Author Topic: help with oscilloscope model 34 (Read 8945 times)

Slowpoke101 · « **Reply #100 on:** October 16, 2018, 01:44:04 AM »

Not wanting to cause any confusion but I find the measured resistance of R63 to be very high even with age drift.
I suggest making sure that no power is applied and all caps have discharged, lift one leg of R63 and measure directly across it. Remember to resolder the resistor back in position when finished.

Word of warning - if R63 is open circuit, C30 may not have discharged. Wait at least 10 minutes after power off and then carefully measure for voltage across C30. Put a 10M resistor in series with the multimeter's positive test lead to help protect the meter just in case C30 is fully charged.

Rob Strand · « **Reply #101 on:** October 16, 2018, 01:58:55 AM »

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Not wanting to cause any confusion but I find the measured resistance of R63 to be very high even with age drift.

No confusion. It's bad for sure. There may be more.

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Word of warning - if R63 is open circuit, C30 may not have discharged. Wait at least 10 minutes after power off and then carefully measure for voltage across C30. Put a 10M resistor in series with the multimeter's positive test lead to help protect the meter just in case C30 is fully charged.

I think whomeno needs to build a simple HV probe. Simple in the sense of handling 2kV and not 20kV to 40kV like some.

I haven't suggested 10M in series with the meter because common 0.25W to 0.5W resistors, the ones about 6.5mm long, are only rated for 250V. For 1500V we really need at least 6 resistors in series covered with a few layers of insulation/heatshrink.

The voltage rating issue also brings up problems with replacing the resistors. You really need to replace the resistors using high voltage rated ones not the common garden electonics store stuff.

Rob Strand · « **Reply #102 on:** October 16, 2018, 04:07:39 AM »

Here's a simple 1.5kV Probe which will *just* do the job.
The circuit has a 60M ohm input impedance which shouldn't load the circuit much.

The earth clip connects to the circuit's 0V/Earth. Nowhere else. You cannot measure across resistors.

The probe touches the point you want to measure.

The resistors are through-hole types and need to be at least rated to at least 250V. Those miniature resistors with body lengths around 3.5mm are too small and won't be 250V. The ones with body lengths longer than 6.5mm are usually 250V. Higher voltage resistors are of course much better is you want to buy them.

Connecting the resistors like this will do,
https://forum.makehackvoid.com/t/1000-1-hv-probe/230

Here's another way to do the resistors but it needs to the tube to make it stiff and not put mechanical stress on the solder joints,
https://www.instructables.com/id/DIY-HIGH-VOLTAGE-MULTIMETER-PROBE/

whomeno · « **Reply #103 on:** October 16, 2018, 07:50:21 AM »

Rob,
how many 10m ohm would I need to be able to measure the mains. I am looking at building an amp in the future .If I am going to build one I want to do it right. After the heat shrink, I could put it in a pvc pipe and seal it

whomeno · « **Reply #104 on:** October 16, 2018, 11:05:41 AM »

1.5kv probe done.

Rob Strand · « **Reply #105 on:** October 16, 2018, 05:14:54 PM »

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how many 10m ohm would I need to be able to measure the mains. I am looking at building an amp in the future .If I am going to build one I want to do it right. After the heat shrink, I could put it in a pvc pipe and seal it

It's got more to do with the construction. Technically, these days you probably shouldn't measure mains inside equipment unless the meter *and leads* have at least at Cat II rating and a suitable voltage rating. For fuseboxes etc you need Cat III or above. The meters before about 1998/1999 don't tend to have markings, not even Flukes, and you need to decipher the manual. A lot of cheaper meters and probes don't comply to the Cat ratings.

https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

So you should only connect the 1.5kV probe to the non mains inside of equipment.
For testing HV inside of equipment, if the earth clip comes off the ground the 6x10M's will limit current to a safe level. For example, Current I = 1.5kV /(6*10MEG) = 25uA. This is would even pass the leakage test for medical equipment.

whomeno · « **Reply #106 on:** October 16, 2018, 05:25:15 PM »

ok, so what do I need to check now?

Rob Strand · « **Reply #107 on:** October 16, 2018, 05:27:05 PM »

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1.5kv probe done.

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ok, so what do I need to check now?

Looks pretty good.

The probe part looks a bit long you might want to slip some heatshrink over the end so only 1/2" (12mm), or less, of the tip is expose. That will prevent accidental shorts on other stuff.

The thing to do now is to test the probe on low voltage then gradually move up. If you find a problem don't continue with the higher voltages.

- Measure a 9V battery without the probe, then measure the 9V battery with the probe. You expect 9/1000 = 9mV.
- Measure a 100V supply without the probe, then measure the 100V supply with the probe. You expect 100/1000 = 100mV.
- Measure a 300V supply without the probe, then measure the 300V supply with the probe. You expect 300/1000 = 300mV.
- Measure a 500V supply without the probe, then measure the 500V supply with the probe. You expect 500/1000 = 500mV.

These don't have to be exact voltages the main point is to get some coverage across the range. Your meter provides the reference. For the higher voltages try to measure the actual power supply voltages not things like tube anodes. The reason is, the tube anodes have high value resistors and your meter will give you a false low reading because its 1M input impedance loads the circuit.

Record the points. If you can, record the value of the resistance feeding the supply rail. You will find the probe might be a little off (maybe +/-5% to10%) because of the resistor tolerances and possibly because your existing meter is loading the circuit.

There's a leakage test but I have to see what your meter can do.

Rob Strand · « **Reply #108 on:** October 16, 2018, 05:35:54 PM »

Actually there are more basic tests to do first:

Using the ohms scale on your meter:
- Measure the resistance between the DMM- lead and the Earth clip. Should be less than 0.5ohm.
- Measure the resistance between the DMM- lead and the DMM + lead. Should be 63.7k +/- 5%.
- Measure the resistance between the Probe TIP and each of the other leads. Wait about 30 seconds between each measurement. It should be about 60M ohm +/- 10%. However your meter might not provide a measurement. If it shows overload, and you have made good contact, then it is at least 20M.
A leakage test will help measure this.

whomeno · « **Reply #109 on:** October 16, 2018, 05:37:33 PM »

battery 9.55v
probe .008

203 volt
probe 0.19 on 20 on meter

346 Volt
probe 0.2 set on 200 on meter

428volt
0.3 set on 200 on meter

whomeno · « **Reply #110 on:** October 16, 2018, 05:44:21 PM »

ok
gr and earth =.53 set on 20 on meter
DMM- lead and the DMM + lead. = 62.9
dimm- and tip =61.7
dimm earth and tip = 61.7
dimm + and tip =61.7

Rob Strand · « **Reply #111 on:** October 16, 2018, 05:46:52 PM »

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DMM- lead and the DMM + lead. = 62.9

Is that kilo ohms?

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dimm- and tip =61.7
dimm earth and tip = 61.7
dimm + and tip =61.7

Is that Mega ohms?

Rob Strand · « **Reply #112 on:** October 16, 2018, 05:49:27 PM »

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battery 9.55v
probe .008

203 volt
probe 0.19 on 20 on meter

346 Volt
probe 0.2 set on 200 on meter

428volt
0.3 set on 200 on meter

Ah, your meter is manually ranged.
For accurate results the meter needs to be set on the 2V range when used with the probe.
With the probe, a reading of 1.2V on the meter now means 1.2kV as the probe divides the voltage by 1000.

whomeno · « **Reply #113 on:** October 16, 2018, 05:52:13 PM »

meter set on 200m 61.7 mohms

whomeno · « **Reply #114 on:** October 16, 2018, 06:00:22 PM »

9.55v
probe = .008

202volt
probe = .197

360 Volt
probe =.361

415 volt
probe =.403

Rob Strand · « **Reply #115 on:** October 16, 2018, 06:20:33 PM »

Those are much better.

I calculated the measurement errors below so they are within say +/-3%.
If the reference meter is loading the supply at all then the true error could
be a bit more in the negative direction.

Meter UTL33T manually set to 2V
Ref Meter   1.5kV probe   Error %
202V     197V    -2.48%
360V 361V     0.28%
415V     403V     -2.89%

OK we need to do a leakage test.
I'll draw-up the set-up.

Rob Strand · « **Reply #116 on:** October 16, 2018, 06:22:10 PM »

For convenience I've linked your UTL UTL33T meter manual:
https://inspectusa.com/uei/UTL33T-PS.pdf

whomeno · « **Reply #117 on:** October 16, 2018, 06:33:35 PM »

is there a little better meter out there that will not kill my wallet?
This one was 19.99 years ago
What would be a good choice under 50.00

Rob Strand · « **Reply #118 on:** October 16, 2018, 06:51:57 PM »

Here's the leakage test (click to enlarge),

Rob Strand · « **Reply #119 on:** October 16, 2018, 07:05:01 PM »

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is there a little better meter out there that will not kill my wallet?
This one was 19.99 years ago
What would be a good choice under 50.00

I'm scared to recommend any $50.00 meters as they vary enormously
and to be honest many are probably of poorer quality than what you have.
There's a stack of meter shootouts on youtube's EEVblog channel.
Unfortunately they are a bit dated not. The general conclusion is the $50 meters
aren't great and you need to spend $100.

A lot of $50 meters don't have gold switch contacts. Also the switch and meter contacts fail quite often.
I have seen this on some of the cheaper meters at work.

Beyond the quality issue is the features you want from a meter. This will further limit the choices.

I have a $30 meter which I bought for $10 20 years ago. I use it for kamikaze jobs.
The switch contacts are all gold and it has survived this long.
The supplied leads are totally crap and the type of leads aren't a modern style.

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Author Topic: help with oscilloscope model 34 (Read 8945 times)