VCM 163 No.1. Problem while calibrating

[HBWOODY]

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?

[Dekatron]

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.

[HBWOODY]

What should be the output across 4 and 5 on the oscillator board?

[HBWOODY]

Is the only way to closely inspect the mA/V switch to loosen the eight screws and drop the front forward?

[Dekatron]

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

Originally Posted by HBWOODY

What should be the output across 4 and 5 on the oscillator board?

[Dekatron]

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

Originally Posted by HBWOODY

Is the only way to closely inspect the mA/V switch to loosen the eight screws and drop the front forward?

[HBWOODY]

Have found the chapter you mentioned and I am working my way through it, very helpful. Thank you

[HBWOODY]

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.

[Dekatron]

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.

[HBWOODY]

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.

[HBWOODY]

Released the front panel to have a better look at the ma/V switch and it looks to be in good order.

[HBWOODY]

I am back to looking at the amplifier board. Checked components and replaced out of specification components. The only thing I have not checked is the transformer T1, what are the specifications of T1.

[Dekatron]

Freely taken from AVO specifications from 1966 below!


Amplifier Transformer:

Use Universal Bridge TF 1313, Marconi, for the following measurements.

PRIMARY:

Resistance: 0,2 Ohm +/-20% at 20°C, Inductance at 10KHz 99.5uH, Q-factor: 2.4 +/-10%

SECONDARY:

Resistance: 45 Ohm +/-10% at 20°C, Inductance at 10 kc/s 99.5mH +/-2%, Q-factor: 14 +/-10%


Oscillator Transformer:

PRIMARY:

Resistance: 85 Ohm +/-10% at 20°C

SECONDARY:

Resistance: 27 Ohm +/-10% at 20°C

Voltage test: Apply 1 Volt RMS at 15 kc/s from a suitable oscillator to the primary winding. Measure open circuit secondary voltage. This should be 0.5 Volt RMS +/-5%. Use a Hewlett Packard VTVM model 400H to measure these voltages.

[HBWOODY]

I do not have any of that test equipment. If I were to remove the amplifier board is there anyone out there that would have the skill and be prepared to service the amplifier board?

[Bazz4CQJ]

Thanks for posting those transformer specs Martin. My recollection is that when I took the amp board out of my 163, the way that transformer was constructed made be wary about the way the enamelled wire of the windings looked to be stretched tight where it emerges to connect with the tinned wires that then go down to the board. My impression was that it wouldn't take much for those wires to break (i.e. be broken ).

B

[Dekatron]

You can set your Fluke 87V to the resistance range and check if the resistances are correct as a first step, you will have to remove the transformer from the PCB to get it out of the circuit so you can measure it correctly.

If the resistances are correct the rest of the values will very probably be correct too.

So unless someone has fiddled with the adjsutment screw in the center of the ferrite transformer you ought to be fine.

Since you have two VCM163s now, you could always do an experiment and feed the signal from one VCM163 to the other with just some wires from the input of one amplifier to the other VCM163 and vice versa - you'll just have to unsolder input pins 1&2 on each amplifier and then connect the wires from the other VCM163 to see how it behaves, this way you'll know if it is the amplifier or not that is the problem. Likewise you can connect the oscillator in a similar fashion from one VCM163 to the other to see if it is the oscillator or the components and switches that are the problem.

[HBWOODY]

Martin I think your last suggestion is the way forward for me. I will give it a try tomorrow.

[Dekatron]

Be careful when you do this as the connections sees the Anode voltage from the tester, so you should set the Anode & Screen voltages to their lowest voltage for this test.

You could always do it in a somewhat more complicated way by swapping the boards completely, more soldering but somewhat safer from the Anode voltage situation.

[HBWOODY]

Decided to go for the full swap of the amplifier boards with encouraging results.
I checked both amplifier boards side by side before installing the known good amplifier board in the non working VCM163 and I can now calibrate the mA/V meter needle to the CAL position. The one anomaly is that the mA/V needle reads 1.5 on the meter in all the other positions on the mA/V switch.
At last some progress. I was on the point of giving up.
Will continue my quest tomorrow.

[Dekatron]

Nice that you got it to work at the Cal. position!

Since it works in the Cal. position I'd check every soldering on the wires of the mA/V switch and the switch itself.

Also check R35 @ 2.4k, mounted at the back of the switch, I've seen several burnt out of that resistor. I've also seen unsoldered wires and dry joints on that switch too.