i have a lm7805 that's powering 74hc04 inverter for differential tda1543 setup; flipflop data signal
the dac goes into massive distortion after 20 minutes or so which is remedied by repowering the circuit. i narrowed the problem to the hex inverter side. there i already replaced the 74hc04 with no avail. all there's left on that part of the circuit is 7805 and salvaged bypass caps.
now the 7805 in this case actually only reads 4.90v on my meter, which i have never witnessed before. they're usually +/- 0.01v from the target and no further.
can faulty regs cause such distortion or should i troubleshoot the salvaged caps?
the dac goes into massive distortion after 20 minutes or so which is remedied by repowering the circuit. i narrowed the problem to the hex inverter side. there i already replaced the 74hc04 with no avail. all there's left on that part of the circuit is 7805 and salvaged bypass caps.
now the 7805 in this case actually only reads 4.90v on my meter, which i have never witnessed before. they're usually +/- 0.01v from the target and no further.
can faulty regs cause such distortion or should i troubleshoot the salvaged caps?
Try to use 74HC14 in place of 04, it has schmitt trigger input, perhaps a noisy input???
78xx are actually being faulty from the moment of start any project, I saw several 7815 where you must short the output to take the regulator start to work.
Motorola's 7824 catch fire.
78xx are actually being faulty from the moment of start any project, I saw several 7815 where you must short the output to take the regulator start to work.
Motorola's 7824 catch fire.
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what else can i look for if it is logic glitch? both 1543 are fine on it's own so i automatically assumed it was the inverter side that's with the issue.
could it be the audio-gd ddc?
I would also replace the decoupling capacitors as these are vital to a stable regulated voltage.
what else can i look for if it is logic glitch? both 1543 are fine on it's own so i automatically assumed it was the inverter side that's with the issue.
could it be the audio-gd ddc?
That's the million dollar question, I wouldn't even like to guess on that one without having it in front of me.
I would replace the 7805 so there is no doubt whatever on that score.
Electrolytics change capacitance with temperature, ie higher temp = higher capacitance and lower esr.
It could be that your reg is on the verge of oscillation at cold, and the cap getting warm sends the reg into oscillation.
Check with a CRO if you have one.
You state that the cap is recovered/used....try a different one and see what happens.
BTW, what is the value of the present reg output decoupling cap...this can be important.
Dan.
It could be that your reg is on the verge of oscillation at cold, and the cap getting warm sends the reg into oscillation.
Check with a CRO if you have one.
You state that the cap is recovered/used....try a different one and see what happens.
BTW, what is the value of the present reg output decoupling cap...this can be important.
Dan.
when the distortion starts, i can kill it by changing the value of the variable resistor for lm317 that feeds the te7022 usb module. so it's not the hex inverter where the distortion is from. my grounding scheme is a mess. i will try to star ground everything. do you think it might be parasitic oscillation?
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