Differential scope probes (Tektronics scope)

[Phil Graham]

Electronics people,

I have a trusty old Tek 2445B scope (with built in DMM). I'm looking for an affordable differential probe for floating measurements. This is for helping two friends playing with vacuum tube circuits in the audio frequency band, and to look at the behavior of some old Philco tube radios I acquired.

Trusting the ancient AC isolation input caps in the 2445B to hold the 250V+ supply voltages makes me nervous.

The frequency response and/or high frequency CMRR doesn't have to that good, I'm hoping one exists that won't break the bank if I accept limited bandwidth. It need to work reliably at 400VDC.

Thanks in advance,

 

[TJ Cornish]

Re: Differential scope probes (Tektronics scope)

The cheapest I've found is this one:
http://www.tequipment.net/CalTestCT25932.html?v=0

There are many copies of this around, available in a couple different attenuations. There are some reviews of these on the EEVBlog forum saying that they work fine. Not sure if $320 fits your definition of breaking the bank or not.

 

[Kevin Graf]

Re: Differential scope probes (Tektronics scope)

What's the voltage rating on the old probes? Most film capacitors have an absolutely huge safety margin when used in mostly DC circuits (not variable speed motor drives or SMPSs). From a film capacitor point of view a 2445B is not all that ancient.

 

[Phil Graham]

Re: Differential scope probes (Tektronics scope)

Kevin,

The old probes are rated at 250V. The 2445B is rated at 400V (dc offset + AC peak) on channels A and B.