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Drake TR-4 has a T/R problem...

Discussion in '"Boat Anchor" & Classic Equipment' started by KG5TW, Oct 12, 2021.

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  1. KG5TW

    KG5TW XML Subscriber QRZ Page

    My old Drake TR-4 has recently developed a problem where, after being on for an hour or so, it (or part of it) goes into what seems like a transmit mode, or fails to come out of transmit mode and back into receive mode. The receiver is quiet but there is no output power and the plate current is pegged. Interestingly, if I put it into Tune mode, it behaves normally with normal power output and normal plate current.

    Is this a transmit/receive relay issue?

    Any and all advice welcome and appreciated.

  2. WA1GXC

    WA1GXC Ham Member QRZ Page

    The plate current should never be pegged, under any circumstances. If you see that, don't try to trouble-shoot the situation--don't start switching modes, adjusting anything. By the time you figure
    out what's going on, you'll have 3 6JB6 final amp. tubes with burned-up screen grids. $100 gone. IMMEDIATELY select power-OFF.

    Things to check:

    1. Before you even start to dig-around your TR-4 innards or check voltages, disconnect and overhaul your AC-3 or AC-4 power supply. Whether you think it needs it or not.
    Your bias-supply circuit is no doubt weak and shot, with dried-out or leaky filter capacitors. replace C5A C5B and D7.

    AC-3 and AC-4 are essentially identical

    2. The relay-control tube has gone flaky. 6EV7 will work perfectly for years, then in the space of 5 minutes go nuts. Weak/gassy. Don't bother checking in a tube tester even of high quality.
    Direct replacement with 1 or 2 substitutions is only valid check.

    3. After all that, check numerous tubes in transmit circuits for internal shorts. Your TR-4 manual has a function chart for which tubes have function in "Transmit" mode. Many
    have TX or RX function only, so can be ruled-out.

    $1 says you have a weak and unreliable bias supply. That means you have also cooked numerous resistors. Check the values and remove & replace, as necessary, the 68ohm screen resistors and
    15ohm cathode resistors on the P.A. Final tubes. $2 says they're more than 25% higher than proper value.

    Good luck.

    Last edited: Oct 13, 2021
    W7UUU likes this.
  3. W7IMM

    W7IMM Ham Member QRZ Page

    $10 says you are SPOT ON! thumb.gif
  4. KK6IYM

    KK6IYM Ham Member QRZ Page

    The 2 ohm meter shunt resistor usually drifts high. Make sure it is correct before setting the bias or you will be off.

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Some good suggestions above if your original post is 100% accurate.

    But be sure it's the cathode current meter that "pegged" and not the S-meter. I only mention that because your description indicates "the receiver is quiet."

    If the RF gain is turned down or the receiver AGC has a problem, the receiver will certainly become quiet and the S-meter can "peg" full-scale. If that happens, it's an entirely different problem than the cathode current meter pegging.:)
  6. KG5TW

    KG5TW XML Subscriber QRZ Page

    All great stuff, guys. Thanks a heck of a lot. I'm boxing it up and sending it to Ron for a complete overhaul. It has needed it for a long time and I have neither the time or know-how to do it myself. He is also going to rebuild the power supply.
    WA1GXC likes this.
  7. WA1GXC

    WA1GXC Ham Member QRZ Page

    Nothing wrong with that. Time for many is something not in long supply. Acknowledging one's capabilities is the safe thing to do and smart for all kinds of reasons.

    Ron WB4HFN is a real gentleman and very honest. . I have never had him do repair work, but the depth and extraordinary generosity of his knowledge, as shown on his carefully produced
    and researched website, is amazing. Nowhere, nowhere, does he pitch or promote his repair business. The expertise is free for the viewing.

    For all those Drake owners or prospective Drake owners who are interested, two essential websites:
    Just Google--

    "WB4HFN Drake"
    "VE3EFJ Drake"

    Unlike most of the crap on the Internet, these two gentlemen actually know what they're talking about.

  8. KG5TW

    KG5TW XML Subscriber QRZ Page

    That is very reassuring to hear. I'm a mechanical engineer and pretty handy with most things. I've even done some light repairs on this rig to include a few tubes but it is like an old friend and it needs a very proper going-thru and that is way beyond me.

    I appreciate everyone who contributed advice here. Super helpful and I do not take it for granted.

  9. W3SLK

    W3SLK Ham Member QRZ Page

    Let me vouch for Ron, WB4HFN here. He helped me out a lot when I was fencing with an L-75 amp. He sold me capacitance boards for the same price he bought them 30 years ago! Great info on his site too!

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