09-02-2020, 02:49 PM
(This post was last modified: 09-02-2020, 02:50 PM by ppppenguin.)
Got back to it today.
First I soldered extra capacitors across the line oscillator timing caps to bring down the frequency. While playing with the line period control the picture disappeared leaving a blank raster. Didn't take long to find that the -25V supply, taken from the LOP, had disappeared. The rectifier D5 (1N4820 on circuit, SF4M1 in reality) had gone open circuit. Replaced with BYV95B and it came back to life. No idea why it failed. OK the LOP voltages were all over the place but it was a 400V diode running with only 250V peak and very little current. Must have died of old age and fright.
Decided to wind down the width/EHT control which brings down the main HT. Gave a small but perfect picture on 625. Nominally +27V it was nearer 16V. Thought this would be safer for the LOP. Played with the line period pot and the picture shows the result. Horrible but things will get better. Winding up the width/EHT stopped it locking, no idea why yet.
Incidentally, I was wrong about the CRT heater current. I misread the analogue meter on my bench supply that's doing the 6.3V for now. It's 300mA, not 150mA.
First I soldered extra capacitors across the line oscillator timing caps to bring down the frequency. While playing with the line period control the picture disappeared leaving a blank raster. Didn't take long to find that the -25V supply, taken from the LOP, had disappeared. The rectifier D5 (1N4820 on circuit, SF4M1 in reality) had gone open circuit. Replaced with BYV95B and it came back to life. No idea why it failed. OK the LOP voltages were all over the place but it was a 400V diode running with only 250V peak and very little current. Must have died of old age and fright.
Decided to wind down the width/EHT control which brings down the main HT. Gave a small but perfect picture on 625. Nominally +27V it was nearer 16V. Thought this would be safer for the LOP. Played with the line period pot and the picture shows the result. Horrible but things will get better. Winding up the width/EHT stopped it locking, no idea why yet.
Incidentally, I was wrong about the CRT heater current. I misread the analogue meter on my bench supply that's doing the 6.3V for now. It's 300mA, not 150mA.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
