Heathkit SB220 Problems

Discussion in 'Amateur Radio Amplifiers' started by K9NRA, Jun 11, 2015.

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  1. K9NRA

    K9NRA XML Subscriber QRZ Page

    I went through this amp about 6 months ago and updated it with all the Harbach mods. Was using it in CW/Tune on SSB last eve. when I hear a noise inside by the diode board RM-220. Shut the unit down and opened it up. D17 was shorted on the board, D16,D15 appeared different also in the diode check with my Fluke . I went ahead and changed all the diodes on the RM220 and checked the resistors. Next I checked the filter caps, all appeared good with cap checker, but D4 on the cap board was also shorted. Anyone have any thoughts on what could have caused this failure ?
     
  2. K9NRA

    K9NRA XML Subscriber QRZ Page

    Jeff at Harbach told me I probably have a shorted tube, but I had not seen any indication of it before. The pair was putting out about 1000-1200W on the cw/tune low power side while on sideband with about 100w drive. Any good way to check out the tubes?
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd add a glitch resistor in the HV+ line before doing anything else, then fire it up again. Should save other components and sacrifice itself if there's a tube short.

    http://www.w8ji.com/fault_protection.htm
     
  4. W8JX

    W8JX Ham Member QRZ Page


    Intermittent shorts from arcing are hard to test out of amp. You can test filament to grid, grid to anode and filament to anode with a ohm meter. I should be open,
     
  5. K9NRA

    K9NRA XML Subscriber QRZ Page

    I looked at the link you listed, so, I have seen a single wire used on the plate supply , single strand type. Is that what you are speaking of?
     
  6. AF6LJ

    AF6LJ Ham Member QRZ Page

    In this case a Glitch Resistor is a series resistor that can handle brief overload conditions.
    Its purpose is to limit damage during a fault condition.
    You will see them in Ameritron amps.

    The resistor must be able to hold off the full plate voltage, and several times the current of the power supply, and handle several hundred watts of dissipation over a fraction of a second.
     
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You add a resistor (typically 10-20 Ohms, 50W, and it must be very well insulated from chassis ground -- I use ceramic pillar insulators about 1" long each to mount the resistor) in series with B+ between the power supply and the tube anode. Close to the power supply is fine; find a convenient mounting place.

    A "high voltage" resistor really should be used, but frankly most of the 50W WW resistors will work just fine because they're quite long (2-3" long usually) and that's long enough to actually be a high voltage resistor for this application.

    In normal operation on CW or SSB, it won't even get warm. Key-down at a kW using RTTY it will get warm, but not harmfully so. But if the tube shorts, the resistor will limit the fault current or if the fault is really bad, the resistor will just fail as an open circuit, thus sacrificing itself and not expensive power supply components.

    A "dead short" should blow the SB-220 circuit breakers, but sometimes other power supply components fail before that happens.
     
  8. K9NRA

    K9NRA XML Subscriber QRZ Page

    Will the 50W sand type resistors work?
     
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

    I would be reluctant to use one of those.
     
  10. K9NRA

    K9NRA XML Subscriber QRZ Page

    So, something more like an ohmite enamel one?
     

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