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Old 2nd Mar 2021, 8:47 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.

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Old 2nd Mar 2021, 9:16 pm   #62
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Default Re: VCM 163 No.1. Problem while calibrating

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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, 9:06 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, 5:55 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"?

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Old 6th Mar 2021, 10:52 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, 3:12 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 14th May 2021, 12:03 am   #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, 12:17 pm   #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, 3:28 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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Old 22nd Oct 2021, 10:38 pm   #70
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Default Re: VCM 163 No.1. Problem while calibrating

Any news on the VCM163?
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Old 24th Oct 2021, 2:02 pm   #71
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Default Re: VCM 163 No.1. Problem while calibrating

Martin
Have a couple of MK2's to sort out and then, with a little apprehension, I will have another try at solving the issues with the 163. Probably in a few weeks time.
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Old 24th Oct 2021, 2:28 pm   #72
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Default Re: VCM 163 No.1. Problem while calibrating

I am looking forward to it!
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Old 12th Nov 2021, 4:57 pm   #73
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Default Re: VCM 163 No.1. Problem while calibrating

Well the Avo VCM163 is back on my work bench. I have installed the amplifier card but the problem remains. I decided to again work my way through the whole circuit of the VCM. I did find the wire to tag no2 had broken inside its plastic covering which would have explained the problem but with the wire repaired the problem remained.
I now have become very familiar with the circuit and will be spending next week double checking everything.
Tomorrow I will be going through " 3.4 VCM163 mA/V measurement " document to check the voltages at each setting of the mA/V switch. I am not totally confident where and how I should make the measurements so a little help on that would be appreciated.
While learning my way round the VCM I have found a few things which needed doing so it has not been a total waste of time.
If the mA/V voltage tests are as they should be then the amplifier card is going to remain number one suspect.
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Old 12th Nov 2021, 10:16 pm   #74
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Default Re: VCM 163 No.1. Problem while calibrating

You can always set the mA/V knob to the CAL. position and check the voltage that you get across pin 1 & 2 on the amplifier board. If you get the correct voltage in that position everything is fine for the CAL. position, if not then you'll have to trace the circuit from the oscillator, via the resistors R28, 29, 30 & 31 and potentiometer RV1 via the mA/V switch S8.

If you set the mA/V control into any other position you'll have to trace a larger circuit as the signal then passes through more switches and also involves more components - you'll also have to do a proper test of a known good valve to get any readings on the gm-meter in those positions.

I've had a few 163's that have had a broken mA/V g2 switch, S7, and also a broken Electrode Selector switch, S4, that resulted in strange faults/measurements as only part of the switch wafers moved correctly.
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Old 14th Nov 2021, 4:45 pm   #75
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Default Re: VCM 163 No.1. Problem while calibrating

I did test a know good valve and and measured 55m/V across pins 1 & 2 which is what I would have expected.
I discovered a problem on the mA switch. Reading is correct in 100 position, 30 position reads about half of what it should, in the 10 position reads high. Sensing resistors on the mA switch measure good
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Old 15th Nov 2021, 11:56 am   #76
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Default Re: VCM 163 No.1. Problem while calibrating

Worked through the mA switch and there was a problem with one of the sensing resistors, another problem solved. The circuit is becoming more familiar to me.
Does the reading 55m/V across pins 1 & 2 suggest the problem is on the amplifier card?
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Old 15th Nov 2021, 2:47 pm   #77
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Default Re: VCM 163 No.1. Problem while calibrating

I don't have enough information to say where the fault lies.

Please include some photos of your measurement of the known good valve that shows the results on the front panel meters and also of the setup of the roller selectors and the front panel switches as well as of the insides of the 163 with the amplifier and back of the switches and I might be able to help out more.
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Old 15th Nov 2021, 2:48 pm   #78
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Default Re: VCM 163 No.1. Problem while calibrating

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Originally Posted by HBWOODY View Post
Worked through the mA switch and there was a problem with one of the sensing resistors, another problem solved. The circuit is becoming more familiar to me.
Does the reading 55m/V across pins 1 & 2 suggest the problem is on the amplifier card?
What was wrong with the sensing resistors?
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Old 15th Nov 2021, 8:01 pm   #79
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Default Re: VCM 163 No.1. Problem while calibrating

R12 is 12 ohms not the 8ohms it should be.
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Old 16th Nov 2021, 2:49 pm   #80
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Default Re: VCM 163 No.1. Problem while calibrating

Martin
I decided to link the faulty VCM163 to the amplifier input pins 1 & 2 of the working VCM163 as you suggested in an earlier post and moved the Anode volts to their lowest setting. With this arrangement I was able to set the mA/V meter to its Calibration point. Am I correct in thinking that this verifies that the amplifier card on the faulty VCM163 is not working?
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