Heathkit SB401, power issue

Discussion in '"Boat Anchor" & Classic Equipment' started by KB1FGC, Jun 14, 2021.

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  1. KB1FGC

    KB1FGC Ham Member QRZ Page

    Hi all,rather than guess and try to take out components that are good I figure I will tell you all what's going on.
    I put in new caps and changed the power cord to 3 prongs.Tested the wiring to make sure it wasn't wired backwards.So I set up the rig to the dim bulb testor and my 150W bulb lit right up.So I figure something is wrong in the P.S. section somwhere.I was pretty careful about triple checking everything that was suppose to go in the correct spots when putting it together.Any suggestion of where to look?
  2. WA2CWA

    WA2CWA Ham Member QRZ Page

    Look and trace again the power supply wiring and the connections of all the parts you replaced in the power supply. Check the electrolytic caps in the bias section, those caps required plus side to ground.
  3. KB1FGC

    KB1FGC Ham Member QRZ Page

    Thanks,I should mention.The light bulb on my dimmer lit up on standby.So If I have this right,The power amp tubes aren't powered at that point so I can at least focas on the P.S. area?
  4. W5RKL

    W5RKL Ham Member QRZ Page

    You don't see to understand how the power supply works in STANDBY or other FUNCTION switch settings other than
    OFF. Here'a a brief statement what happens to the final amplifier tubers in STANDBY and when the SB-401 is not transmitting
    but powered UP.

    We will assume the FUNCTION switch is in STANDBY

    First, look at the SB-401 schematic and find the 2 final amplifier tubes (2 x 6146A).

    Find pin '3" on either final tubes, it doesn't matter which one as both final tubes are wired in "parallel" so the screens in both final tubes
    are connected in parallel.

    Now trace the wire from pin 3 of either final tubes. You will see it connects to the junction of C22 and R17. R17 then connects to relay
    RL1 terminal "4". The relay is NOT energized so terminal 4 connects to terminal "7".

    Notice terminal "7" is labeled "NC" which is short for "No Connection". Therefore the final tubes are still receiving plate and grid voltage
    BUT no screen voltage. This means there is no current flowing in the final tubes.

    Why no current flowing? It's simple.. With no screen voltage, the screens are now "negative" in respect to the "positive" potential on the PLATE.
    This negative potential presents a negative field to electrons emitted from the cathode that try to flow towards the plate. A negative screen and
    the negative electron doesn't like each other so the electron is repelled back to the cathode, hence no plate current flows in the final tubes.

    If the light bulb lights up bright with the FUNCTION switch is set to STANDBY, not "OFF", then there is a problem in the power supply
    or elsewhere. The light bulb will glow "dim" if there are no problems. I suspect you have a wiring problem in the power supply.
    The power supplies do NOT shut down in STANDBY, they continue to supply HV to the final amplifier plates, B+ to the other circuits except
    the final tube screens, grid, and filament voltage as long as the FUNCTION switch is NOT set to OFF. The final tube screen voltage is controlled
    by relay RL1 terminals 8, 4, and 7 not through any FUNCTION wafer switch.

    Mike W5RKL
  5. KB1FGC

    KB1FGC Ham Member QRZ Page

    Mike thanks,While I struggle with signal flow in details I try to get a crude concept of electronics.So that was nice and detailed for me.So I had somewhat of an understanding but not enough.I knew part of the power amp was not powered up but knew that the P.S. was powered up.I remember reading about no screen current in stand by and took that to mean the entire finals were off.How do you know all this!-lol.Wish you lived closer and could take a look at these rigs.I'll check the wiring.Not sure what I could of don't as I check so much before reinstalling.

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