I dug out a STFM with this fault, though this machine does not boot, so I am unable to try much..
But in relation to the composite signal...
The scope on the left is what it is normally, the one on the right is with 60R resistor to gnd. While my STFM doesn't boot, it does "cure" the video sync fault.. but I don't know if it is simply killing the video totally or helping solve the issue because this machine doesn't boot.
I tried a 68R resistor on my working STFM, its not "upset" anything... I don't think loading composite with 68R is a good idea though. I'm trying to see the the "video amp" can be changed to see if the output level can be reduced that way next...
The Output Stage
The output amplifier normally produces about 2.0 Vpp and is intended to be loaded with 150 Ω as shown in Figure 16.
This provides about 1.0 Vpp into 75 Ω, an industry standard level (RS–343). In some cases, the input to the monitor may
be through a large coupling capacitor. If so, it is necessary to connect a 150 Ω resistor from Pin 9 to ground to provide a low
impedance path to discharge the capacitor. The nominal average voltage at Pin 9 is over 4.0 V. The 150 Ω dc load
causes the current supply to rise another 30 mA (to approximately 60 mA total into Pin 14). Under this (normal)
condition the total device dissipation is about 600 mW. The calculated worst case die temperature rise is 60°C, but the
typical device in a test socket is only slightly warm to the touch at room temperature. The solid copper 20–pin lead
frame in a printed circuit board will be even more effectively cooled.
So loading the pin with 150R seems to be the "correct" way... Not happy about that thought because pulling another 30mA doesn't sound good to me..
The video amp is located on pin8. A 2.7K resistor down to gnd has the same effect as the 68R on pin9. Looking at the composite signal, it only lowers in voltage a fraction. Though this *could* explain the borderline situation where some boards are ok and some are not.
R11 which is 1.2K, with 4.7K in parallel it becomes 950R. So changing R11 for 950R should be the same as adding 4.7K on pin 8. Maybe someone who has this video fault can give it a try and report back ?
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