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Old 25th Nov 2020, 7:44 pm   #1
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Default VCM 163 problem while calibrating

While going through the setting up procedure everything was going as it should until section "(t) Set the mA/V switch to Cal" the meter needle remains on the zero point on all positions of the mA/V switch. I have checked the meter and it is functioning perfectly. Is there any obvious reason why this is happening? Though it worthwhile to ask before dismantling it to check the amplifier board.
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Old 25th Nov 2020, 8:16 pm   #2
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Default Re: VCM 163 problem while calibrating

Has the cut-out relay activated so that transformer T1 is without primary voltage? If so the amplifier is not powered so the needle can't move.

Another common reason is that RV1, Cal. mA/V, potentiometer has gone open circuit so no signal from the oscillator is fed to the input of the amplifier.
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Old 25th Nov 2020, 9:14 pm   #3
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Default Re: VCM 163 problem while calibrating

I have the correct voltages on the "oscillator and power unit" and RV1 on the potentiometer board is ok. The mA/V meter needle does move a little when switching the 163 on and off
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Old 25th Nov 2020, 9:26 pm   #4
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Default Re: VCM 163 problem while calibrating

Is this the same VCM 163 that you fixed in another thread here on the forum or another one?
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Old 25th Nov 2020, 9:52 pm   #5
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Default Re: VCM 163 problem while calibrating

Yes. This was to be the final calibration.
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Old 25th Nov 2020, 10:09 pm   #6
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Default Re: VCM 163 problem while calibrating

Have you rotated the Electrode Selector switch, the Circuit Selector switch and pushed the two buttons mA g2 and mA/V g2 to see if that "frees" something up?

Can you do an ohms check across pin 1 and 2 on the amplifier board to see that they aren't shorted?

I have seen broken wafers and broken center pieces on the rotary switches making a mess of the measurements as they sometimes become open circuit and in some cases permanently short some positions.
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Old 26th Nov 2020, 8:24 am   #7
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Default Re: VCM 163 problem while calibrating

When I test a valve I get the correct readings on the M1 meter but nothing on M2.
Did an ohms check across pin1 and pin 2 on thew amplifier board and got a reading of 12ohms.
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Old 26th Nov 2020, 12:50 pm   #8
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Default Re: VCM 163 problem while calibrating

That ohms check is about right, there is a 10 Ohm resistor across 1 and 2.

The next step would be to hook up a meter across the red and brown connections on the oscillator board and see if you have the correct signal there.

Then you'll have to check if you have any signal across the output transformer of the oscillator on pins 4 and 5.

If all is correct this far you'll have to try and trace the fault in the amplifier, it could be as simple as a lose wire to/from the amplifier, perhaps to the meter, or a wire that has been pegged beneath any of the standoffs so that it has become shorted to the common ground.
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Old 26th Nov 2020, 2:00 pm   #9
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Default Re: VCM 163 problem while calibrating

I have now gained access to the amplifier and oscillator circuit boards.
First things I checked on the amplifier board were D1 and D2, one of them has failed. I have ordered some new diodes and some new transistors and intend to replace the existing ones.
I did get the correct signal at the RED and BROWN connections on the oscillator board.
I will report back as soon as the replacement parts are fitted.
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Old 29th Nov 2020, 1:52 am   #10
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Default Re: VCM 163 problem while calibrating

While you are able to check the boards do measure all resistors and the electrolytic capacitors, replacing them as needed.

I've seen a lot of resistors on these boards increasing their resistance over the years (very few decreasing their resistance) and almost all of the electrolytic capacitors have dried out working more as a high ohms resistor than as a capacitor, some with cracked/dried seals.

However if you replace any resistors, and in some cases any electrolytic capacitors, you will have to re-calibrate the tester again.
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Old 1st Dec 2020, 12:27 pm   #11
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Default Re: VCM 163 problem while calibrating

Replaced diodes D1 and D2 and transistors VT2 and VT3 on the amplifier board. On switching on the needle on the mA/V meter rose to near the CAL mark. When switching from the CAL position on the mA/V switch the needle on the meter does not drop to the zero point , it goes no lower than the 1.5 mark (1/4 scale). What does this mean?
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Old 1st Dec 2020, 12:30 pm   #12
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Default Re: VCM 163 problem while calibrating

When I turn the Ma switch to the DOT above the 100Ia position the needle on the mA/V meter does drop to zero.
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Old 1st Dec 2020, 1:06 pm   #13
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Default Re: VCM 163 problem while calibrating

What transistors did you use for VT2 & VT3?

The mA/V (gm) meter is shorted in the "dot" position on the mA switch.

Did you check all other components on the amplifier/oscillator boards and the voltage driving them?
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Old 1st Dec 2020, 1:42 pm   #14
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Default Re: VCM 163 problem while calibrating

VT2 was replaced with a BC237B
VT3 was replaced with a BC237Q (was sold to me as a BC237A)
I only checked the diodes and transistors.
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Old 1st Dec 2020, 8:11 pm   #15
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Default Re: VCM 163 problem while calibrating

I usually use BC549C for all transistors and then recalibrate the tester if necessary.

Did you check the voltage for the amplifer and oscillator to see that it has been properly smoothed by the electrolytic capacitors? They are a common fault as they dry out and don't smooth the voltage then.

Please check all wires and components so you have a chance to find the fault.
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Old 2nd Dec 2020, 1:43 pm   #16
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Default Re: VCM 163 problem while calibrating

Voltage measured across 3 and 6 on the amplifier board is 11.64 DC
Voltage measured across 1 and 2 on the oscillator board is 24.6 rms
All the capacitors checked ok on the amplifier board. Should C1 be 1uf? On the circuit diagram it looks to be marked 1.99uf.
I will check the oscillator board this afternoon.
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Old 5th Dec 2020, 12:49 pm   #17
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Default Re: VCM 163 problem while calibrating

Have checked the amplifier and oscillator board, replaced the transistor and electrolytic. Got a bit confused when checking continuity on the oscillator board as I could not get continuity from R14 to R19 and there appears to be two R14 on the circuit diagram. Needle does not move towards the calibration mark. The needle does move slightly when turning the mA/V switch and when turning on the 163.
Will now start checking continuity leading from the amplifier and oscillator boards.
Starting to loose heart but will persevere.
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Old 5th Dec 2020, 1:57 pm   #18
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Default Re: VCM 163 problem while calibrating

The second "R14" that you see is actually the component value for Thermistor 2 - "Th.2". Thermistor 1 "Th.1" has a component value of "A14". These thermistors can be found in the STC / ITT Thermistor guide I've included.

RV1 at 500 Ohm on the output from the oscillator is known to go open circuit.

The mA/V range switch which is used in the calibration position is also known to break, sometimes the center contacts damage the outer contacts and break.
Attached Files
File Type: pdf STC_THERMISTORS_BW-Adobe-OCR.pdf (2.22 MB, 7 views)
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Old 5th Dec 2020, 3:40 pm   #19
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Default Re: VCM 163 problem while calibrating

Would I be correct in assuming that the oscillator board is ok if I am able to achieve the 15mV by adjusting RV1 on the oscillator board?
Thank you for pointing out my confusion over R14 and the thermistors.
Where does the signal which is adjusted to set the CAL position on the meter come from?
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Old 5th Dec 2020, 5:36 pm   #20
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Default Re: VCM 163 problem while calibrating

If you mean that you got the 15mV across the red & brown connectors on the oscillator board, then it works ok, however you can't be sure that the output transformer is working unless you measure a signal across pin 4 & 5.

The signal comes from the oscillator board and is then passed through RV1, R28, R29, R30 & R31 and the mA/V rotary switch to the input of the amplifier board producing a signal that should make the meter move to the CAL position.
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