16-12-2021, 10:30 PM
Another war and peace.
I had a free day, so decided to see if I have been making any measurement errors that I could find also to measure the current flowing from the secondary of the transformer to the line scan coils, so a 0.5 Ohm resistor was placed in series with the scan coils and a scope probe connected up across it.
Another scope probe was connected across the scan coils to display the applied voltages. Everything was DC coupled.
I set the vertical trace to give a reasonable height, 75mm and marked the CRT glass at the ends of the trace with a marker pen.
The scope display is here.
The straight line is my Reference Volts either the HT rail for the primary side or 0 Volts for the secondary.
The tallest trace is the voltage across the scan coils and was set to 5 Volts per division, the more rounded trace is the voltage across the 0.5 Ohm resister and was at 20mV per division. The horizontal divisions are 1 mS, so the sweep rate is a bit slower than it should be at around 3.5 mS ,running at approx 285 Hz.
Looking at this suggests 28 Volts p-p across the scan coils, with 92mV across the 0.5 ohm, so current at 184mA p-p. I think I have too much damping on the transformer, but was not sure if that could be the issue.
I re did measurements with just the DC power supply across the Line scan coils and used it to deflect the trace to the CRT marks and measured DC current and volts with a couple of multimeters. The change in DC to get the same height of deflection was 11.29 Volts and the current measured at 178.1 mA. So the current appears to be the same as expected between a line scan deflection and DC deflections, Line scan coils DC resistance at 63.3 Ohms, 63 mH.
After this I decided to play with the damping of the primary circuit, as the above was very heavily damped.
Horizontal trace now at 0.5mS per division, after resetting the scan rate to 400 Hz.
If I looked at the voltages without any damping this is what I got : -
If I damped what I would guess is still heavy but stopped any overshoot I got this with a 6K8 and 0.1uF across the primary: -
The tallest peak is the primary volts at 50 V per division, 290 V peak-peak.
The secondary is the next sawtooth (smallest trace) at 20 Volts per division, 40 peak-peak
The rounded waveform is again through a 0.5 Ohm resistor at 20mV per division showing the scan coil current at 184 mA.
The primary and secondary volts provide for nice sawtooth waveforms, but the Scan coil current which I would say is what drives the trace is not good, like in the over damped picture above.
So starting again, this time just focusing on the Scan coil current, I have a 56K Ohm resister across the primary, this stops the ringing after the initial flyback spike.
This has a totally different look, but if I neglect the flyback pulses things are starting to make a lot more sense to me looking at this.
Both primary and secondary voltages have the big flyback spikes, still on 50 and 20 volts per division for them, with the secondary spike approx 68 Volts reference, the primary must be at least 600 Volts above HT.
But looking at the secondary voltage after the initial flyback it is now down to approx 10 volts swing.
So to check I moved the HT trace out of frame and just looked at the secondary scan voltage trace and the scan coil current trace.
The voltage trace is now 10 Volts per division and the current trace still at 20mV per division.
This is showing a 10 Volts peak -peak ignoring the flyback. The current trace at 84mV across 0.5 Ohm resistor, so 168 mA Scan current for a trace of approx 75 mm. This being more what I would expect after doing the DC tests.
So the over damping in order to get good Voltage sawtooth traces was my error, it needed the pulse. I guess the higher voltage at 40 Volts peak to peak I was seeing and confusing the hell out of me, was due to the secondary voltage having to build up the scan coil current to peak then having to reverse the current as I had damped out the flyback pulse which would normally take care of it.
Adrian
I had a free day, so decided to see if I have been making any measurement errors that I could find also to measure the current flowing from the secondary of the transformer to the line scan coils, so a 0.5 Ohm resistor was placed in series with the scan coils and a scope probe connected up across it.
Another scope probe was connected across the scan coils to display the applied voltages. Everything was DC coupled.
I set the vertical trace to give a reasonable height, 75mm and marked the CRT glass at the ends of the trace with a marker pen.
The scope display is here.
The straight line is my Reference Volts either the HT rail for the primary side or 0 Volts for the secondary.
The tallest trace is the voltage across the scan coils and was set to 5 Volts per division, the more rounded trace is the voltage across the 0.5 Ohm resister and was at 20mV per division. The horizontal divisions are 1 mS, so the sweep rate is a bit slower than it should be at around 3.5 mS ,running at approx 285 Hz.
Looking at this suggests 28 Volts p-p across the scan coils, with 92mV across the 0.5 ohm, so current at 184mA p-p. I think I have too much damping on the transformer, but was not sure if that could be the issue.
I re did measurements with just the DC power supply across the Line scan coils and used it to deflect the trace to the CRT marks and measured DC current and volts with a couple of multimeters. The change in DC to get the same height of deflection was 11.29 Volts and the current measured at 178.1 mA. So the current appears to be the same as expected between a line scan deflection and DC deflections, Line scan coils DC resistance at 63.3 Ohms, 63 mH.
After this I decided to play with the damping of the primary circuit, as the above was very heavily damped.
Horizontal trace now at 0.5mS per division, after resetting the scan rate to 400 Hz.
If I looked at the voltages without any damping this is what I got : -
If I damped what I would guess is still heavy but stopped any overshoot I got this with a 6K8 and 0.1uF across the primary: -
The tallest peak is the primary volts at 50 V per division, 290 V peak-peak.
The secondary is the next sawtooth (smallest trace) at 20 Volts per division, 40 peak-peak
The rounded waveform is again through a 0.5 Ohm resistor at 20mV per division showing the scan coil current at 184 mA.
The primary and secondary volts provide for nice sawtooth waveforms, but the Scan coil current which I would say is what drives the trace is not good, like in the over damped picture above.
So starting again, this time just focusing on the Scan coil current, I have a 56K Ohm resister across the primary, this stops the ringing after the initial flyback spike.
This has a totally different look, but if I neglect the flyback pulses things are starting to make a lot more sense to me looking at this.
Both primary and secondary voltages have the big flyback spikes, still on 50 and 20 volts per division for them, with the secondary spike approx 68 Volts reference, the primary must be at least 600 Volts above HT.
But looking at the secondary voltage after the initial flyback it is now down to approx 10 volts swing.
So to check I moved the HT trace out of frame and just looked at the secondary scan voltage trace and the scan coil current trace.
The voltage trace is now 10 Volts per division and the current trace still at 20mV per division.
This is showing a 10 Volts peak -peak ignoring the flyback. The current trace at 84mV across 0.5 Ohm resistor, so 168 mA Scan current for a trace of approx 75 mm. This being more what I would expect after doing the DC tests.
So the over damping in order to get good Voltage sawtooth traces was my error, it needed the pulse. I guess the higher voltage at 40 Volts peak to peak I was seeing and confusing the hell out of me, was due to the secondary voltage having to build up the scan coil current to peak then having to reverse the current as I had damped out the flyback pulse which would normally take care of it.
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
