My_Ref problem: DC on output

[erocfx]

I was testing a VU meter circuit on my working My_ref rev C amp.

Now I have -29 volts on my outputs and inputs. This is with the LM318 in or out of the circuit. I don't see any burned components, and it happened to both my channels. The +-12 volt is working.


Anyone have any ideas what to check, or what causes this?

Thanks,
Eroc

 

[Mark Whitney]

Unplug everything and check the resistance of the 1 Ohm resistors. It is R11 if I remember correctly.

 

[erocfx]

Good call Mark, R11 was open on both of them. I will replace those and see what happens. Thanks.

 

[Mark Whitney]

If you want to prevent this from happening again add two back to back diodes across R11.

 

[erocfx]

Do you have a schematic for that? Any suggestion for which diodes?

 

[erocfx]

Are those in series or parallel with R11?

 

[Mark Whitney]

Like this.
You could use 1N4001 or similar. Just need adequate surge overload rating.

 

[erocfx]

Thanks again for the help Mark!

 

[tomchr]

If you had enough current flowing in R11 to fry it, a pair of diodes won't help. They'll prevent the resistor from frying, but that'll likely just cause something else to fry. There isn't supposed to be any current running in R11, so I'd figure out where that current is coming from first.

Tom

 

[Mark Whitney]

Normally it is only R11 that has blown. My guess is that a high enough potential between the grounds when you plug in or switch on the amp is the cause.

 

[tomchr]

I'd still look at where that current is coming from. You shouldn't have any significant difference between ground potentials.

Tom

 

[erocfx]

Thanks Tom, the problem came when I tried to hook up a vu meter driver board. Since the ground was tied to the AC ground. I think that's where the problem is. Not sure how I can hook this up, I may have to skip using them.