Checking Bleeder resistors on Heathkits SB301/401

Discussion in '"Boat Anchor" & Classic Equipment' started by KB1FGC, Jun 4, 2021.

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  1. KB1FGC

    KB1FGC Ham Member QRZ Page

    I want to check the bleeder resistors on my radios.I think I found one at 10K R208 for the Sb301.Also 3 100K resistors on the SB401 R44,R41,R411Although I thought the bleeder would be after the recitfier diode.Excuse my small brain-lol.
    I'll will provide a schematic upload if you don't have easy access to one.
  2. KJ4YEV

    KJ4YEV Premium Subscriber QRZ Page

    I don't have the schemos, but bleeders (if it has them) would definitely be after the rectifier. Usually, directly across one of the filter caps, but not necessarily the first one.
  3. W3SLK

    W3SLK Ham Member QRZ Page

    I'm turning the 'Wayback Machine' back a few years to my Vo-tech training. The purpose of the bleeder resistors was twofold: 1)To discharge energy remaining in the power supply circuit filter caps when the unit was turned off, and 2)They were supposed to maintain ~10% of total circuit current to help stabilize voltage regulation. Usually companies kept #2 to a minimum in an effort to save $$$. Nowadays, B+ sag has been incorporated into the normal operation.
  4. N2EY

    N2EY XML Subscriber QRZ Page



    In the SB-301, R208 and R207 form a voltage divider to reduce the voltage applied to the bias rectifier.

    Why only those?

    They do.

    What's the problem that makes you want to check only those resistors?
  5. KB1FGC

    KB1FGC Ham Member QRZ Page

    Thanks for the input.I'm pretty limited knowledge.I'm not sure what a bias recitfier is,The bias I know is applied to amps, Transitors,tubes.If you were closer I would ask if you wanted to take a look at some problems I am having with my 301-lol.I got an article online about restoring the Heathkit 301.401.And the guy had advised about making sure the bleeders are working correctly.I think I got confused with the 301, as I see the bleeders are after the rectifier on the 401.
  6. N2EY

    N2EY XML Subscriber QRZ Page

    I suggest you read the parts of the manual that explain the SB-301 and SB-401 design.
  7. AG5RJ

    AG5RJ Ham Member QRZ Page

    Hi Rich,

    I agree that reading the circuit descriptions and theory of operation in the Heathkit manuals is a good step to take. One finds a lot of threads on QRZ where some "wandering" sorts of questions turn into complete confusion.

    Some power supplies, particularly older ones than found in the 1960s Heathkit SB-series, use BIG, high wattage power resistors connected from the power supply output directly to ground. These are bleeder resistors. They shunt part of the supply's output power straight to ground for the reasons W3SLK mentioned above: discharging capacitors on power-down, and regulation improvement during operation. Click this link--Unknown antique homebrew transmitter--to go to my QRZ page, and look at the lower right corner of the second picture. You'll see a vertical dark grey ceramic rod the size of a small stick of dynamite, located among four tube sockets. That's a bleeder resistor, a big one, in a power supply for a 1940s AM transmitter. A Collins 516F-2 power supply has bleeder resistors the size of my little finger, housed in a ventilated cage.

    The Heathkit SB rigs don't have bleeders to ground. They have series resistors. It goes like this: rectifier, filter capacitor to ground, series resistor, another filter cap, another series resistor, another filter cap, another series resistor, etc. The resistors are wired in series between the rectifier and the load(s), not to ground. As N2EY said, they form parts of voltage dividers that step down the B+ voltage, a notch at a time, to power different parts of the circuitry in the rig. These resistors aren't usually any bigger than 2 watts, and usually dwindle in size as you proceed down the chain, away from the rectifier.

    Might they be bad? Possibly. I haven't found a bad one yet in my Heathkit radios. You can desolder them from the capacitors and check them if you want.

    Got to run now--lightning coming.

    Alan AG5RJ
    N4FZ likes this.
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    If the voltage divider network should not only provide the proper voltage division but also eventually discharge the filter capacitors this network must end with a resistor to ground.

    The whole network then becomes the bleeder. In receivers, the voltages are
    relatively low and the filter capacitors small so the stored energy is quickly dissipated when the receiver is turned off.

    In transmitters, the energies may be large enough to present some danger quite long after turn-off, why bleeders need to be dimensioned accordingly.

    N4FZ likes this.
  9. KB1FGC

    KB1FGC Ham Member QRZ Page

    That's why I will do things,Wait a very long time and discharge the necessary capacitors using a Chicken stick.Then check with my meter.
    Thanks all
    N4FZ likes this.
  10. SM0AOM

    SM0AOM Ham Member QRZ Page

    The time constant of a realistically dimensioned voltage divider network
    in a receiver is in the order of a few seconds.

    When you have waited, say, 10 seconds at least two time constants have passed, and the voltage over the capacitors then are ten percent or less of the original values, and decreasing.

    I believe that there are more pressing things to worry about...

    N4FZ likes this.

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