Drake T-4XB No Idle Current

Discussion in '"Boat Anchor" & Classic Equipment' started by K0OKS, Nov 25, 2021.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I have a B-Line that has been running fine for a few months now after a general service (replaced all the caps, checked resistors, cleaned the band switches, etc.)

    All of a sudden last night I noticed that I get no indication of idle current (bias setting) on the T-4XB. I can still Tune in Lower (x) mode and get full power output. However if I just key the PTT, or put the transmitter in Tune in upper sideband mode (per the manual) I see a reading of zero on the meter. The meter resistor was replaced with that initial service.

    Prior to last night I would always see the requisite current at idle on that meter. Now the meter does not move at all until I modulate.

    I tried adjusting the bias pot on the AC-4 of course. This made no difference. I can measure the bias voltage (-40 thru -60) all the way at pin 2 of the finals. So I know I am getting the bias voltage from the AC-4 and it is making it to the grids.

    Screen voltage is ~255V at each final tube.

    The resistances around the finals seem to match the chart in the manual, as do the voltages. (Within some variance for old equipment and modern line voltages.)

    It does not matter if I use the VFO in the Transmitter or Receiver, or the xtal mode in the receiver.

    Any clues would be appreciated. Thanks in advance.

    Edit: I do see that I am only getting an indication of about ~190mA on the meter when tuning full power. So maybe something is up with the meter circuit, but I would still think it would move a little bit at idle. Right now I don't see any movement. Also, the current level has been about that (with line voltage variance) for the past few months.
     
    Last edited: Nov 25, 2021
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It sounds more like a metering issue than an actual bias current problem given that it seems to work normally at full drive power. In addition to the meter shunt, have you checked or replaced the meter multiplier resistor as that's part of the current metering circuit?

    When I suspect current meters I'll turn the rig off, unplug it and actually inject a known current through a DMM in current metering mode to the current shunt to see if the front panel meter agrees with the DMM. The test supply can be as simple as an AA battery in series with a current limiting resistor. IOW, inject something close to a known 50mA of current through the metering shunt and see if the front panel meter reads about the same or close.
     
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Good point. I have done this before with my Swans as well (using a current limited lab supply makes it easy).

    Do you mean R38 in the attached schematic clipping? I believe I replaced R36, which was a little out of spec. I will go over all of those I suppose. It's been a few months since I looked at it closely.

    Thanks.

    Screenshot 2021-11-25 142438.png
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yeah, generically speaking that's the meter multiplier and is hand chosen for accurate metering readings. If you inject current at the top end of R36 with the negative terminal of the battery on the bottom of the meter/R36 junction shown in that schematic you can test whether the meter is reading correctly.
     
    K0OKS likes this.
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thanks.

    FWIW, R36 was literally split in half before I replaced it. I will take a look at the new one. Perhaps there is some other issue.

    The top of R36 at the non-final end up RFC2 is at about 145V when in receive mode. Drops to 0V on transmit. I think this is normal, but the voltage chart is only in Tune mode. The meter should only read a voltage difference across R36 of course. It shouldn't really care what the DC offset is. (Hence the meter does read nothing in receive as it should.)
     
  6. WA1GXC

    WA1GXC Ham Member QRZ Page

    Transmit/receive control relay routes multiple cathodes to ground on transmit.

    I'll bet you have a dirty/intermittent set of relay contacts. Depending on date of production run, you have either Clare plastic-cased plug-in relay
    or open-frame relay bolted to chassis.

    Spray the heck out of the multi-multi-pole switch contacts on the "Function" switch and cycle thru SSB-Tune-CW-AM

    If open-frame relay, spray and clean all contact sets on "Transmit" ("Make") with contact cleaner and clean high-quality paper
    If plug-in relay, replacement is available but very expensive. It's time to learn to clean it.

    Remove from socket. V E R Y carefully pry the plastic case off using two thin metal tools, one under each detent.
    Clean all contact sets with contact cleaner spray and paper.
    Re-assemble.

    Back on the air in 30 minutes.

    73
     
  7. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Thank you both for the input.

    I had cleaned the relay thoroughly to resolve some other issues.

    It turns out the issue was much simpler, though it was dirty contacts. It was simply the push/pull switch for the Plate Current/Relative Output-- the little knob you use to switch to relative output. I never use this because I have a nice big analog power meter in line. Nevertheless cleaning that seems to have fixed the issue.

    Thanks again.
     
    WA1GXC and K7TRF like this.

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