[GreenKeys] HAL ST-5000 adjustments

[Keith Lueck]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20190101/99f95370/attachment.html>