA7PRC likes this.
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    When you get home Pat you might want to discconnect your FTDX5k for the night.
    Lightning is moving up from south of the border
     
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Better, ensure your ground system is adequately bonded to... well... ground. Tom W8JI in GA sez his tall towers each get several direct strikes per year, and he has yet to experience any significant damage. Broadcasters and many other systems don't disconnect either. They're either very lucky or, they're doing something right. I'm betting on the latter. :)
     
    AF6LJ likes this.
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    It's all about doing something right.
    When I was in two way radio a rep from Polyphaser gave a presentation on lightning proofing your commercial radio site.
    They don't screw around, lots of ground rods, surge protectors on all the lines entering and exiting the building. EVERYTHING IS GROUNDED.
     
    WA7PRC likes this.
  10. WA7PRC

    WA7PRC Ham Member QRZ Page

    I never trusted disconnecting my feedlines and putting them into a pickle jar. Or, was that a mayonnaise jar? :p

    It's good to be well-grounded... in all ways. ;)
     

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