28-05-2018, 10:06 AM
Report on yesterday travails on the Pye CTM4. The vertical amplitude control had little or no effect on the picture height. The 500Kohm control itself was found to be OK but the linearity control was found to have an open circuit track. Fortunately it was an easy fix, just a simple matter of changing the connections of the outer tags around.
A mysterious 1.2Mohm resistor was found to be wired across the linearity control, presumably an attempt to solve the excessive height fault. Tracing through the circuits revealed that C85 was missing, clipped out by a previous repairer for some reason or other. C85 is wired in series with the linearity control, it's function will be described later in this essay. A new 0.05mfd capacitor has been soldered in.
Testing the receiver: after all this work one would expect an improvement to the picture height adjustment and linearity, not so. Check those controls again, test OK. The fault was traced to the new capacitor C85 having a bad soldering joint on the cathode bias capacitor. Resoldering the tag connection has solved the problem. The height and linearity controls now work perfectly.
The linearity control works on the pre-distortion principle, the shape of the waveform supplied to the grid of the output valve is altered to correct any non linearity in the output stage.
The donor line output transformer has an isolated winding which could be employed as the source of the reference pulses for the flywheel sync circuits. However when it was connected into the circuit the low impedance primary winding of the phase splitter transformer T11 damped the performance of the line output stage. A solution to that problem will have to be found.
The second attachment shows the connections on the 819 line transformer.
The frame sync fault was traced to a leaky waxie C80 connected between the grid of V22A and ground.
Geordie McBoyne.
A mysterious 1.2Mohm resistor was found to be wired across the linearity control, presumably an attempt to solve the excessive height fault. Tracing through the circuits revealed that C85 was missing, clipped out by a previous repairer for some reason or other. C85 is wired in series with the linearity control, it's function will be described later in this essay. A new 0.05mfd capacitor has been soldered in.
Testing the receiver: after all this work one would expect an improvement to the picture height adjustment and linearity, not so. Check those controls again, test OK. The fault was traced to the new capacitor C85 having a bad soldering joint on the cathode bias capacitor. Resoldering the tag connection has solved the problem. The height and linearity controls now work perfectly.
The linearity control works on the pre-distortion principle, the shape of the waveform supplied to the grid of the output valve is altered to correct any non linearity in the output stage.
The donor line output transformer has an isolated winding which could be employed as the source of the reference pulses for the flywheel sync circuits. However when it was connected into the circuit the low impedance primary winding of the phase splitter transformer T11 damped the performance of the line output stage. A solution to that problem will have to be found.
The second attachment shows the connections on the 819 line transformer.
The frame sync fault was traced to a leaky waxie C80 connected between the grid of V22A and ground.
Geordie McBoyne.
