Hi all,
turns out that I have two nice Asteroids PCBs to play with (one -02 and one -04). Now, I wanted to make a matching CRT display, so I picked up an old 11" TV (that was working, I tuned it to a DVD rf output and displayed some good movie).
Here's the first big mistake: I didn't check the actual EHT supply before having ripped off all electronics. I have an XY driver board so my plan was rewiring the vertical yoke coils and make a new EHT supply using the original high voltage transformer of the old TV.
The CRT is a Philips A28-14W and according to all the other brands datasheets (seems the Philips one has not been scanned by anyone), this tube runs at 11kV anode voltage.
At first I've tried to run the original flyback circuit by removing the horizontal yoke connection and experimenting with different values of the flyback capacitor, but results were very poor, the driver transistor's collector and base waveforms were really terrible with lots of ringing, no matter how I would try to adjust drive frequency and duty cycle.
Then I replaced the horizontal yoke with a similar inductance big coil (multi layer air coil, 1mm wire diameter) and that worked a lot better, the collector waveform has the usual double peak, no ringing at all and the peak flyback voltage can be tuned by tweaking the drive frequency and duty cycle. However the coil gets hot soon, so clearly that can't be a permanent solution.
I am measuring both the G2 supply and the anode supply (with a 1Gohm HV probe).
Now, before tearing the original TV chassis into parts, I had measured the G2 supply and that was 460V (with a displayed picture though).
With the replacement coil, I can get easily the G2 supply to be 460V or even higher, however the anode supply can only reach 7.6kV
So question for the experts: what could be the reason for nominal G2 supply but way less than "expected" anode supply (ok, I could have a bad transformer/rectifier, but the picture on the old TV looked normal to me).
Second question: is there any proven circuit to generate the EHT supply using the original transformer?
I've examined the Electrohome XY monitor schematics and they have a very clever regulated drive to the transformer, but that requires a separate small winding for the base feedback on the driver transistor and the usual TV transformers don't have that winding.
The vectrex anode supply is instead (as far as I understand) using a normal TV flyback transformer, and my next attempt will be replicating that circuit, but of course I have a different flyback transformer and base-driver transformer, so I expect to need to tweak that circuit quite a lot.
Another option I'm going to try is rigging up a current-mode PWM drive using an UC3843 based circuit. However, in this case I would get the voltage feedback from the G2 supply, so if that voltage doesn't track well the anode voltage (as it seem the case?) then I'm screwed again.
So, what the experts suggest?
Thanks!
Frank
turns out that I have two nice Asteroids PCBs to play with (one -02 and one -04). Now, I wanted to make a matching CRT display, so I picked up an old 11" TV (that was working, I tuned it to a DVD rf output and displayed some good movie).
Here's the first big mistake: I didn't check the actual EHT supply before having ripped off all electronics. I have an XY driver board so my plan was rewiring the vertical yoke coils and make a new EHT supply using the original high voltage transformer of the old TV.
The CRT is a Philips A28-14W and according to all the other brands datasheets (seems the Philips one has not been scanned by anyone), this tube runs at 11kV anode voltage.
At first I've tried to run the original flyback circuit by removing the horizontal yoke connection and experimenting with different values of the flyback capacitor, but results were very poor, the driver transistor's collector and base waveforms were really terrible with lots of ringing, no matter how I would try to adjust drive frequency and duty cycle.
Then I replaced the horizontal yoke with a similar inductance big coil (multi layer air coil, 1mm wire diameter) and that worked a lot better, the collector waveform has the usual double peak, no ringing at all and the peak flyback voltage can be tuned by tweaking the drive frequency and duty cycle. However the coil gets hot soon, so clearly that can't be a permanent solution.
I am measuring both the G2 supply and the anode supply (with a 1Gohm HV probe).
Now, before tearing the original TV chassis into parts, I had measured the G2 supply and that was 460V (with a displayed picture though).
With the replacement coil, I can get easily the G2 supply to be 460V or even higher, however the anode supply can only reach 7.6kV
So question for the experts: what could be the reason for nominal G2 supply but way less than "expected" anode supply (ok, I could have a bad transformer/rectifier, but the picture on the old TV looked normal to me).
Second question: is there any proven circuit to generate the EHT supply using the original transformer?
I've examined the Electrohome XY monitor schematics and they have a very clever regulated drive to the transformer, but that requires a separate small winding for the base feedback on the driver transistor and the usual TV transformers don't have that winding.
The vectrex anode supply is instead (as far as I understand) using a normal TV flyback transformer, and my next attempt will be replicating that circuit, but of course I have a different flyback transformer and base-driver transformer, so I expect to need to tweak that circuit quite a lot.
Another option I'm going to try is rigging up a current-mode PWM drive using an UC3843 based circuit. However, in this case I would get the voltage feedback from the G2 supply, so if that voltage doesn't track well the anode voltage (as it seem the case?) then I'm screwed again.
So, what the experts suggest?
Thanks!
Frank
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