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I got a burnt Dragon 64 computer from a work mate some years ago. I never tried one of these before. When I switched it on for the first time the TV signal was missing and I didn't know what was wrong.

Recently I decided to try to fix it. I started checking the PSU and found that the DC voltage levels were all incorrect. Actually they were null. It looked that the TRF was damaged. I found three fuses inside the TRF package and all of them were burnt. I got new ones and the DC levels were right (+5V, +12V, -12V, -5V) but the TV image was still missing. Interestingly I could hear a noise every time I switched the power on.

I found some schematics on the Internet. Most of them were scanned in low resolution but they were better than nothing. A friend suggested me checking the computer was running typing a MOTOR ON/OFF command... and it worked: I could heard the cassette relay switching on/off. So I concentrated on the video section. I replaced the VDU (MC6847P) and the color modulator (LM1889) but it didn't work. Then I replaced the transistors in the output video stage, TR2 and TR3 and... it worked!! But, the image was in B/W, not color. I checked the modulator connections and found a broken track.

Now I can see the image on "blue", can anyone tell me if that is right?

Blue as in the text screen that appears is blue? That would be wrong (should be green) - perhaps one of the two chroma lines or the chroma bias line (between the 6847 and the LM1889) is broken too?

Nice job getting it that far though!

Have you tried using composite video connector round the back? This should tell you whether the fault is in the TV modulator or in the video circuitry.

sixxie wrote:Blue as in the text screen that appears is blue? That would be wrong (should be green) - perhaps one of the two chroma lines or the chroma bias line (between the 6847 and the LM1889) is broken too?

Mmm you may be right. Actually the broken track connected a resistor (R60, 15K) and one of the chrome lines... the B-Y line (blue minus luminance). I used a wire to replace the defective track and got the screen blue. I am assuming that the schematic I got from the web has the same circuit that my pcb.

I checked the chroma bias (CHB) and seems to be rightly connected to the VDU (pin 9). However, the R-Y signal is not directly connected to the VDU. It goes through two ICs (4053, 2318) before it finally gets to the R-Y input of the LM1889. I have checked the power pins (+5V) and seem to be right. You are right, I should also check all the connections of the OA signal from the VDU to the modulator.

Alastair wrote:Have you tried using composite video connector round the back? This should tell you whether the fault is in the TV modulator or in the video circuitry.

Yes I am using both of them and I get the same result. I built a SCART lead connecting the video signal from the DIN connector (pin 3) to pin 20 (composite video input) of the SCART connector. Is that right? I also connected the GND pins at the SCART and DIN

I forgot to mention that I am drawing the schematic of the video section with KiCAD (http://kicad.sourceforge.net/wiki/index.php/Main_Page). I will post it when it be finished.

I am using the schematics on the web and the pcb to complete the information. Unfortunately I am unable to read the capacitor values on the original schematic. Does anyone have a clearer schematic of the Dragon 32 or 64 video section?

I'll check my A3 photocopies of the Dragon 64 one when I get home on Monday if nobody else finds something before then. No guarantees though - the overall clarity is low, and I've never paid much attention to the analogue side (analogue is hard ).

Thanks!

Rinconete wrote:

Alastair wrote:Have you tried using composite video connector round the back? This should tell you whether the fault is in the TV modulator or in the video circuitry.

Yes I am using both of them and I get the same result. I built a SCART lead connecting the video signal from the DIN connector (pin 3) to pin 20 (composite video input) of the SCART connector. Is that right? I also connected the GND pins at the SCART and DIN

If you mean pin 3 as shown on this page http://www.grempc.demon.co.uk/dragon/info/pinouts.html then that is correct. I would have to check on the SCART connector as I use the Composite In socket on my TV. But if you have the same problem with both video outputs then the fault probably does not lie with the modulator.

Have you seen this page http://www.grempc.demon.co.uk/dragon/info/psu.html ? Does the last sentence with regards to the D64 video circuit offer any help?

Alastair wrote: If you mean pin 3 as shown on this page http://www.grempc.demon.co.uk/dragon/info/pinouts.html then that is correct. I would have to check on the SCART connector as I use the Composite In socket on my TV. But if you have the same problem with both video outputs then the fault probably does not lie with the modulator.

Yes it is that pin. mmm I should check if my TV has an alternate "composite video in" socket apart from the SCART connector.

Yes, I think so, the RF modulator seems to work properly.

Alastair wrote: Have you seen this page http://www.grempc.demon.co.uk/dragon/info/psu.html ? Does the last sentence with regards to the D64 video circuit offer any help?

I have checked it and it is right: the regulator works and provides -5V DC.

Thanks for your remarks.