[SOLVED] Tenma 72-5015 Function Generator waveform issue.

[anewmanx]

Greetings,

I have a Tenma 72-5015.

For the life of me I cannot find a schematic for it anywhere online or any kind of manual telling me how to properly calibrate it. I have an issue that causes the leading and trailing edges of the square wave to be deformed. When I set the trigger source on my Tek 5111A to composite instead of the ext that is connected to the CMOS output of the generator the Triangle and Sin wave are perfectly generated. It's only square wave (CMOS) that's off. When I set the CMOS output of the Tenma as the trigger source for my Tek scope, I get the distortion on all wave forms. So it's definitely the square wave generation that's taking a hit on the unit, as it is capable of producing clean triangle and sin waves when that it's my trigger source.

This is what the waveform looks like at various frequencies:







Here is a closeup of the distortion to the triangle wave when CMOS is being used as the trigger source:



Finally, here is a large blowup of the circuit board:



I find it hard to believe that this would be normal behavior and suspect it is simply out of adjustment. There are a multitude of PCB mounted pots to adjust on the board, but I have no bloody idea which one to start with and don't want to throw other stuff out of adjustment in the process.

Suggestions from wiser minds? I deeply appreciate any input you may have to offer. I can live with it like it is, most of what I am doing is way lower than 2mhz, but it would be nice to have a clean signal to use as a quick clock source for microcontrollers.

All of the capacitors seem to read good on the board and I couldn't find any bad diodes, etc...

[anewmanx]

Would the next step be to turn it on and probe around with the scope until I find the source of the square wave signal and see where between source and output it distorts?

[anewmanx]

Well, I may have solved my own problem, but I will post my results here as I go along in case it helps someone else in the future.

In the lower left hand side of the control board there are two TC4011BP NAND gate IC's.  Datasheet:



The circle on the left reads a much cleaner square wave than the circle on the right. Both have similar input signals coming into their logic pins.

This leads me to believe that either the chip is bad, or that perhaps the signal that is going into either pin 1 or 2 is slightly off on the IC on the right. I am tempted to take the two IC's out and swap them to see if it exhibits the same behavior with a different IC, but I fear of breaking the pins on an old IC. I'm going to try next to get a closer look at the input signals on pins 1 and 2.

Update:

Swapping the IC's didn't help, so apparently the problem lies before that point. Tomorrow I need to trace around, take this thing further apart, and find out what path the signals take to get there.

Update 2:

Curious.

On the back of the board that contains the two NAND gates that produce the square wave I found an oddity.



There appears to be a filter cap placed over two of the pins that control one of the nand gates on the IC that gives a cleaner signal... which admittedly makes sense and solves the mystery as to why one is different. Judging by the traces all the band hates that I saw signal on share pins except the one that is messed up looking. Weird. Why two signals? Why two chips with multiple nand gates?

Now the question is why there isn't one on the other NAND gate? Does anyone know? Should I add one?

Here is that board in full:



Update 3:

Problem not solved, but when I put it all back together it was producing a cleaner waveform. I’m guessing lots of these connections were in poor condition and needed reseated.

[anewmanx]

Well, it appears the problem is solved.

It was mechanically loose and dirty connections on the PCB. Taking everything apart, reseating the nand gate I’ve and putting it all back together solved the distortion.

I suppose the simplest solution is sometimes the best. The rise and fall still seems out of spec, but it is smooth. Perhaps someone else can add insight to that.