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So I Bought A Swan 350

Discussion in '"Boat Anchor" & Classic Equipment' started by AF6LJ, Jul 11, 2014.

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  1. W2VW

    W2VW Ham Member QRZ Page

    I would think lack of grid bias would produce a whole lot of idling current and in the case of a pot problem plenty of power out for a short time.

    Maybe the cathode metering is flaky. Extra resistance in the cathode return would reduce power out.

    Almost makes me wish I didn't give my Eico 753 away to a new ham.
  2. AF6LJ

    AF6LJ Ham Member QRZ Page

    I have given the cathode resistance some thought...

    The cathode circuit consists of two bypass caps, two 4.7 ohm resistors a series resistor of selected value that leads to the positive meter terminal through a set of relay contacts.
    With that said with 60 ma flowing though those 4.7 ohm resistors. that is 2.35 ohms that actually measures 2.2 ohms. For one of those resistors to open the idling current reading of 60 ma would double. Unlike Heathkit these cathode resistors are two watt units, fairly beefy for the job at hand.

    If I had a tube with a screen to control grid short this would yield the same symptoms.
    No power out and high idling current.
    Same with an intermittently shorting electrolytic (C-406) (for those playing along at home)
    The driver blocking cap (C-310) being leaky could also cause the problem.
    A less likely situation a fault in the ALC input circuit could also cause the problem.
    All of which can be seen from a test point at the cold end of the PA grid choke.

    I am going to test the pot theory tomorrow. Where the idling current is set now is just above 50 ma, I am going incrementally increase it and see if the voltage jumps ether negative or positive, indicating a possible bad bias pot.

    I have the luxury of not working on the clock, which makes for nice accurate troubleshooting and almost no excuse to throw parts at the problem.

    When I worked at Loral corp we were expected to troubleshoot to the nearest stage without opening a returned unit. Yes it was possible.
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    Well the intermittent high cathode current is being stubborn, so I guess the multimeter will stay wired to the radio for a time until it fails..............
  4. W5BIB

    W5BIB Premium Subscriber QRZ Page

    Sue,... How about an up-date ???:)
  5. AF6LJ

    AF6LJ Ham Member QRZ Page

    So far so good the 350 worked well this weekend.
    There a few things left to do, I want to do something about the power supply cable.
    It sounds good on the air. :)

    Stay tuned more to follow....
    I am going to look for a new piece of cable and a new speaker to go in the power supply / speaker cabinet.
  6. W5BIB

    W5BIB Premium Subscriber QRZ Page

    :):) Let me know when you want to "take on" a 500C (works with same pwr sply).... It's been interesting following your progress.

  7. AF6LJ

    AF6LJ Ham Member QRZ Page

    I'll do that.......
    I'm a little tight for money until October, then it shouldn't be a problem.
  8. W5BIB

    W5BIB Premium Subscriber QRZ Page

    Same deal,... just shipping.
    I gotta "downsize" !!! Let me know when & I'll just ship it, ...& I'll just, "charge it to the dust, & let the rain settle it" !!!

    check PM
  9. AF6LJ

    AF6LJ Ham Member QRZ Page

  10. N2EY

    N2EY Ham Member QRZ Page

    All good news!

    Good to see a classic brought back to life and actually on the air.
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