Re: [Amps] SB-1000 problem

[R.Measures <r@somis.org>]

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 
> brought the
> caps up on a variac fine. With the tube removed (running on 240) the 
> 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 
> blows the
> fuses (10a in each side of the line) every time.
>
> Two things come to my nearly virgin (to amps and troubleshooting) mind 
> - a
> short in the tube, which I think is unlikely;

  Brad ---  Anode to (grounded) grid shorts in late production 3-500Zs 
are fairly common due to bad spot-welds in the anode cooler.  If the 
anode looks tilted, this is likely the problem.

> or the tube just needing
> "burned in"; hooking up the filament to ionize (??) it properly (this 
> last a
> suggestion from a local ham).

  I have yet to see a gassy 3-500Z getter itself by lighting the 
filament.
> Any ideas about where to begin looking?

  The first thing I would do after pulling the mains-plug is to remove 
the tube and slap it hard into the palm of one hand.  If the anode 
tilts from the force of deceleration, the tube is kaput.  If the anode 
welds hold, I would unsolder one end of the parasitic suppressor 
resistor and measure its resistance.  If the resistance is more than 
50% higher than it should be, there is a chance that the amplifier is 
marginally stable c. 150MHz.  I would also inspect the output 
band-switch contacts.  Since the SB-1000 is quite similar to the 
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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