Heathkit SB-200
This is a set of notes on the Heathkit SB-200 amplifier.
It’s mostly for me to make sure I don’t operate the thing
incorrectly.
General notes
- It’s rated at 600W CW and 1000W PEP SSB. Anything between 500-600W is good.
- Don’t overdo it! It’ll not be acting linearly.
- A well maintained rig will output 700-800W on the lower bands but the grid current is pretty high. Don’t do it - limit drive output to whatever you need to be able to peak/load at 600W output.
- The HV should be around 2300V - if it’s not then (after carefully discharging everything) check all of the meter resistors for correct values and replace accordingly
Using it with a solid state rig
- Note the ALC voltage is .. very negative, and it could damage your radios. Kenwoods for example don’t need more than around -9v for ALC to shut down transmit.
- So either build a circuit to clamp the ALC at -9v (and realise it could swing quite a bit more, so be careful on your choice of zener) or don’t hook up the ALC line and be double careful.
- The relay circuit is also -110V DC; that’s fine for a TS-440S but other radios using transistors will be unhappy. Make sure you install the soft-key module.
- Check the service bulletin section
Service bulletins
This includes a note about ALC and using an 8v zener.
Tuning notes
From the manual:
- Set load at minimum (left), set tune at the centre of the band
- Set low output from exciter, look for around 200mA plate current on the meter
- Adjust Tune to maximise output power - this should be dipping the plate
- Adjust power up a little
- Re-adjust Tune to maximise output power - should dip the plate again
- And start adjusting Load to peak power; should be dipping plate each time
- Repeat until you hit 600W output power
- If grid current peaks above the white section (ie, 100mA) - increase load to back it off
- If plate current peaks above 500mA then decrease exciter power
Limits:
- Maximum grid current for a pair of 572Bs is ~ 100mA (50mA each)
- Normal grid currnet operation is around 70ma - 80mA
- Maximum plate current for the pair is ~ 500mA (250mA-275mA each)
- Grid current should peak when plate tuning causes a plate current dip
Maximum power issues on different bands
- Aging tubes can manifest as not being able to load down on 80 and 40m
input - the load cap is fully counterclockwise for max power and can’t
dip.
- BUT - the amp also doesn’t present a 50+j0 impedance on all bands.
So although tube transmitters will just output
full power, solid state transcievers will begin reducing power above 1.5:1
and thus you may not be outputting 100w.
- I’ve had to use my tuner on the TS-440S for 15 and 10m to stop the SWR to
be terrible, but it’s touchy - the SWR at the exciter changes as tune/load
happens so the autotuner needs to be kicked in and re-run a few times during
tuning.
AM usage
- Tune the amplifier up at 100W CW, like any other mode
- Then reduce AM carrier power down to the SB-200 outputs 100W AM carrier
- Use this; don’t key down for more than a couple minutes at a time and all
should be OK.
TODO items to investigate
- Look at what it’ll take to make a proper ALC reducing circuit - see if
ALC can be asserted above 600W to keep the amplifier from being overdriven.
- Add HV fuse, glitch resistor.
- Maybe add a series input resistor to limit the input power - ie, 100W exciter
but only 50-60W into the amplifier input tank.
- Write up something on checking the metering resistosr to ensure that they’re still
within spec.
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