[Test-Equipment] Sound Technology 1710A switch problems...

[Steven Bender]

Hi again friends and Tech’s,

I’ve also got a pair of fairly clean inside Sound Technology 1710A THD / IMD Distortion Analyzers, one that has stuck and sticky switches. Its one of my priority projects.  Actually, of the two, so far, in Unit #1 I found the +17 volt power supply is hovering at around 21-22 Volts when powered up, while the other three PS (-17, +/-15 volts) measure quite fine.  This units’ array if 34 front panel pushbutton switches seem to be OK.  The meter quivers, and vibrates, and the front panel leds light up regardless of the settings of the rotary and pushbutton switches, so it doesn’t actually “work”.

At this point, I’m not sure if all the internal IC’s have been blown up by the excess voltage present (I suspect a its due to a shorted, or very low beta = failed 2N3055 pass transistor in the +17 volt PS) all four power supplies are based on the LM-725 IC and are almost identical in circuitry.  Other things that might account for the jittery meter besides blown IC’s - include a bad or open VCR2N JFet (as is present in the HP-339A - this set was designed by “EX" HP Engineers) and maybe other blown or failed diodes or parts causing the meter’s bad jitter.

ST-1710A unit #2 has yet to be powered up, as several of the bottom row of switches are jammed in and stuck (this is in the 34 switch pushbutton assembly).  The rotary switches seem fine, the Generator / Analyzer pushbutton is similarly jammed in the “in” position.  Anyone know something about a cure?  Personally, all I could come up with is maybe  removing the whole assembly (quite a job) and soaking it in 99% alcohol for an hour or so, and see if that helps or unstickys’ these parts! 

Stickers plastered all over the inside of the set warn -do not to spray the switches, but in this condition something must be done for the fault to be corrected!

So can anyone chime in on what else to try?

Otherwise, I was thinking maybe pull the working switch assemblies out of unit #1 and implanting them in Unit #2.  Basically would take a week or more, of labor, but I’d get good experience pulling the sets apart. 

Any Comments?

Can anybody familiar with the ST-17xxA Series chime in on other possible failure modes here?

Has anyone actually worked on these units who can have some insights?

Unfortunately, these sets are not identical internally due to a six switch pushbutton array of pushbuttons related to the IMD Generator present in Unit #2 (situated right next to the meter) which isn’t present in Unit #1.  So the internal circuit boards are different, also with different placements (and seemingly this is not documented anywhere).  It seems Unit #2 has an option not present in the other set, making its' repair more desirable.  Yes, I do have the ST-1710A OP / Service Manual. 

Many thanks,


Steven L. Bender