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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 29th Mar 2021, 4:34 pm   #21
Mark1960
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Default Re: 2 dead CBM 8032-B 'PETs'

Leon’s meter is labelled true RMS, so it shouldn’t matter if its reading AC or DC, both should be the same.
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Old 29th Mar 2021, 5:04 pm   #22
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

That's a new one on me, I've never really thought about that.

I have just been on a voyage around the Internet to find either a manual or a decent walkthrough on Youtube and I did see one video where the presenter selected 'Volts' and it immediately lit up a 'DC' symbol on the display before any voltage was connected to it.

We already know from the transformer output tests that it correctly reads AC volts, so the simplest way to reassure ourselves that it also reads DC voltage is for Leon to try measuring the voltages of a few different batteries he may have lying around, especially something like a 9V-rectangular type.

When you think about it, it wouldn't take a lot of intelligence in the instrument to notice whether the input is constantly changing polarity or staying the same, and then indicate 'AC' or 'DC' accordingly. It's just not the way we are used to multimeters working. We expect, because experience has led us to expect it, that we should have to select DC or AC volts as and when needed.

My Fluke 83 is True-RMS but it still expects me to know whether I am trying to measure AC or DC and to select the correct voltage mode range.

Last edited by SiriusHardware; 29th Mar 2021 at 5:31 pm.
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Old 29th Mar 2021, 5:32 pm   #23
ortek_service
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Default Re: 2 dead CBM 8032-B 'PETs'

True RMS is designed to cope with non-sinusoidal waveforms (rather than just assuming 0.707 x peak), but they will still have a frequency response range - particularly at the upper end but maybe also at the lower end and may not always go down to DC (which could be useful, if you wanted to measure ripple on a DC signal).


The problem with it deciding whether it should read AC or DC maybe down to ripple (which you may want to measure) on a DC supply, or if you're trying to measure a pulsing DC supply.
And they also need to ensure it is very reliable when measuring AC mains etc. voltages (I recall a software? bug in an old Fluke, that when you went over 1kV, the display went to 0V!)

Maybe my Fluke 23 is too old / lower-spec to have True-RMS, that's been on most Fluke's for quite a while - And I was surprised it read 0V on ACV, when measuring 9Vdc.

I wonder what your Fluke 83 actually reads on ACV, when measuring 9Vdc?
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Old 29th Mar 2021, 5:43 pm   #24
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

Once quick test later with a PP3 battery:-

On DC: 9.20 VDC

On AC: Nothing, a brief pulse of 'something' which suggests that there is a series capacitor in the way, subsiding quickly to 0V. I don't have a circuit or service manual for the Fluke (83 Mk 1), I have so far managed not to need one, probably a fatal thing to say.

I agree that autodetection of DC / AC could be more of a curse than a blessing when you actually do want to measure the AC component of a rough DC voltage, or the average voltage of a pulsed-DC waveform.

Everything I've read about these LOMVUM T28s suggests they do come with a manual so maybe we should just ask Leon to read that bit when he resurfaces. At the moment he will still be subject to new-user moderation which should pass after a few more posts from him.

Last edited by SiriusHardware; 29th Mar 2021 at 6:00 pm.
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Old 29th Mar 2021, 6:20 pm   #25
ortek_service
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Default Re: 2 dead CBM 8032-B 'PETs'

It does seem that most / all Fluke's read 0Vac, when measiring a DC voltage.

And I do remember one of their later (Mark V Fluke87?) ones that only has a Volts position on rotary Function switch. It defaults to ACV, so you have to press a button to change it to DCV, otherwise you get 0V. So can be a bit annoying having to ensure display is always showing DC Volts.
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Old 29th Mar 2021, 6:29 pm   #26
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Default Re: 2 dead CBM 8032-B 'PETs'

I've seen a hint that there is a manual way to intentionally select between AC and DC volts range on the Lomvum T28B, but not how. Logically, it has to be a quick press on the Function / Hold button when on the Volts position on the main dial.

The 'Big Brother' of that meter, the 28C, has a conventional looking dial with separate ACV and DCV positions on the dial.

Let's wait and see what Leon says when he comes back.
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Old 1st Apr 2021, 3:57 pm   #27
Leon_Night
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Default Re: 2 dead CBM 8032-B 'PETs'

Quote:
Originally Posted by SiriusHardware View Post
If the meter does not autodetect AC/DC then most likely you get from AC to DC mode and back with a single press of the Function / Hold button.

The Ohms / Continuity / Diode position of the meter clearly shows these functions on a circular track suggesting that these functions do have to be cycled through.

The 'Volts' position does not have a similar circular diagram with 'DC' and 'AC' on it, so my default assumption is that the meter decides for itself, but you may be right, it may have to be manually selected.
Sorry I haven't been involved in all of this, I only really get weekends to play. I just wanted to point out that the multimeter can be switched from AC to DC with the function key and I have been doing that.

Also, sorry, disregard the picture of the power connector, I checked again and it does seem that PET 1 and PET 2 have their connectors wired differently even though they are apparently identical models.

Many thanks for all the involvement that you have all been putting into this!
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Old 1st Apr 2021, 4:59 pm   #28
SiriusHardware
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Default Re: 2 dead CBM 8032-B 'PETs'

Thanks for clarifying the meter situation. Assuming it's PET-1 you are working on, can we have a picture of that machine's power connector wiring and maybe a wider view of the wiring to the capacitor / transformer block?
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