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Old 12th September 2013, 01:17 PM   #1
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Default effects of faulty lm78xx?

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?
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Old 12th September 2013, 01:25 PM   #2
Mooly is online now Mooly  United Kingdom
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Regs can go faulty but it wouldn't be the first suspect. 4.9 volts output is absolutely fine and well within tolerance. They are so common and cheap that if there is any doubt then just replace it.

My guess is that a logic glitch is occurring somewhere.
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Old 12th September 2013, 01:45 PM   #3
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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?

Last edited by PreSapian; 12th September 2013 at 01:48 PM.
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Old 12th September 2013, 01:55 PM   #4
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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.
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Last edited by Osvaldo de Banfield; 12th September 2013 at 01:58 PM.
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Old 12th September 2013, 01:56 PM   #5
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Quote:
Originally Posted by PreSapian View Post
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.
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Old 12th September 2013, 02:03 PM   #6
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the circuit plays fine up to 20 minutes or so when it starts distorting periodically until it completely goes noisy. so, some kind of current build up issue= faulty caps?
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Old 12th September 2013, 04:57 PM   #7
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by PreSapian View Post
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.
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Old 12th September 2013, 05:02 PM   #8
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Something is breaking into oscillation. Check your power supply decoupling on the power pins of each IC.
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Old 13th September 2013, 05:07 AM   #9
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The Lm7805 has a minimum load current of 5ma. If you are just powering the 74hc04 with the regulator you are probably not drawing enough current ( this would explain the 4.90V ). Try adding a 1K resistor from 5V to ground and see if this helps your problem.

Last edited by RJM1; 13th September 2013 at 05:10 AM.
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Old 13th September 2013, 09:22 AM   #10
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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.
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