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Old 6th Dec 2020, 7:11 am   #21
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Default Re: VCM 163 problem while calibrating

Would it be possible to temporarily bypass the mA/V rotary switch to verify whether the amplifier board is working?
What signal should I be getting across the output transformer pins 4 and 5?
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Old 6th Dec 2020, 2:12 pm   #22
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Default Re: VCM 163 problem while calibrating

Please read chapter 3.4 on the VCM 163 mA/V measurement circuit here: https://www.vintage-radio.net/forum/...ad.php?t=86262

Now you'll see that the oscillator signal is too high for the amplifier input, some 81mV where maximum input should be some 51mV so you'll need to keep RV1, R28-R31 in the circuit to get the CAL voltage at the input of the amplifier, or replace them with something similar.

If you look at the circuit diagram you will see that you can simply bypass the mA/V switch with some wires from the left side of R31 & R28 (left as in the left side in the circuit diagram) to the amplifier inputs - R28 left side to pin 1 on the amplifier and R31 left side to pin 2 on the amplifier.

However the mA/V switch can still affect this so best is to unhook 4 & 5 on the oscillator and 1 & 2 on the amplifier and make your own voltage divider that you hook up between the oscillator and amplifier.
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Old 8th Dec 2020, 12:45 pm   #23
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Default Re: VCM 163 problem while calibrating

What should be the output across 4 and 5 on the oscillator board?
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Old 8th Dec 2020, 2:27 pm   #24
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Default Re: VCM 163 problem while calibrating

Is the only way to closely inspect the mA/V switch to loosen the eight screws and drop the front forward?
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Old 8th Dec 2020, 4:53 pm   #25
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Default Re: VCM 163 problem while calibrating

All voltages and circuit description is in the chapter I mentioned above.

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What should be the output across 4 and 5 on the oscillator board?
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Old 8th Dec 2020, 4:54 pm   #26
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Default Re: VCM 163 problem while calibrating

Unless you have a very good cameraphone to stick inside the tester that is the best way.

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Is the only way to closely inspect the mA/V switch to loosen the eight screws and drop the front forward?
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Old 8th Dec 2020, 7:45 pm   #27
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Default Re: VCM 163 problem while calibrating

Have found the chapter you mentioned and I am working my way through it, very helpful. Thank you
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Old 9th Dec 2020, 9:03 am   #28
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Default Re: VCM 163 problem while calibrating

Have measured output across 4 and 5 of the oscillator circuit and get a reading of 81mA/V. Does this mean that I should get readings at the input 1 and 2 of the amplifier circuit of 5.1 mV with the mA/V switch set to 0-6mA/V range, 15.2mV with mA/V switch set to 0-20mA/V range, 50.5mV with mA/V switch set to 0-60mv range.
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Old 9th Dec 2020, 9:55 am   #29
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Default Re: VCM 163 problem while calibrating

No, these values are used when you are checking a valve and applied to the grid voltage, the amplifier never sees them directly, only indirectly as it measures the amplified voltage in the anode circuit.

However in the CAL position you should see close to 15mV across 1 & 2.
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Old 30th Dec 2020, 1:55 pm   #30
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Default Re: VCM 163 problem while calibrating

With a valve under test I get a reading of 51mV across 1 and 2 of the amplifier board when the the ma/V switch is at positions 0-6, 0-20, 0-60. At the Cal position on the switch I get a zero reading.
Will now release the front of the 163 so I can have a closer look at the ma/V switch.
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