23-05-2017, 05:04 PM
(This post was last modified: 23-05-2017, 05:05 PM by Geordie McBoyne.)
Connecting up the signal generator to the grid coil of the W61 IF amplifier reveals that the sound IF is 11Mc/s, that's a long way from the original EMI frequency of 4.5Mc/s.
So what's the vision IF? Before that could be determined the 33Kohm screen feed resistor of the Z66 final vision IF amplifier valve was found to be open circuit, likely the result of a leaky 0.047mfd decoupling capacitor.
That fault put right he signal generator was connected to the mixer grid. It turns out the vision IF is 14.5Mc/s, it is 8Mc/s in the original EMI circuit. As the sound IF is below the vision confirms that the local oscillator operates below the carrier frequencies. Later on I'll turn my attention to the RF and frequency changer stages.
The local oscillator is working. This was proved by connecting a voltmeter via a 22Kohm resistor to the grid of the EC52 triode. A reading of 15 volts negative was noted.
This morning more exciting pyrotechnics. lots of smoke pouring out from the underside of the chassis. This time it is one of the vision IF transformers that has broken down. Later today the transformer will be removed from the chassis for examination.
Geordie McBoyne.
So what's the vision IF? Before that could be determined the 33Kohm screen feed resistor of the Z66 final vision IF amplifier valve was found to be open circuit, likely the result of a leaky 0.047mfd decoupling capacitor.
That fault put right he signal generator was connected to the mixer grid. It turns out the vision IF is 14.5Mc/s, it is 8Mc/s in the original EMI circuit. As the sound IF is below the vision confirms that the local oscillator operates below the carrier frequencies. Later on I'll turn my attention to the RF and frequency changer stages.
The local oscillator is working. This was proved by connecting a voltmeter via a 22Kohm resistor to the grid of the EC52 triode. A reading of 15 volts negative was noted.
This morning more exciting pyrotechnics. lots of smoke pouring out from the underside of the chassis. This time it is one of the vision IF transformers that has broken down. Later today the transformer will be removed from the chassis for examination.
Geordie McBoyne.
