MacDougall
Member
Hello, I have this macplus which seems to be working perfectly except when I first start it up. I believe something in the horizontal scan circuit is going wrong in a quite alarming way. :O Here's a quick clip showing what I mean -
On the video it is quiet but in person the sound is quite loud. What you can't see in the video is that there are visible sparks discharging at the CRT. I can't quite tell if they are inside or outside the tube. Some further information -
If I disconnect just the horizontal scan from the tube this doesn't happen, no clicking, no sparks, immediate bong and a nice straight vertical line on the screen.
If I disconnect just the vertical scan, I get sparks etc, the horizontal line is not so straight, it has a small sinusoidal wave to it. This isn't noticable in the full picture but I don't know if this is normal?
Notice on the video, at first power on, it takes a while and bunch of clicks to get going, subsequent power ons take less time to settle down.
The machine appears to be working perfectly once it is going, all the analogue board adjustments are correct.
All electrolytics caps have been replaced, all yoke connectors and flyback resoldered. Flyback seems to measure correctly according to the analogue repair guide, though it's hard to be sure with such low resistances.
What I think is happening is that the horizontal scan is up and running before the CRT has warmed up causing a build up and discharge of voltage in the tube. I've read this kind of thing can happen in vacuum devices if HT is applied before the cathode has warmed up. I only know a little about <400V audio valve circuits and this isn't really a big issue there though it supposedly is what the standby switch is for. I gather the clicking is the crowbar protection in the PSU resetting. It certainly seems like it is clicking for about the time it might take for a vacuum element to heat up. What I don't know is if this indicates a CRT on the way out or if there is supposed to be some kind of warm up timer to delay the startup of the horizontal scan. I guess a drifted resistor value could cause this fault in such a circuit. Any thoughts?
It is a european voltage analogue board by the way. I have only been able to find schematics for the US version. I read the circuit is much the same after the AC step down but the layout and component numbering is just different enough to make it difficult for me to find things.
On the video it is quiet but in person the sound is quite loud. What you can't see in the video is that there are visible sparks discharging at the CRT. I can't quite tell if they are inside or outside the tube. Some further information -
If I disconnect just the horizontal scan from the tube this doesn't happen, no clicking, no sparks, immediate bong and a nice straight vertical line on the screen.
If I disconnect just the vertical scan, I get sparks etc, the horizontal line is not so straight, it has a small sinusoidal wave to it. This isn't noticable in the full picture but I don't know if this is normal?
Notice on the video, at first power on, it takes a while and bunch of clicks to get going, subsequent power ons take less time to settle down.
The machine appears to be working perfectly once it is going, all the analogue board adjustments are correct.
All electrolytics caps have been replaced, all yoke connectors and flyback resoldered. Flyback seems to measure correctly according to the analogue repair guide, though it's hard to be sure with such low resistances.
What I think is happening is that the horizontal scan is up and running before the CRT has warmed up causing a build up and discharge of voltage in the tube. I've read this kind of thing can happen in vacuum devices if HT is applied before the cathode has warmed up. I only know a little about <400V audio valve circuits and this isn't really a big issue there though it supposedly is what the standby switch is for. I gather the clicking is the crowbar protection in the PSU resetting. It certainly seems like it is clicking for about the time it might take for a vacuum element to heat up. What I don't know is if this indicates a CRT on the way out or if there is supposed to be some kind of warm up timer to delay the startup of the horizontal scan. I guess a drifted resistor value could cause this fault in such a circuit. Any thoughts?
It is a european voltage analogue board by the way. I have only been able to find schematics for the US version. I read the circuit is much the same after the AC step down but the layout and component numbering is just different enough to make it difficult for me to find things.
