[Heathkit] SB-1000

[Mark S Graalman]

  Hi Carl,

  I have never checked as far a linearity, but I always found
that regulation was poor at low current levels. Something I did not
think about however is the simple fact with the presence of
idle current, there would never really be a point of real low
current draw. At any rate I do see and understand your points,
and I'll stand corrected. I do wonder however, I have never
had a zener fail in any of my amps during any kind of a fault, but I
have always shunted the bias zener with a 100 ohm 10 watt resistor,
maybe that makes the difference.

Mark  WB8JKR


On Tue, 10 Feb 2004 10:36:54 -0500 "Carl" <km1h@jeremy.mv.com> writes:
> 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" <wb8jkr@juno.com>
> To: <cburk001@juno.com>
> 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
> > <cburk001@juno.com> 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