Tips for restoring Heathkit SB-200

Discussion in '"Boat Anchor" & Classic Equipment' started by N5OLA, Jul 6, 2019.

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  1. N5OLA

    N5OLA Ham Member QRZ Page

    Hello folks -- I just acquired a Heathkit SB-200 amp. It runs fine with the exception that output power is maxing at only about 400 W on CS. Specs indicate more than twice that. I haven't yet looked under the hood but interested in hearing from anyone who has successfully tackled this problem. Cap replacement needed? I'm not too confident with a multimeter but hoping this will give me a good opportunity to learn.

    Thanks,
    Will N5OLA
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    First of all, the Heath SB-200 is NOT an 800-watt output amplifier! It should put out around 600-watts on 80-meters through 20-meters, about 500-watts output on 15-meters, and about 450-watts output on 10-meters. Some get around 650-watts output on the lower bands. That is with new 572B tubes and key down CW. You can push the amplifier to more power output but you are really only going to decrease the life of the tubes and increase how much you pay the electric company. The station, on the other end, won't be able to tell the difference in your power output but the tubes, and power supply, are going to know the difference!

    The reason for the lower output on the higher frequency bands is because the efficiency starts to drop off as you go higher in frequency.

    If you look at Page 4 of the SB-200 manual, you will see that there is no power output specification given. The only power specifications are INPUT power and driving power.

    What do you mean by "CS"? In 60-years of being licensed, I have never seen that abbreviation having to do with power output from any transmitter.

    On an average reading wattmeter, on SSB voice peaks, when the amplifier is putting out the full 600-watts when tuning, you should only see from 120-watts to 150-watts. At that point, the peak output is actually around 600-watts. The analog meter just cannot follow the voice peaks.

    What power is the driver putting out? If using a "modern" solid-state transceiver, there is a good chance that the driver power is being "folded back" because of a mismatch between the exciter and the amplifier.

    Are you running the amplifier from 120 VAC mains or 240 VAC mains? Especially if 120 VAC mains the input voltage may be sagging because of the higher current draw of the amplifier. On 120 VAC the amplifier draws at least 16 amperes which is slightly more than most household 120 VAC circuits are rated.

    What is the high voltage reading when in the key up and then in the key down condition? Have you replaced the series resistors used with the high voltage reading on the meter? If not, replace them immediately because they age, over time, and the meter reading is then inaccurate.

    The SB-200 is a very good amplifier. I had one back in the early 1970s and have another one now. There are original Cetron 572B tubes in the amplifier and I have a couple more original Cetron tubes for spares.

    Glen, K9STH
     
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    This note suggests you should review working with high voltage circuitry and pay special attention to safety considerations such as methods for safely discharging the HV capacitor bank before you open up the rig.

    As posted above the rig is roughly 1000 watts DC input power (plate current when tuned and loaded properly multiplied by plate voltage) but that results in roughly 600 watts RF output power. You shouldn't expect much more than that and shouldn't tune up for max power as that implies the amp is under loaded which can create signal distortion and splatter. Typical procedure would be to tune for max power but then increase the Load capacitor setting till the power drops by 10% or so, the grid current is less than 100 mA and if you use a monitor scope in trapezoid or signal monitoring mode the waveform is appropriate (straight sides on a trapezoid and no flat topping on SSB voice peaks).

    Most likely your tubes are a bit soft with age but don't go in there and swap in new tubes unless you're 100% clear on discharging the HV supply and making sure the capacitor bank is fully discharged as that power supply could easily kill you.
     
  4. KC8VWM

    KC8VWM Ham Member QRZ Page

    The very first thing to learn about is safety.

    Even when your SB-200 amplifier is unplugged from the wall, sticking your hands inside without knowing exactly what you are doing can and will kill you.

    This is not a ham radio amplifier, but here's some basic tips to follow:

     
    N4RBZ likes this.
  5. N5OLA

    N5OLA Ham Member QRZ Page

    All the tips thoroughly appreciated. Thanks, guys. BTW meant to say CW, not CS (typo). 73
     
  6. K5RT

    K5RT Ham Member QRZ Page

     
  7. WQ4G

    WQ4G Ham Member QRZ Page

    If the Electrolytic Caps in your amp have not been changed you should go ahead and change them - all of them. Also, change all the carbon comp resistors. It also might be a good idea to update the amp with a Soft Start and Soft Key circuits. Harbach Electronics makes Soft Start, Soft Key, and Power Supply kits for the SB-200. The kits are fairly easy to assemble and install.

    Dan KI4AX
     

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