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4-400 turning blue.

Discussion in 'Amateur Radio Amplifiers' started by KB4MNG, May 15, 2015.

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  1. KB4MNG

    KB4MNG Ham Member QRZ Page

    No, I had no connection regarding the 4 pin connector on the rear. Since then, I have installed a relay to switch the grid between blocking and operating bias. I have seen no change in the operation of the amp switching the relay, this includes when the tubes were going red. Going to try to track down the bias issue. Thanks
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    Just use a jumper, And measure the voltage at it, No need to complicate things.

    What controls your relay ? Does its contacts connect to the 4 pin P102 conections 2-3 and 4 ? 4 being common.

    Good Luck.
  3. KA9JLM

    KA9JLM Ham Member QRZ Page

    I think you have the proper info needed.

    Some of the stuff I brought up, Steve already pointed out.

    I do not see the problem that you are having reading the Bias Voltages. What kind of meter do you have ?

    Connecting Pin 2 to 4 on P102 should Bias the tubes off. If the voltage makes it to pin 4. Chances are there is none.

    You can read the voltage at that connection, Pin 4. I think Steve posted that early.

    If you do not have the tools, or do not feel comfortable working on Tube Voltages, Just do not do it.

    Be safe, Save some Smoke for later.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I think this was a mistake. It's best to take smaller steps and see if everything actually works prior to using relays and stuff.

    Measure the bias voltage coming from the 4-pin socket. Period. Write down that number and let us know what it is. A relay won't do anything if the bias isn't there to begin with.
  5. KB4MNG

    KB4MNG Ham Member QRZ Page

    Your right, no bias. I get a reading of .006 ac. So I'll start looking for where I'm losing it. What voltage should it be?

  6. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    What kind of meter are you using? You are looking for a DC voltage, which will be negative to "ground". If you measure with an AC meter, your reading could be normal.
  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page


    Get rid of the relay and strap pins 3 & 4 of P102 (4-pin plug) together.

    The bias at pin 3 of P102 should be -165Vdc when the MODE switch is set to CW and -75Vdc when set to LINEAR (or TUNE).

    Until you see that happen, you don't have functioning bias nor a functioning amplifier.

    The bias supply is a winding on transformer T103, rectified by V104 (6BY5GA) and then filtered and the unregulated bias is -165Vdc. In the LINEAR (or TUNE) switch position, that same bias is regulated down to -75V by V109 (VR75/0A3). If you have no bias at Pin 3 of P102, something is wrong in that chain and must be fixed. But V109 is only switched into the circuit in LINEAR (or TUNE) and is out of the circuit in CW. The amp will work without V109, but the bias voltage will be -165Vdc and the amp will only work in Class C, which produces high efficiency but also requires more RF drive.

    In LINEAR with a functioning V109, bias is reduced to -75V and the amp will be slightly less efficient but also require less drive.
  8. KB4MNG

    KB4MNG Ham Member QRZ Page

    I guess Ill first check the voltage coming from T103. Looks like the supply goes to 1, 2, and 8 of v104. About how much voltage should I expect from the transformer?

    I forgot to add in the other post, 0 dc voltage.

    Will I be able to read the voltages with only the filament switch on?

    thanks, brian
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Can't be. Pin 2 is one end of the heater. The bias voltage from the transformer T103 should go to pins 1 & 8 and should be a few hundred volts AC. However, if the screen voltage regulators are "lit" (glowing red/blue/purple or something) then the transformer T103 should be good, since they get their voltage from the same transformer secondary. So, if those are glowing, the transformer is extremely unlikely to be bad.

    Make sure the heater (filament) in the rectifier 6BY5GA (V104) is glowing. If it's not, then of course it won't produce any output for bias.

  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    If you or someone re-caped that amp, You may want to check the Polarity of C104.

    If your relay setup was shorting the Bias Supply line, Or C104 is backwards then L103 could be open.

    You may end up finding why it was Popping in the first place.

    Good Luck.

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