Vintage Synth Repair

From my experience, most scopes have 3 or more input modes:

AC coupled via capacitor - not good to measure DC signals
DC coupled with high impedance (megaohm) for DC and AC signals
DC coupled with low impedance (50 ohm) for DC and AC signals.

Related to the polarization issue, if the scope frame is at ground potential, but the device is at 'hot' (mains) potential (due to opposite mains polarity), then there will be a high voltage difference at the ground lead which may give a spark when connected. 

However in your case, the probe tip gave the spark, so I am not sure what is going on here.   Perhaps the scope is using the DC low impedance input setting?  

I would use a multimeter with high impedance input to measure the AC (and DC) voltages between:

 -    scope ground and equipment ground 
 - scope probe tip and equipment ground
 -    scope ground and equipment signal test point
 - scope probe tip and equipment signal test point

By noting any voltage differences, you may be able to find out why the spark is occurring.

Good luck,
Mike

--- In vintagesynthrepair@yahoogroups.com, "r_j_d_2.phila" <r_j_d_2.phila@...> wrote:
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> thanks, alan. sometimes its good to question the assumptions we have....i guess its possible i spaced and had ground and + probes swapped at the scope terminals?
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> anyway, checking away on a unit, and seeing what i should see. thanks-rj
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