SB-220 Find

Discussion in 'Amateur Radio Amplifiers' started by AF6LJ, Dec 19, 2012.

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  1. AF6LJ

    AF6LJ Ham Member QRZ Page

    I am going to use a step start circuit anyway to keep the wear and tear down on the primary power switches.
    I like that board better than some of the alternatives I have seen.

    As for C-52 .02MF is overkill for the application when a .005MF will do just as good with a much smaller current spike. They must have found a good deal on those caps. :)

    The Harbach capacitor replacement kit has all the parts needed to rebuild that cap bank, and the end result looks good, less hassle for me.

    The W7RY QSK board is a good solution to a few issues, not the least of which is a stable bias source other than the bias solution used in the SB-220. The circuit is also not overly complicated like some I have seen.
    The only issue I have with his QSK board, has to do with it's mounting, I am not happy with single hole mounting schemes, but there is a workaround for that that I find acceptable.

    I want to stick to the orginal wiring colors if for no other reason than it makes it easy for someone including me to have a reference to where those wires are going. Just in case the documentation gets separated from the amplifier. No blue wire for the B+, that I never cared for test lead wire and some Teflon sleeving will do nicely.

    So it looks like I will be picking up W7RY's two boards and the reed relay...
     
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    We have a couple of surplus stores here, I am hoping a trip this weekend will take care of my wire needs.
    irradiated PVC is nearly as good as Teflon wire, and it doesn't cold flow like the Teflon insulation will if one gets too ambitious with tie wraps and cable clamps.
     
  3. W1BR

    W1BR Ham Member QRZ Page


    Hi Bryan

    My understanding was that using short leads to the PCB improved heat transfer for power diodes. At least that is what I have read in the past. I'd guess the copper PC board runs are better at heat conduction and dissipation? Just curious as I have no fixed opinion either way.

    http://www.interfacebus.com/semiconductor-diode-packages-DO-35.html

    Pete
     
  4. AF6LJ

    AF6LJ Ham Member QRZ Page

    What Bryan says I have heard before.
    And it does make sense, so does what you say provided the copper pads are of reasonable size.
    In the case of the SB-220 the PCB pads are small. Mounting the diodes a little off the surface of the PCB, say the thickness of an orange stick 1/16" to 3/32" this provides a little room for air movement around the body of the device. Each diode would under voice operation would dissipate something around a watt, maybe a little less on the average. This doesn't take into account the inrush current which a step start (soft start) circuit will already limit. Even though the only air flow in this part of the SB-220 is from convection currents generated by the filament transformer's heat, this is plenty of air movement.
     
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    Here is a picture of the PC board in question.

    [​IMG]
    The four resistors over on the left side are the HV meter multipliers. The 5.6K resistor is a shunt to protect the meter in case the resistor chain open circuits...

    The fourteen diodes are across the bottom, as I understand it they were 600V diodes. I don't know what the current rating of the original diodes is but they would have to be around 1.8A I would think. In this amplifier the diodes are mounted with no air gap, and you can see the pads are not very large

    The .82 ohm resistor is the grid current shunt. below that are the plate current shunt and a multiplier resistor.
     
  6. W1BR

    W1BR Ham Member QRZ Page

    Hi Sue

    That board looks to be in very nice shape. Usually they are pretty sad looking after 30 years of service! I broke down and bought the Harbach board (big decision for a cheap New England Yankee type ham!)

    Pete
     
  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I think this board will clean up really nice. While I may be picky...
    The grid shunt resistor was improperly installed. note the strain on the right hand lead.

    The amp has been gently built and gently used, I should get many years of good service out of it.
     
  8. WA7PRC

    WA7PRC Ham Member QRZ Page

    Don't forget the 4M7/1W HV meter multiplier resistors. If they're not out of tolerance, they will be. I used Vishay VR68 series (datasheet) "Metal Glaze" types. They're good for WAY over 1KV each. You want pn VR68000004704JAC00. Digi-Key has them for $1.26 each (a nonstock item for Mouser). The 5K6/½W shunt resistor should be fine. It just keeps the bottom end of the 3 x 4M7 string from soaring to HV when the meter switch isn't set to read HV.
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    They are and I have a list of parts to order from Digikey next month.

    I am thinking all the resistors and diodes will go....
     
  10. AF6LJ

    AF6LJ Ham Member QRZ Page

    New fan motor arrived from Granger on Wednesday I installed it yesterday however after obtaining the proper spacers it will be removed and reinstalled then. The back of the fan rotor housing was a little different than the ordinal and I had to improvise spacers, a minor issue and easily remedied.
    It took a little gentle persuasion to get the fan blade to fit and it was on perfectly straight.
    More air and only a little more noise, which with the radio volume at a comfortable isn't even noticeable.
     
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