[Amps] SB-1000 problem
R.Measures
r at somis.org
Fri May 7 23:07:40 EDT 2004
On May 7, 2004, at 6:28 AM, Brad Hedges wrote:
> I've got a Heathkit SB-1000 KW amp; uses a single 3-500z. It's got an
> original Eimac. The unit hasn't been used in about six years. I=20
> brought the
> caps up on a variac fine. With the tube removed (running on 240) the=20=
> unit
> fires up fine - the high voltage meter showing about 3350 volts.
>
> The problem comes when attemting to start it up WITH the tube. It=20
> blows the
> fuses (10a in each side of the line) every time.
>
> Two things come to my nearly virgin (to amps and troubleshooting) mind=20=
> - a
> short in the tube, which I think is unlikely;
=F0 Brad --- Anode to (grounded) grid shorts in late production =
3-500Zs=20
are fairly common due to bad spot-welds in the anode cooler. If the=20
anode looks tilted, this is likely the problem.
> or the tube just needing
> "burned in"; hooking up the filament to ionize (??) it properly (this=20=
> last a
> suggestion from a local ham).
=F0 I have yet to see a gassy 3-500Z getter itself by lighting the=20
filament.
>
> Any ideas about where to begin looking?
=F0 The first thing I would do after pulling the mains-plug is to =
remove=20
the tube and slap it hard into the palm of one hand. If the anode=20
tilts from the force of deceleration, the tube is kaput. If the anode=20=
welds hold, I would unsolder one end of the parasitic suppressor=20
resistor and measure its resistance. If the resistance is more than=20
50% higher than it should be, there is a chance that the amplifier is=20
marginally stable c. 150MHz. I would also inspect the output=20
band-switch contacts. Since the SB-1000 is quite similar to the=20
AL-80B, replacement band-switches are available from MFJ-Ameritron.
good luck
>
> I have the manuals and can trace a
> circuit, but really have no idea where to begin.
>
> Thanks,
> Brad
> K0BHC
>
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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