[Heathkit] SB-1000

Tue, 10 Feb 2004 10:36:54 -0500

Mark, why would you consider diodes to be "poor engineering practice"?

I have used them for decades in place of the zener in many amps. A zener
is a very fragile device and subject to failure from surges, spikes, gas
arcs, etc  In the SB220 series for instance a failed zener can also take
out the filament xfmr in short order. At its 10W rating it has no
margin.

OTOH installing a reverse polarity string of robust diodes such as the
1N5408 you add another level of reliability.
Diode drift due to temperature is completely unimportant here. First the
1N5408  drifts very little in the SB220 and the bias point on the 3-500Z
is so broad so as not to affect linearity. Simply install enough diodes
to obtain the required 90-100 ma of idle current and enjoy life. In the
SB1000/AL80 series, shoot for half that.

I have confirmed the linearity with various HP Spectrum Analyzers I have
owned over the years.

Carl
KM1H

----- Original Message ----- 
From: "Mark S Graalman" <[email protected]>
To: <[email protected]>
Cc: <[email protected]>
Sent: Sunday, February 08, 2004 10:53 AM
Subject: Re: [Heathkit] SB-1000

>   Hi Charlie,
>
>   Subbing in a string of diodes will work but its poor
> engineering practice, bias regulation will be considerably
> poorer. After replacing the zener, I would also suggest
> placing a 20-50 ohm 50 watt wirewound in series with
> the B+ to the 3-500, and place a back to back pair
> of parallel connected diodes across the meter movement.
>  Diodes are cheaper than meters.
>
> 73,  Mark  WB8JKR
>
>
> On Sun, 8 Feb 2004 09:27:26 -0600 Charles W BWW Burkhardt
> <[email protected]> writes:
> > Gentlemen; I have a Heath SB-1000 amp which is giving me some
> > trouble.
> > The  7.5 volt Zener has gone bad, along with some components on the
> > meter
> > board. Someone had sent me information years ago about substituting
> > 10 1
> > amp rectifying diodes in place of the Zener. Does anyone have
> > experience
> > with this mod. Unfortunately I didn't save the original notes.
> > Thanks.
> >
> >         Charlie, K0GPF
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