-5V drops to 0 after a few seconds (solved - C3 reversed)


It´s late. I´m tired. And I need some help I think.

Finished my Shruti YM. It looks good but I don´t get a sound out of it. So I unpacked it again and measured the voltages.

There is a -5V problem: Right after power-up there is a negative voltage around -5V. But it drops quickly to 0. A few seconds only. In these first seconds I was able to play a sound.

Is it normal that C3 gets very hot?

I power the Shruti with a 7,5V switching AC/DC adapter.

Thank you in advance! Sleep well!

No its not normal for caps to get hot? Is it oriented in the right way? Is you 7905 getting hot too?
Looks like you have a short…

No, 7905 is cold. But I just saw that I might have inverted C2. Wait… Iron heating up.

Yes. It was soldered the wrong way. Now I have a steady -4.30V… Better, but not really good. May I have fried something?

You should probably replace that cap.

I´ll do that tomorrow. Thank you and good night!

After replacing the tantal cap nothing changed. A new electolytic cap and another LT1054 were no success also. I found the “Troubleshooting the power connection”-section in the Wiki and measured the L7905CV which I used. It had 6.22V in and 4.30V out. Next I replaced it with a MC7905ACTG from mouser and voila: 6.22V in and 5.00V exactly out. So it seems to me I fried the 7905.

YM is working.

That new 7905 is designed to work with a lower-than-usual voltage drop (an LDO regulator), it might be that the old one just didn’t work well with the 7.5V PSU. You’re pretty close with 1.22V between the input and output of it.

Sounds logical. I thought about something like this too. But I am still a newbie regarding electronics. I could have tried to switch the PSU to 9 V then. But I like the LDO-way more I think. So I keep the old 7905 as a spare part and try it somewhere else… somewhen… :wink: