Tek scope question (off topic somewhat)

[Pars]

Hi all,

I finally got around to picking up a scope yesterday (Tek 465B), and although I have alot of experience using scopes as a tech in the semiconductor industry, I have a somewhat off-topic question (hopefully jeffreyj is around).

I want to use this not only on headphone amps, but on some upgrading that I am doing to my Counterpoint tube preamp (SA5.1). In looking thru the specs in the manual, it states that the max input voltage (for either DC or AC coupled) is 250Vp plus the peak AC voltage, or 500Vpp. I would like to take a look at AC ripple off the filter caps in the PS sections, one which is at 425Vdc, the other which is at 250-260Vdc or thereabouts. I know the second one should be OK, but what about the first? When I was a tech, I didn't worry much about it, since it wasn't my scope <g>. Now that it is, uhhh...

TIA

Chris

 

[zzz]

if you have 10x probes, it'd be 43v and 25v respectively. just don't short the ground lead to anything (that's how my first scope died)

.

probes have max voltage ratings as well, you should look them up too, but I believe most 10x probes go to 500v with no problem.

 

[jeffreyj]

Quote:

Originally posted by Pars ... In looking thru the specs in the manual, it states that the max input voltage (for either DC or AC coupled) is 250Vp plus the peak AC voltage, or 500Vpp. I would like to take a look at AC ripple off the filter caps in the PS sections, one which is at 425Vdc, the other which is at 250-260Vdc or thereabouts. I know the second one should be OK, but what about the first? ...

Well, at the risk of giving potentially dangerous advice, I have to say that I've yet to experience a probe failure from overvoltage and I routinely subject my 300V rated ("Cat II") Tek probes to peaks in the kV range. To be fair I should mention that the scope I use 99% of the time is a portable Tek DSO that is fully isolated and built to withstand 1500V common and differential mode across any of its inputs, but I've still yet to cook a probe from overvoltage.

That said, the real problem with overvoltage applied to the probe is because most resistors exhibit a change in resistance with a change in applied voltage. Carbon resistors have the highest Voltage Coefficient, but all types exhibit it to some degree. This error begins to substantially affect the value of carbon types at around 250V, but since it is frequently non-linear, a 2% error at 250V often turns into a 20% error at 500V.

At any rate, don't worry too much about exceeding the dissipation rating of the probe's series resistor (assuming the probe is a x10, or set for x10). At 1kV input with a 9M series resistor and 1M scope input impedance the probe will have to deal with a sweltering tenth of a watt.

For this type of situation - where a small AC voltage is riding on top of a large DC voltage - you can simply connect a small film capacitor in series with scope probe to block the DC component. 0.1uF in conjunction with 10M produces a low frequency cutoff point of 0.16Hz - more than adequate for measuring the ripple in a 50/60Hz power supply

 

[Pars]

Quote:

Originally posted by zzz if you have 10x probes, it'd be 43v and 25v respectively. just don't short the ground lead to anything (that's how my first scope died) . probes have max voltage ratings as well, you should look them up too, but I believe most 10x probes go to 500v with no problem.

Thanks zzz and jeffrey for your replies. Yes, it came with two x10 probes (P6105 and P6109 I believe).

When you say don't short the ground lead to anything, do you mean don't ground the probe to the chassis (via the ground lead)? Or just be careful if the ground lead is not connected to anything to not drag it across something in the preamp while doing measurements?

Thanks,

Chris

 

[bigcat39]

Oh boy, you should try having a catI probe blow up some time it's an experience you won't soon forget.
Jefferyj has given very sound advice, just make sure your cap has a high enough voltage rating, better safe than sorry!

 

[jeffreyj]

Pars - What zzz was talking about is that line-operated scopes, like the Tek 465B (a fine scope, btw - the first "real" scope I purchased for my personal use), are grounded - ie, at the outlet. If you then connect the ground clip on the probe to something that isn't grounded, but is ground-referred, then you will short that source to ground. Fireworks often ensue at this point. The work-around for this is to float either the scope or the D.U.T. (that's "Device Under Test", btw) with an isolation transformer OR to buy a differential scope probe.

bigcat39 - Oh yeah, I know about blowing up Cat I stuff... in fact, it was from just such an experience that I immediately went to the Tektronix dealer 2 hours away and plunked down the $3000 for the fully insulated, fully Cat III rated DSO that I now love so much I can't imagine how I got along without it.

 

[Pars]

Exploding scope probes, sounds like fun...

Jeffrey, yes the scope is grounded. The DUT chassis is also grounded via a 3-wire cord, and at least for the PS (its a dual enclosure preamp), the board grounds and chassis grounds are at the same potential. This being the case, I should be OK grounding the probe to the chassis if I wish? Or would your advice be to leave it off just to be safe?

Dealing with high voltages (I'm sure to you guys this isn't that high) is something I haven't really done alot of, even when I was a tech.