Question: re ....Work around to 1 meg in Heath IM-28 probe?

[Fred Olsen]

Hi Tom,

The short answer is "yes and no".  The probe resistor is there not so
much to isolate the probe from the VTVM, but to isolate the circuit
under test from the probe's cable capacitance and the meter's input
capacitance.  That is one factor in reducing circuit loading.  Think of
perhaps measuring a grid bias, as an example.  The resistor is also an
integral portion of the meter's input resistance, often 10% or so, but
the 'internal' mod will not harm that aspect if done correctly.

By the way; if one doesn't have the original probe for a VTVM it's often
not a problem.  There's only one 'simple' (not H-P) VTVM anyway: a 12AU7
and a 6AL5 (or if you and your equipment are as old as I am, a 6SN7 and
a 6H6).  Just about anybody's (Heath, EICO, B&K, Sencore, etc.)
switchable probe will work with the right value and voltage rating of
resistor, a cable of similar C/ft, and the right connector.  It's also
not that hard to roll your own probe out of chunks.

Anyone else please feel free to amplify; or tell me how stupid I am.

Fred

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"