Working my way back in time - Drake TR-4

Discussion in '"Boat Anchor" & Classic Equipment' started by K0OKS, Feb 16, 2018.

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  1. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I don't seem to be able to get the bias (plate idle current) set right. The power supply has been rebuilt with the HeakitShops kit. The voltages check out correctly.

    I can only get about 650mA of plate current in the Bias setup procedure (with the pot on the power supply full clockwise).

    I changed the only 2 Ohm resistor I could find. I am 90% certain it was the meter shunt ;-) However that didn't help me get the bias to set properly. I think I should check R13 as well. That seems to be some resistor in parallel with the shunt resistor whose value is "chosen at install time."

    I will check R13 and also check the PSU voltages again.

    The screen and cathode resistors are within their specs, though slightly off. I will likely eventually change those, but I need to figure out this bias issue first.
  2. K3XR

    K3XR Ham Member QRZ Page

    Was lucky to find a Drake TR-4cw RIT which had been in storage for some time the physical condition outstanding needed some electrical work alignment etc. to return to specs. As to the rebuild kits they should last for many years and easy to install.
  3. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Oh... The bias output from the AC-4 only goes from -53 to -94V. I think this need to get more positive to increase the plate current. So I need to check out the power supply. Odd since it has the rebuild kit. But I bet some old resistors were used. Will have a closer look and check the pot as well.
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I hope you mean 65mA is the maximum idle current you can get :) If you're really seeing 650 mA then your tubes are conducting wide open and operating above recommended full power plate dissipation all the time.

    FWIW, on my TR4-CW the meter bias resistor is on a small PCB next to a relay not too far from the three large sand resistors and filament fuse.

    On old rigs like this I'll turn off the power, rig a small adjustable power supply current limited with a suitable resistor and wired in series with a DMM set to measure small current up to 500-600 mA or so. With that set up slowly increase the supply voltage and compare the DMM current reading with the front panel current meter reading. If they don't agree you'll want to check the meter shunt or meter multiplier resistor (hung off the front panel Ip/Rel Pwr meter switch). I just did this about a month ago on the TR4CW and the meter shunt was off by about 30% which was enough to warrant replacement. I just used a 9V battery as the supply rigged in series with a 2 watt potentiometer and the DMM. Pretty easy to test meter accuracy.
    K3XR likes this.
  5. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Obviously I meant 65mA of current is the most I can get... not 650!

    So I found the 10K pot in the AC-4 would only go to 7K. So I replaced that. This got the bias down to -48V. However, WB8VGE, who makes the power supply rebuild kit that is installed says -70V is about normal. So I think I may need to keep looking. I forgot to check that R13.
  6. K0OKS

    K0OKS Premium Subscriber QRZ Page

    I like this. I will try this too. Thanks... Back down to the workshop...
  7. K0OKS

    K0OKS Premium Subscriber QRZ Page

    Ok. So the meter is off, even with the new 2Ohm resistor (measured with a calibrated 61/2digit Keithley).

    100mA indeed reads around 65mA on the meter. So do I change the resistors to make it read correctly, or is there some zero adjustment for the meter hidden inside the little meter box? The needle rests left of zero when off. Is that normal?
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'm not aware of a mechanical zero adjustment but there probably is one as it's pretty much standard in analog meter movements. The question is whether the meter tracks correctly throughout its operating range (~100mA - 500mA). I'd test current through that range compared to the DMM. If tracking is correct but all offset the same amount I'd consider looking into a mechanical zero. If it doesn't track I'd change the meter shunt till it does track.
  9. K0OKS

    K0OKS Premium Subscriber QRZ Page

    OK. I got the meter accurate. It is dead on at 100mA and 400mA. I did not try 500mA, but I am sure it is pretty darn close.

    But... I still only get about 65mA of plate current showing when I perform the bias adjustment.The bias pot is at its stop at this point. It keeps going up until I get to the stop. I measured -48V from the no-load power supply at this stop.

    I replaced R48. I had to destroy it to get it out; so I am not sure what its value was exactly. The resistors in red are all within tolerance.

    Any other thoughts on where to look would be appreciated. I guess I should measure the screen voltage during operation next.

    I am still not ruling out the power supply. According to WB0IQK's troubleshooting for the power bias supply this should operate at a higher (less negative) voltage.

    Attached Files:

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    Last edited: Feb 22, 2018
  10. KK6IYM

    KK6IYM Ham Member QRZ Page

    Hallicrafters used a 10 ohm 1 watt resistor in series with the HV on the PS-150-120 power supply. A DMM is connected across the resistor and the voltage is measured. 1 volt is 100 milliamps, .65 volts is 65 milliamps etc. Besides following the procedure in the manual, I seem to remember that the final output tuning should not be at resonance because it can screw up the meter reading. Have you nulled the carrier? The manual calls for 100 ma. Is the sideband switch counterclockwise? I doubt it is a problem with the power supply. Those numbers sound like they are okay.


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