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Old 2nd Mar 2021, 7:47 pm   #61
Radio Wrangler
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Default Re: VCM 163 No.1. Problem while calibrating

The oscillator and the meter have to remain in tune or else the meter result will fall from the peak and this will affect calibration of the Gm ranges

The tuned nature of the Gm meter, I suspect is a ploy to reduce its sensitivity to mains harmonics. It might be best if it were tuned to a frequency which is not an integer multiple of the local mains frequency, and that the oscillator is tuned to match.

David
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Old 2nd Mar 2021, 8:16 pm   #62
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Default Re: VCM 163 No.1. Problem while calibrating

Quote:
Originally Posted by Radio Wrangler View Post
The oscillator and the meter have to remain in tune or else the meter result will fall from the peak and this will affect calibration of the Gm ranges

The tuned nature of the Gm meter, I suspect is a ploy to reduce its sensitivity to mains harmonics. It might be best if it were tuned to a frequency which is not an integer multiple of the local mains frequency, and that the oscillator is tuned to match.

David
Could I seek clarification of a couple of points you make; it's the oscillator and the meter amplifier that need to be on the same frequency of thereabouts, though the meter itself plays no role in achieving that?

My recollection is that the system works up at a little over 14kHz (and may perhaps vary a little from one instrument to the next?). Are harmonics from 50Hz really likely to interfere with the ~14kHz signal?

I wonder if the day may be coming when someone may finally say, "Let's ditch those two boards and build something with modern components and incorporate a dedicated DC supply". Trading originality for reliability might be a temptation?

B
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Old 3rd Mar 2021, 8:06 am   #63
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Default Re: VCM 163 No.1. Problem while calibrating

I made a few simple tests with other circuits involving IC's but found that it would probably be easier to rewind the transformers and replace the thermistors with something modern if they ever broke.

Modern isolation IC's also required more voltages from a power supply so that it also got more complicated, some of the voltages also had to be isolated from the other due to the isolation design which meant more secondary windings (or isolated DC/DC modules).

All of this meant it cost a lot and would also take up a lot of space which made it complicated to put in the space available, I even split the power supply from the amplifier/oscillator, so I didn't go beyond some tests. Nor did I have the time I needed for proper design and tests to spend on it as my guess is that the purists would never accept anything else than the original design.
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Old 3rd Mar 2021, 4:55 pm   #64
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Default Re: VCM 163 No.1. Problem while calibrating

Yes, Martin, I take your point that the purists would never accept it.
That said, I am one of those owners who has a Gm meter which has shorted turns, so I've already had to install an op-amp driver and a small 9-0-9 psu, so I'm already in trouble with the purists.

I'm just wondering whether HBWOODY's problems are something very peculiar to his 163, or a "sign of things to come"?

B
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Old 6th Mar 2021, 9:52 am   #65
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Default Re: VCM 163 No.1. Problem while calibrating

I have removed the amplifier board from the VCM.
Can I test the amplifier board by applying 15mV across 1 and 2 with 12v across 3 and 6 with a 50 uA meter attached to 4 and 5 to see if the needle moves to the CAL position?
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Old 6th Mar 2021, 2:12 pm   #66
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Default Re: VCM 163 No.1. Problem while calibrating

Yes!

Just hook up a sinusoidal oscillator with a frequency of 15kHz across a potentiometer and adjust it to read 15mV on your Fluke 87V and then hook it to the amplifier input.

You can just put a 1500 Ohm resistor in place of the 50uA instrument and measure the voltage (mV) across it with your Fluke 87V, this frees up your Fluke 87V for other measurements, but then you'll have to divide the voltage with 1500 to get the current flowing. Now, dividing the current flowing (in uA) by 50 will give you the percentage of FSD swing, multiplying that by 6 will give you the figure of the outer scale that the needle would land on.

So for full swing you would get 50uA or 75mV which would result in 50uA/50uA*6 = 6 FSD, or 75mV/1500Ohm = 50uA which then also will give you 6 for FSD. To reach the CAL point at 4.5 you'll need 37.5uA or 56.25mV. With 37.5uA/50uA*6 = 4.5.
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Old 13th May 2021, 11:03 pm   #67
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Default Re: VCM 163 No.1. Problem while calibrating

@HBWOODY: Any news on the repairs, heard your board was working just fine and returned to you?
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Old 16th May 2021, 11:17 am   #68
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Default Re: VCM 163 No.1. Problem while calibrating

Martin
Ed kindly checked out the amplifier board and it is waiting patiently to be installed in the VCM163 I have had some surgery so the amplifier card will have to be patient for a while longer. All is going well so hopefully I will soon be inspired to start getting stuff done.
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Old 16th May 2021, 2:28 pm   #69
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Default Re: VCM 163 No.1. Problem while calibrating

Thanks for the update and I wish you a speedy recovery!
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