[GreenKeys] HAL ST-5000 adjustments

[Don Cunningham]

Keith,
 From the original article on the ST-5 in Ham Radio "With no input 
signal, or with the input grounded, adjust the pot on the limiter for 
zero volts at pin 6".   As my old mind remembers, that is a TOUCHY 
adjustment with that one turn pot, but doable if the 709 is good.  I 
hope that helps, as the ST-5000 is JUST like the old ST-5 and just as 
reliable as those were.
73,
Don, WB5HAK

On 12/31/2018 6:26 PM, Keith Lueck wrote:
> Hi guys -
>
> My usually reliable HAL ST-5000 has been giving me some trouble on the 
> 2019 calendar.  Luckily, I recorded the audio for it and I'm able to 
> replay it at will.  Anyway, I've noticed some dropouts or noise in the 
> beginning when the big, "ITTY NEWS" banner is being printed.  I've 
> noticed that the unit responds to noise on the line when no audio 
> signal is present.  Looking at the schematic, I see the input limiter 
> is a 709 opamp running open loop.  This is a tremendous amount of gain 
> and amplifies the noise on the (relatively) quiet line, which doesn't 
> seem to be a good idea to me.  OTOH, I never noticed this phenomenon 
> before - it could be due to the "limiter balance" adjustment, which 
> looks like it's simply there to counteract the 709's DC offset - I 
> imagine this is rather finicky, and is related to the age of the opamp 
> and more importantly, the temperature, due to Vos thermal drift.
>
> The manual only says that everything has been adjusted at the factory, 
> and no further adjustments should be necessary.  That might've been 
> true 36 years ago when it was manufactured, but I'm thinking it may 
> need some tweaking.  All the DC supplies look stable and noise free, 
> and I'm reluctant to go mucking about with the adjustments w/o any 
> guidance.
>
> Does anyone on the list have any wisdom on the adjustment procedures 
> for this guy, and a possible mod to reduce the input limiter gain?  
> Tacking in a feedback resistor would be relatively simple, but I'm 
> thinking the designers must've had a reason for wanting so much 
> gain...  The 709 has a typical large signal open loop voltage gain of 
> 93 dB - I'd think that maybe 20 dB would be more than adequate for a 
> reasonable audio level input.  That much more gain just sounds like 
> asking for trouble.
>
> What say ye, Greenkeys?
>
> Thanks,
>
> Keith
>
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