[Amps] SB-1000 problem

[R.Measures]

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