SB-220 question

Discussion in 'Amateur Radio Amplifiers' started by KC9JYW, Apr 10, 2021.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    If you are saying you see this with the amp off then it must be AC related. Somewhere you have a leakage driving current into the chassis. Does this amp have a soft start?
    WQ4G likes this.
  2. NU4Y

    NU4Y Ham Member QRZ Page

    With the amp off ? Unplug the power cord for the amp.

    Wait 10 minutes. If the meter is still going backwards. you might have damaged the movement.

    If it zeros out with it unplugged. Problem is electrical.

    Just my WAG.

  3. KC9JYW

    KC9JYW Ham Member QRZ Page

    Lou this app does not have a soft start but let me tell you what had happened. The amp was running great Just a little noise in the fan So I decided to swap it out. Whenever I took the ample part and disconnected the fan I put the new one in Then fired it up Without the Spring cage grounded . My bad All it did was blow a fuse I thought no big deal Put another in an all seemed good except now I am getting the negative grid when I switch to that position. I can shut everything down And it still gives my grid a negative Reading when switched. This is with or without tubes installed
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If the meter is offscale "negative" with the amp completely shut off, sounds like the meter was damaged. However, "switching" to grid current (with the amp powered off) wouldn't change anything. If the meter's damaged it would be damaged regardless of the meter switch position.

    Where's the fuse in an SB-220? I haven't owned one in many years, but mine had two circuit breakers.
  5. KC9JYW

    KC9JYW Ham Member QRZ Page

    The fuse blew on the aftermarket bios board. AG6YJ The amp has updated parts and this bios board and power supply was two of them including a peter dahl HV transformer
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    It's "bias," and wow, those changes mean analyzing the SB-220 schematic won't really work.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Ah ha so you shorted the B+ to ground. Often times that would blow the grid shunt resistor, the .82. Check it, I’ll bet it’s open. I’m not crazy about the aftermarket rectifier board. If there are meter protection diodes on that board ( probably are) they to might be bad. You’ve got some checking to do.

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