Heathkit SB-221 low plate amps Help? Please.

Discussion in 'Amateur Radio Amplifiers' started by WG7D, May 20, 2020.

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  1. WG7D

    WG7D Ham Member QRZ Page

    Good morning all ! I have search the forums and have not seen anything that relates to this problem.
    A little history of the SB-221 and my problem. I bought the AMP a week ago and was told it was working good.(I did not see it in operation) (my bad) The SB-221 has the following Harbach mods: The soft start,Soft key and the metering board. They are the original Eimac 3-500Z tubes from 1982. Now a little history of me with AMPS...never owned one in my 50+ years of ham radio. So this is new territory for me (WG7D) . I have the original Heathkit manual and I am following the tuning procedure to the letter and the first problem I saw was the plate current was low (.09amp) with either 25,50,or 100 watt drive) No idle current is shown at all (zero). I am hoping I don't have a tube problem as those guys would double the cost of this amp...Any ideas AMP gurus? I will be forever indebted to you ! Thank you all ! ...........What a great forum 73, Mike WG7D
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    You didn't detail much about the setup.

    Is the amp clearly "keying" when you transmit? Sounds like it probably is, as the plate current should be "zero" before the amp is keyed.

    The .09A is low even for idling current, although it might be down in that range with the amp set to "CW" rather than "SSB."

    What does the plate voltage indicate? Does it drop slightly when you key the amplifier? (It should.) Do you see the high voltage increase when switching from CW to SSB? What does it indicate each way?

    What band have you selected to run this test? Does it behave the same way on all bands? Does the amp have the 10m modification, or is it 80 through 15m only?

    Do you see RF output power from the amplifier, irrespective of indicated plate current? With the amp "OFF," does your transmitter power (up to 100W) appear at the amplifier OUTPUT (antenna connection)? (It should.)

    When you drive the amp with "25, 50 or 100W," is that CW key-down drive power and actually indicated someplace or just a "selected" power on your transmitter?
     
  3. WG7D

    WG7D Ham Member QRZ Page

    I will try to give some more details . Yes the relay clicks when keying and the plate current is zero ,then 90 ma after key down. CW HV at 2400 .SSB HV at 3000.
    Only have tried 20 and 40 meters so far. 80-15 meter only amp (no conversion) I will try to measure power out of the 7300 to the amp,I have not confirmed that yet.
    Thanks for your help STEVEN ...looking at your QRZ page looks like you are a busy guy!
    Mike WG7D
     
  4. WG7D

    WG7D Ham Member QRZ Page

    Oh..one question more STEVEN...The relay in this amp looks like the contacts are a little burnt..could that cause these symptoms? If so ,what would
    be the best method to clean those relay contacts?
    Thanks again, Mike WG7D
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Can you answer the other questions posted, one by one? Read each one, they address different things.
     
  6. WG7D

    WG7D Ham Member QRZ Page

    Ok . I will do that.

    Is the amp clearly "keying" when you transmit? Sounds like it probably is, as the plate current should be "zero" before the amp is keyed. (yes its keying)

    The .09A is low even for idling current, although it might be down in that range with the amp set to "CW" rather than "SSB." (was set at CW tried RTTY also)

    What does the plate voltage indicate? Does it drop slightly when you key the amplifier? (It should.) Do you see the high voltage increase when switching from CW to SSB? What does it indicate each way? (Plate voltage CW 2400 v.SSB 300 volts) (yes drops to 1800 v on transmit) (on transmit SSB mode drops to 2400 volts)

    What band have you selected to run this test? Does it behave the same way on all bands? Does the amp have the 10m modification, or is it 80 through 15m only? (ran 20 meter and 40 meters so far) and same HV readings)

    Do you see RF output power from the amplifier, irrespective of indicated plate current? With the amp "OFF," does your transmitter power (up to 100W) appear at the amplifier OUTPUT (antenna connection)? (It should.) (some small indication of power out) (with AMP off I see full 100 watts at the output of the amp. )

    When you drive the amp with "25, 50 or 100W," is that CW key-down drive power and actually indicated someplace or just a "selected" power on your transmitter? ( yes I have an inline watt meter)

    Hope that clears it up a bit. Mike WG7D
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Sounds like you have very bad HV regulation. Something is caving in under load.
     
  8. WG7D

    WG7D Ham Member QRZ Page

    What to check in that case please?
    Thanks! Mike WG7D
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree, but also seems the Ip meter isn't registering properly. No way the HV can drop from 2400 to 1800V without substantial current being drawn, somewhere...unless the power supply is seriously degraded.

    My first recommendation would be to contact the seller who previously had this amp and ask him about it.
     
    WG7D likes this.
  10. WG7D

    WG7D Ham Member QRZ Page

    I am doing that and he is helping with the troubleshooting. Thanks!
     

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