I'm new on the forum and have been helping Lastspaceship trying to get this MTC9000A in Robotron up and running.
We've had R187 smoking almost immediately upon switch on, after about 20 seconds it burns out. I've attached a section of the schematic that shows R187 and the voltages measured either side of it.
I was surprised to see -1100V on the flyback side of R187, I didn't realise my multimeter could even read that high, it also explains the R187 smoke! Would I be right that the normal voltage at that point should be between 0 V and -190V, nominally -50V ish depending on where brightness preset RV106 is set?
I'm thinking that the only way we could get -1100V here is if there is a flyback problem on the EHT coil. I thought it shows the flyback must be being driven (or these voltages couldn't be generated). I wondered if there is a short somewhere in the EHT coil it may pull that point to ground and cause the pin 7 end of the coil to go very negative. B+ is badly affected but I think that's a symptom not the cause.
I thought I'd ask for some opinions before just blindly getting a new flyback to try. In-circuit resistance from flyback pin 7 to ground is only about 500 k.
Sorry for the poor image quality. Here's a link to a full MTC9000A schematic.
https://www.ukvac.com/forum/data/uploads/2544/20190803210718_001.zip
We've had R187 smoking almost immediately upon switch on, after about 20 seconds it burns out. I've attached a section of the schematic that shows R187 and the voltages measured either side of it.
I was surprised to see -1100V on the flyback side of R187, I didn't realise my multimeter could even read that high, it also explains the R187 smoke! Would I be right that the normal voltage at that point should be between 0 V and -190V, nominally -50V ish depending on where brightness preset RV106 is set?
I'm thinking that the only way we could get -1100V here is if there is a flyback problem on the EHT coil. I thought it shows the flyback must be being driven (or these voltages couldn't be generated). I wondered if there is a short somewhere in the EHT coil it may pull that point to ground and cause the pin 7 end of the coil to go very negative. B+ is badly affected but I think that's a symptom not the cause.
I thought I'd ask for some opinions before just blindly getting a new flyback to try. In-circuit resistance from flyback pin 7 to ground is only about 500 k.
Sorry for the poor image quality. Here's a link to a full MTC9000A schematic.
https://www.ukvac.com/forum/data/uploads/2544/20190803210718_001.zip
