Dumb Scope Probe Q...

[Justin Thomas]

Hello all,

I'm ready (I think... I hope...) for the next step, as I'm having to hunt down noise & oscillations in some of my amps. I've been given a Heathkit IO-4235 35MHz scope, and can get a dot and move it around. I've read what I can find here about buying probes. So, here's what I'm wondering.

My scope manual says it'll handle 400V. I'm confused about probe ratings. Given that I could see about 1000V worst-case in a cranking 100W amp, should I get 100:1 1000V+ probes? I'm just learning, I'm probably gonna screw up, and probably won't score another free scope! So say I measure the B+ of my Bassman 100 with my 1KV 100:1 probe. My scope only sees ~4.5V, correct? If I were using a 10:1 probe, it'd be 45V... Or, as it's an "older" piece of gear, it might be a bit more robust, and a rapid transient probably won't kill it? It's dual trace/channel, so once I figure it out, I can get another finer probe for the other channel, I'm thinking... most probes in 10:1/1:1 seem to handle 600V in 10:1 & 300V in 1:1. I don't mind if a probe blows up, but I don't wanna blow up my scope!

Thanks,

Justin

[eschertron]

I scored a free scope many years ago, and with it probes. It was just earlier this year I fried a probe while it was on the plate of a new build.

So I have just gone through this. Yes, the 10:1 or 100:1 will reduce the voltage seen by the scope; the limiting factor then becomes the probe itself. the probes I just bought are rated at 600v ONLY with the 10:1 switch engaged, and are good to 200v in 1:1 mode. Check the specs on the probes that you're looking to buy, as it looks like you have.

[Justin Thomas]

Thanks, that's what I was wondering! I figure one of each for each channel will do me good...

Justin

[g1]

Something about still being limited to the 400V scope rating when on DC coupling, regardless of probe?
Fuzzy on the details, I'll try to find the link.

[g1]

Here it is, it took me a while to wrap my head around it, the problem occurs on AC coupling, not DC:
http://music-electronics-forum.com/t31492/#post285463
(see also post #15 in that link)

[Justin Thomas]

I'm still trying to wrap my head around that thread... I'm okay "in theory..." actual math not so much. Especially impedance. I have the manual; I'll hafta wait tilI have my laptop to check the specs in further detail... I know there's lots of scopes out there being used for tube amps with less than 400V capability, so I know there's a way around this! Even if I hafta rig up something.

Justin

[g1]

The problem occurs when you are on AC coupling looking at something with a high DC component. For example, checking AC ripple on your B+.
With the scope set to AC coupling, there is a cap in series with the input. This blocks any DC current, so the voltage divider only acts on the AC, not the DC. To DC it is open circuit, and voltage in an open circuit is the same at all points.
So, for example, you have 500VDC B+, and you want to see the AC ripple on it. Your input coupling cap on your scope (rated 400V) gets charged up to 500VDC. Not good, fireworks may follow.
I believe there are probes that get around this issue by having an internal resistor to ground, but it is not the standard 10X type probe.

[olddawg]

100x P4100 High Voltage 2KV 2000V Oscilloscope Scope Passive Clip Probe 100MHz | eBay

[Mick Bailey]

I made up a high voltage unit with a resistive divider string with the bottom end grounded. That way the scope only sees the voltage at the divider point. I sized it to run to 10kv (1000:1) so I can measure HT off my DMM too. Works really well, though the stacked tolerance of the resistors affects precision. When I'm probing high voltage circuits It's generally just the waveform I'm interested in and not any absolute measurement, though.

[Justin Thomas]

Hey all,

Thinking what I'll do is get a set of each 10:1 & 2kV 100:1. I've got two channels, right? One for each and a spare. Just don't forget to switch em... And I'll keep reading; eventually I'll get it.
Planning to buy beginning of the week, but from Amazon. I'm mostly gonna be doing my 50W or less to learn on, Bassman is oscillating on Normal channel and still have that Jet that's losing signal somewhere...

Thanks again!

Justin

[Justin Thomas]

Opinions?

For x1/x10:
Hantek 2 X 100MHz Oscilloscope Clip Probes with Accessory Kit: Amazon.com: Industrial & Scientific

Or

Hantek 2 X 60MHz Oscilloscope Clip Probes with Accessory Kit: Amazon.com: Industrial & Scientific

And for 100:1
Hantek 2 X 60MHz Oscilloscope Clip Probes with Accessory Kit: Amazon.com: Industrial & Scientific

Thanks!

Justin

[Justin Thomas]

Hey guys,

I know I'm a bit slow, but I just ordered probes today, the second two above. Hopefully this opens a new chapter in my building/repairing hobby, noisekilling, and reduce the number of fires I start (just kidding on that last part!). Thanks again to all who have helped (and continue to do so) me out over the years, knowj or unknown. And here's to finally getting that Jet outta here...

Justin

PS I did manage to get the Bassman to quit oscillating - I had inadvertently moved some wires around last time I was in it.

[TomCarlos]

Justin...

Find yourself a couple YouTube videos... some of them offer good explanations of O probes. I too will review these as a refresher on the subject.


Oscilloscope operation 10X probe compensation

#9: Basic 1X and 10X Oscilloscope Probe tutorial

Footnote... the second video has a link to a document that shows the notes used in the video.