Oscilloscope

[Richard]

Hi

For general amp testing I usually use made-up leads to connect directly to the amp output which is only a few volts but sometimes probe for faults.

I have 2 scopes and 2 sets of x10 probes with instructions for both probes and 1 scope. The scope is German Hameg. The 2 probes are German non-branded and RS, £5 a set NOS ex-WD.

This is quite a serious consideration so I checked the equipment books and welcome any comment.

The scope has max input of 20V/cm scale so 100+Vpp can be read directly on the screen without attenuation.

The scope book says,
"If the applied signal is superimposed on a DC level, the total value (DC + peak value of the alternating voltage) of the signal across Y input must not exceed +/-500V. This same limit applies to normal attenuator probes x10, the attenuation ratio of which allows signal voltages up to approx 1000Vpp to be evaluated. Voltages up to approx 3000Vpp may be measured by using the HZ37 high voltage probe which has an attenuation ratio of 100:1."
(The x10 probe limit seems to be in reference to their own of which they show different types.)

The German probes say,
"Position x10.......Working Voltage; 600V DC including peak AC"

The RS probes say,
"x10 Position.....Max input voltage 600VDC inc peak AC derating with frequency (see fig1)"

I read all this to mean that this scope itself, unattenuated, will be OK looking at, for example, a 50Vpp AC signal on a 475VDC HT line. If these probes were then used on x10 setting this could be 100Vpp on 550VDC for example (as the scope would only see 1/10th input so the max input would apply to the probes).

[NickG]

I would guess that all that relates to the scope in AC mode, it may not be able to handle the same level in DC.

You can always cliplead a 2kv cap between the probe and the thing being probed.

Sorry, complete novice at this. Take my WAD 300B (please..........only joking), I need to find where the signal goes. As I will be touching, or potentially touching 600+ volts with the probe what sort of probe would I need?? Like I say, complete newbie and fishing around at the moment!

[Greg.]

Hi Stuart,

Max HT on the 300B is around 565v. Mine is slightly higher but never up around 600v.

Best wishes,

Greg

Yes but mine has a mysterious problem, i have to cover all bases!! Unless someone fancies having a poke inside it??

[Martin213]

I will collect Sunday PM,after Cambridge Autograss Racing,if that's ok Keith?
Not forgotten you mate,no worries

[Martin213]

Please PM contact details mate,looking forward to seeing you again

[Martin213]

P.S. i know where the 'black cat' roundabout is,if that helps

[Martin213]

Hi Keith,

You have a new PM.

Kindest Regards

Martin

[Richard]

Hi Stu, Nick

I think the max voltages mentioned relate to what is allowed to travel down the probe and into the machine; to the insulation, blocking cap and the circuit limits.

(For anyone not familiar, a scope has a switchable AC/DC input selector. You'd switch it to AC to look at AC on a line which may be carrying DC and it would then block the DC and allow you to select a sensitive input setting to view a lower level AC wave.)

For higher voltage use, as Nick says, you could connect via a high voltage cap (say a 3000V .1uF film cap) on the tip of the probe and it will block DC from entering but allow you to meter AC. The AC in a 300BPP is unlikely to exceed the equipment limits mentioned unless there is a serious(!) problem.

All this said it's only on the output stage we may be encountering voltages near the limit for ordinary probes. Checking voltages with a dmm is part of the routine so the scope won't be connected blindly and will usually be switched to, and looking at, the AC component.

Stu, refresh my memory, what's playing up in your amp?