Dim bulb test

Discussion in '"Boat Anchor" & Classic Equipment' started by KB1FGC, May 27, 2021.

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  1. KB1FGC

    KB1FGC Ham Member QRZ Page

    I just fired up my Heathkit SB301 after installing new caps from Hayseed hamfest.I put a 150W bulb to be safe and run it for a while.So far the tubes are just dimly lit and the bulb isn't on so i hope that is good at least so far.I noticed that the S meter is pegged.That seems usual. Does that indicate a problem.Any advice?
  2. W5RKL

    W5RKL Ham Member QRZ Page

    Hi Rich,

    It appears one or all 3 of the following may be causing the S meter to peg fully CW

    1. Missing MUTE socket shorting RCA plug (required if the SB-301 is not connected to the SB-400/401 or compatible
    transmitters MUTE socket). The rear panel MUTE socket, for normal receive, must be shorted to ground. When operated
    with the SB-401 or compatible transmitter, the transmitter's internal relay provides the MUTE socket shorting to the ground.
    Remove the short and the receiver mutes and S meter pegs fully to the right.

    2. RF gain control is not set fully CW. Required for normal receive.

    3. The FUNCTION switch is set to "STBY". Normal operation is with the FUNCTION switch set to OPR.

    The dim-lit bulb indicates there doesn't appear to be any problems with the power
    transformer and/or power supply filter capacitors.

    Mike W5RKL
    Last edited: May 27, 2021
  3. KB1FGC

    KB1FGC Ham Member QRZ Page

    Mike thanks alot.I set the RF gain and it is working now.Strange because when I first turn it on the control didn't work.But the bulb I was using was defective.The filiment was weak and the radio shut down.I put a new light bulb in.I have had it on for 50mintutes now and it seems to be OK.I guess I'll try a 100W bulb next.
  4. W5RKL

    W5RKL Ham Member QRZ Page


    If the "new" light bulb is not glowing bright, it's glowing dim, and as you said, the receiver is working, then you can remove the bulb test setup.
    Replacing the "new" light bulb with a 100W light bulb will accomplish nothing.

    You should have a 3/4 amp fuse in the fuse holder which should protect the receiver if any shorts occur.

    Mike W5RKL
    WA1GXC likes this.
  5. KB1FGC

    KB1FGC Ham Member QRZ Page

    Mike thanks,I have a 2A fuse in there.t's working on 4 bands so far-hihi.Just posted the question on this forum.10 and 20meters are dead:(.But exxcellent sounds!!I'm hooked
  6. WA9PND

    WA9PND XML Subscriber QRZ Page

    A DBT is used to protect the load. If you had no issues using a 150W bulb, then you are good to go.
  7. W5RKL

    W5RKL Ham Member QRZ Page

    You have a "2 amp fuse" in the fust holder instead of a 3/4 amp fuse????? You should never do that!!
    Always use the correct rated fuse even if you have to put off powering up the radio until you get the correct rated
    fuse!!! If you don't have a 3/4 amp fuse, buy one from either Mouser or eBay.

    Mike W5RKL
  8. KB1FGC

    KB1FGC Ham Member QRZ Page

    Hi Mike.In the manuel it says the radio uses a slow blow 2 amp fuse.While I have your ear,Do you use a chicken stick and a Isolation transformer?I would like to do some tests but am reluctant now as the radio has been powered up.
  9. W5RKL

    W5RKL Ham Member QRZ Page


    I have no idea why Heathkit says to use a 2 amp fuse. That a bit high for the SB-301 especially when it only dissipates 50 watts maximum at
    120VAC main power. It's possible that Heathkit believed the initial surge in current that occurs when a
    fully discharged capacitor is applied voltage would require a 2 amp fuse. I haven't had any
    problems using a 3/4 amp fuse.

    The SB-301 is only slightly different than the SB-300. The power in watts dissipated in the power transformer is 50 watts with an AC power of
    120VAC. If you do the math you will come up with slightly over 1/2 amp at a maximum of 50 watts in the primary winding. You can calculate the fuse
    by using the formula:

    I = P / V

    "I" is the unknown primary current
    "P" is the 50 watts (not audio output power)
    "V" is the 120VAC primary AC voltage.

    50 / 120 = 0.416 amps

    0.416 x 1.5 = 0.625 amps

    The closest fuse is 3/4 amp. This will protect the SB-301 plus allow for the initial surge in charging current in the power supply
    filter capacitors when power is initially turned on.

    Here's a link that discusses "RC time Constants" relating to capacitor charging and discharging. It also includes a simple "chicken
    stick" discharging schematic. You can learn how to calculate the time it takes to charge and discharge a capacitor using the
    RC Time Constant. A good "chicken stick" will have a resistor in series with the tip and ground connection.


    I do not use an isolation transformer. Why would you use one, the SB-301 doesn't require using an isolation transformer?

    Replace the 2 blades non-polarized power cord with a 3 blade grounded power cord then remove and discard the two 0.005ufd 1.6KV
    capacitors in the power transformer's primary winding to the ground.

    Mike W5RKL
  10. KB1FGC

    KB1FGC Ham Member QRZ Page

    Mike thanks again for your help.I replaced the power cord to 3 prong and replaced the caps.I do remember the RC time constant from the 1 year tech school although not from memory-hihi. Probably should put it to use .But I didn't worry to much as I checked the fuse just to make sure they didn't install something to high.I'm in over my head.I have not one but 2 old rigs.A lot has been done but now I have problems with 2 bands not working on the 301.Plus the S meter has problems with low reading.Not sure if my equipment is correct for tesing.
    I have a:
    decent multimeter
    Tecktronix Osciiloscope.
    Simple signal generator-from school
    So not sure if that is enough for HV rigs.

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