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SFG-1013 GWINSTEK DDS Signal / Function Generator, how to hack/mod it so TTL OUT finally works

Here is how it looks with main board taken out, The main problem with its design is TTL output is ONLY active when TTL is selected !!
This means the main output is also square wave, but with amplitude and offset and duty cycle adjustments, while the TTL is just a 5V version of this.
This Modification will make TTL OUT active in ANY wave mode, so it can be used as a trigger source for your scope.
The DUTY cycle adjustment is offcourse also working. I explain how this works below, see the pictures too.

Whole main board

5V Powersupply area, YES all 3 regulators deliver +5V, the C125 side, I see 8-9V

Hi voltage and hi powersupply for output section, and TTL mode relays (those we trick abit)
Check points for service : C115 -15V and C114 +15V and C121 -5V

Output transistor amplifier, be warned !! those are VERY hot !! due to huge bias current, this is needed to perform from 0 - 3MHz

Bottom side of the main board, note the TTL out track

DDS section

Inside the case with main board out, it is easy to take out, but you dont need to do that, when you know what to do.

The display frontplate board and main CPU

Here is THE MODIFICATION Lift pin 2 of U303 (LM311 opamp) and solder a thinn wire to the DDS output R312 as shown
NOW the TTL output driver U101 (74HC20) will give you a nice signal no matter what wave mode you are using..
R101 (RED ARROW) can be removed after this mod, so the unused relay RL101 is not pulled for no reason.
Pin 3 of U303 is an analog DC voltage from the duty pot, about -1V to +1V,
the footprints R315A and R314A (BLUE CIRCLE) can be used to adjust the min and max dutycycle if needed, default is only 20% to 80%
I mounted 120 K on both footprints, and got now a much better duty cycle adjustment. (2% to 98%)

After the mod you now get TWO signals out at the same time, YES they are inverted, very usefull

In sine mode the DUTY adjustment is very usefull to move the trigger

Thomas Scherrer OZ2CPU 2014
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