Falcon Csync video fault.
Posted: Wed Mar 20, 2019 10:33 pm
I had a email a few days ago from Steve asking about a capacitor bulging problem after re-capping his Falcon motherboard.. But this actually gets more interesting later...
This particular capacitor is in the video circuit, or more explicitly, it is the coupling capacitor for the composite sync signal.
My first simple test to check the voltage across the capacitor to see what was there...
The particulate meter does register HF pretty well, though for a quick test, we can see the voltage is backwards across the capacitor and we are measuring negative 3.89V!
Next up I swapped the capacitor around and retested..
So now we have a proper polarity and the voltage is now 3.49V.
I then went on check the voltages on the scope...
Above we see a maximum voltage of 4.44V and a minimum of 2.08V.
On the other side of the class to we see 0.64V max and (negative) 1.48V!
So between the two negative voltages, we actually have a 3.56V difference, which basically matches the meter readings of about 3.49V.
So in both tests, the voltage across the capacitor is reversed!
I went back in the ST designs and looked at the STE schematic....
Where the same capacitor also seems in backwards! I even checked the MSTE and that is also the same circuit.
I went further back to the original STFM design...
Where the composite video output as the capacitor in the opposite direction of STE/Falcon based machines! So in actual fact, the STFM capacitor seems to be the correct direction.
As to why capacitor ended up being backwards I really don't know. Though for some reason Atari decided to swap this capacitor around, either on purpose, or by some huge screw up and carried on that mistake to or from other machines.
The interesting thing is that I had a Falcon in for repair a few months ago which I mentioned in this thread...
https://www.exxosforum.co.uk/forum/viewt ... 70&p=11759
Where it was exhibiting some video sync fault, which I ultimately gave up trying to diagnose.. At the time, my test Falcon was working fine but recently started exhibiting the exact same problem as in that thread. At the time I could not find any real difference between my working Falcon and the Falcon I had in for repair. But basically I was comparing 2 faulty machines. Only at the time, my own Falcon was actually working! So this was a huge spanner in the works of trying to diagnose this problem as I did not realise my own machine is actually faulty as well! I actually thought it was something to do with my monitor going faulty at one point, but I never had the sync issues with my STFM's or STE's in fact. So it actually remained one of those mysteries to look at another time.
Though since swapping around the capacitor in my test Falcon, my monitor has not had any sync faults in the 2 or so hours it has been powered up for . Previously it would lose sync every 30 seconds or thereabouts easily.
So I am actually thinking now is going to be a common fault on Falcons and other machines that use the composite signal (or sync). I have seen it happen on two machines myself. Though Steve reported this as a issue that the capacitor was bulging after just three days use. Steve did not mention any video issues, though clearly there is a issue that is capacitor was self-destructing after just a few days. and 3V or higher backwards at a high frequency would certainly not do the cap any good!
Oddly the original capacitor (wangs caps etc ) seems to be mostly happy in working like this, though of course my test Falcon has recently started developing issues which seem to be cured by turning the capacitor around.
So I thought I would document this as I assume this will be a problem for the people in the future. Of course thanks to Steve for bringing this to my attention!
This particular capacitor is in the video circuit, or more explicitly, it is the coupling capacitor for the composite sync signal.
My first simple test to check the voltage across the capacitor to see what was there...
The particulate meter does register HF pretty well, though for a quick test, we can see the voltage is backwards across the capacitor and we are measuring negative 3.89V!
Next up I swapped the capacitor around and retested..
So now we have a proper polarity and the voltage is now 3.49V.
I then went on check the voltages on the scope...
Above we see a maximum voltage of 4.44V and a minimum of 2.08V.
On the other side of the class to we see 0.64V max and (negative) 1.48V!
So between the two negative voltages, we actually have a 3.56V difference, which basically matches the meter readings of about 3.49V.
So in both tests, the voltage across the capacitor is reversed!
I went back in the ST designs and looked at the STE schematic....
Where the same capacitor also seems in backwards! I even checked the MSTE and that is also the same circuit.
I went further back to the original STFM design...
Where the composite video output as the capacitor in the opposite direction of STE/Falcon based machines! So in actual fact, the STFM capacitor seems to be the correct direction.
As to why capacitor ended up being backwards I really don't know. Though for some reason Atari decided to swap this capacitor around, either on purpose, or by some huge screw up and carried on that mistake to or from other machines.
The interesting thing is that I had a Falcon in for repair a few months ago which I mentioned in this thread...
https://www.exxosforum.co.uk/forum/viewt ... 70&p=11759
Where it was exhibiting some video sync fault, which I ultimately gave up trying to diagnose.. At the time, my test Falcon was working fine but recently started exhibiting the exact same problem as in that thread. At the time I could not find any real difference between my working Falcon and the Falcon I had in for repair. But basically I was comparing 2 faulty machines. Only at the time, my own Falcon was actually working! So this was a huge spanner in the works of trying to diagnose this problem as I did not realise my own machine is actually faulty as well! I actually thought it was something to do with my monitor going faulty at one point, but I never had the sync issues with my STFM's or STE's in fact. So it actually remained one of those mysteries to look at another time.
Though since swapping around the capacitor in my test Falcon, my monitor has not had any sync faults in the 2 or so hours it has been powered up for . Previously it would lose sync every 30 seconds or thereabouts easily.
So I am actually thinking now is going to be a common fault on Falcons and other machines that use the composite signal (or sync). I have seen it happen on two machines myself. Though Steve reported this as a issue that the capacitor was bulging after just three days use. Steve did not mention any video issues, though clearly there is a issue that is capacitor was self-destructing after just a few days. and 3V or higher backwards at a high frequency would certainly not do the cap any good!
Oddly the original capacitor (wangs caps etc ) seems to be mostly happy in working like this, though of course my test Falcon has recently started developing issues which seem to be cured by turning the capacitor around.
So I thought I would document this as I assume this will be a problem for the people in the future. Of course thanks to Steve for bringing this to my attention!