Hi,
I have a faulty Dragon 32. It is displaying colums of @ characters and flashes between this and a totally scrambled screen.
I have replaced the ram chips, IC16 and IC19 but still get the same results.
The MOTOR ON and MOTOR OFF commands active the relay so something must be working. However sound commands do not produce any sound.
Any ideas?
Thanks
Faulty Dragon32
Re: Faulty Dragon32
Interesting fault - the machine is generally working, and the VDG is obviously generating a display. Sounds like video data might not be making it to the VDG.
Maybe check "IC13" (I think - it's a 74LS273 anyway) and the lines from it to the VDG and RAM. Checking that clock pin 11 goes to RAS0 on the SAM - pin 12 - would be a simple first check. That chip latches data from RAM to the VDG at the appropriate time, so if the RAM's ok, and the VDG is ok, it sounds like a good candidate.
Could also be worth checking the VDG to SAM connections, in particular DA0 (pin 22 on VDG, pin 8 on SAM), although I would imagine if that were broken you'd just get a green screen.
The lack of sound is a bit more odd - are you listening through a TV (ie through the RF modulator)? Sometimes slightly off tuning can drop the sound. If it's from the monitor port, there's very little between the PIA outputs and it to mess up. "IC34" is a simple buffer (4050) between the PIA and the DAC. "IC20" is the analogue mux (4529) which would prevent audio output (and joystick measurement) if dead.
Good luck!
Edit: hm, columns of @ characters? Anything like this screenshot? Or just part of it repeated? Does it flash up garbage at regular intervals? I wonder if maybe the SAM is getting no HS signal - a little more difficult to check as the signal isn't directly connected... (and looking at the D32 schematic, I can't see exactly where the "NHS" signal (New HS I guess) is generated).
Maybe check "IC13" (I think - it's a 74LS273 anyway) and the lines from it to the VDG and RAM. Checking that clock pin 11 goes to RAS0 on the SAM - pin 12 - would be a simple first check. That chip latches data from RAM to the VDG at the appropriate time, so if the RAM's ok, and the VDG is ok, it sounds like a good candidate.
Could also be worth checking the VDG to SAM connections, in particular DA0 (pin 22 on VDG, pin 8 on SAM), although I would imagine if that were broken you'd just get a green screen.
The lack of sound is a bit more odd - are you listening through a TV (ie through the RF modulator)? Sometimes slightly off tuning can drop the sound. If it's from the monitor port, there's very little between the PIA outputs and it to mess up. "IC34" is a simple buffer (4050) between the PIA and the DAC. "IC20" is the analogue mux (4529) which would prevent audio output (and joystick measurement) if dead.
Good luck!
Edit: hm, columns of @ characters? Anything like this screenshot? Or just part of it repeated? Does it flash up garbage at regular intervals? I wonder if maybe the SAM is getting no HS signal - a little more difficult to check as the signal isn't directly connected... (and looking at the D32 schematic, I can't see exactly where the "NHS" signal (New HS I guess) is generated).
Re: Faulty Dragon32
Try replacing the SN74LS783N (MC8663) SAM chip.
Re: Faulty Dragon32
Hi
OK I have checked the connections and have tried with a spare SN74LS783N and get the same result. Also I made a mistake before and I am getting sound.
The display pulsates between these two screens staying on each for around a second.
OK I have checked the connections and have tried with a spare SN74LS783N and get the same result. Also I made a mistake before and I am getting sound.
The display pulsates between these two screens staying on each for around a second.
Re: Faulty Dragon32
The second of those screens is exactly what I see in XRoar if I stop the HS signal getting to the SAM (ie disable HS and FS signalling). I don't see the first one at all, but that's probably due to shortcuts taken in emulation (well, maybe). So I think that's displaying uninitialised RAM, and combined with the fact it's not displaying nice 12 line high characters I would say that's likely the problem.
I think I'd be focussing on the path of the HS signal to the SAM. If you look at page 3 of the Dragon 64 schematic, you can see HS coming in to a flip-flop, the output of which (called NHS) is connected to the SAM's HS input. Now it's possible this only goes through the latch to clean up the signal (ISTR reading about early CoCo 1s that suffered over time if the HS pulse wasn't wide enough), but I can't see that on the Dragon 32 schematic, even though it says NHS goes to the SAM pin.
Do you have an oscilliscope? Can you check the falling of HS at the VDG pin against the same at the SAM pin? The HS signal is obviously making it to the generated video signal...
I think I'd be focussing on the path of the HS signal to the SAM. If you look at page 3 of the Dragon 64 schematic, you can see HS coming in to a flip-flop, the output of which (called NHS) is connected to the SAM's HS input. Now it's possible this only goes through the latch to clean up the signal (ISTR reading about early CoCo 1s that suffered over time if the HS pulse wasn't wide enough), but I can't see that on the Dragon 32 schematic, even though it says NHS goes to the SAM pin.
Do you have an oscilliscope? Can you check the falling of HS at the VDG pin against the same at the SAM pin? The HS signal is obviously making it to the generated video signal...
Re: Faulty Dragon32
Another thing to check (if you have not already), that I have seen cause a similar fault is the connection between DA0 on the VDG (Pin 22) and DA0 on the SAM (Pin 8).
Cheers.
Phill.
Cheers.
Phill.
Re: Faulty Dragon32
OK I have replaced the 74LS273 IC13 and tested the signal as best I can for the HS although the schematics I have are not very easy to read. As far as I can tell the signal getting to the VDG and SAM are equal.
Can anyone confirm which pins I should be testing too?
Thanks
Can anyone confirm which pins I should be testing too?
Thanks
Re: Faulty Dragon32
Whats the chance this could be a faulty VDG?