VIDEO DAC BRIGHTNESS ISSUE

[exxos]

So something still is not quite right with the video circuit. The TV is still on whiteout where as plugging in an original STFM into it its ok, see pics of both below:

You monitor isn't expecting CSYNC for some stuff is it ? Maybe check CSYNC is working ok ? Not sure really if that be the problem, but worth checking...

[PaulJ]

Found the issue. When scoping around the level out of ROUT and BOUT where the same but GOUT was only half. Checked the digital input on all the DAC inputs and all were transitioning at a TTL level except MUX_G2. G2 into the MUX U4 was also ground... as was pin 24 on the shifter. Removed shifter pin 24 from the socket and it now had a good TTL level swing. Removed U4 and it had a solder bridge underneath. Replaced it and put a 10k pot in and now I have an excellent color display. The optimal contrast is at 2.65 volts.

GC1.jpg (370.53 KiB) Viewed 4253 times

GC2.jpg (199.08 KiB) Viewed 4253 times

[exxos]

The optimal contrast is at 2.65 volts.

"user faults" are the best ones

I'm assuming if the voltage is higher, the brightness goes down ?

[Icky]

You monitor isn't expecting CSYNC for some stuff is it ? Maybe check CSYNC is working ok ? Not sure really if that be the problem, but worth checking...

Could well be . Will have a look.

[PhilC]

Yes I am using VGA, not got a scart tv at the moment.

[exxos]

Could well be . Will have a look.

Also try a 10K pot instead, the bias could still be off...

[PaulJ]

The optimal contrast is at 2.65 volts.

I'm assuming if the voltage is higher, the brightness goes down ?

Approaching 0 volts its almost all just a white screen. When you approach 3.0 volts its way to dark to use. Its pretty good at 2.5 volts so it would be easy to make a divider.

[exxos]

Approaching 0 volts its almost all just a white screen. When you approach 3.0 volts its way to dark to use. Its pretty good at 2.5 volts so it would be easy to make a divider.

I guess future builds could even have a small pot for this... Though if everyone finds 2.5V works best, then sure can just use like 2x 1k resistors or something.

[PhilC]

May be worth having a jumper again with two brightness settings as the 4.7k and 10k looks fine on my VGA but will have to test this on two of my other monitors to verify

[PaulJ]

The video appears to fairly easy to troubleshoot. Check the output level on ROUT, GOUT, and BOUT of the DAC. They should all be the same level. Mine are approx. 1.5 volts pk to pk(ignore dc here). Initially my ROUT and BOUT were the same while GOUT was down significantly. If you have a color that is not the same level check that all MUX_XX signals are toggling TTL signals. They should all have signals on them. My MUX-G2 was sitting on group. Look at the input to the mux chip for the signal not toggling. If bad there and on the shifter bend the in question shifter pin out of the socket. Check again and if the pin now has a signal you most likely have a short on that line. Mine was under the mux chip.