Mackie LM3204 PSU

[Siegfried Meier]

Hey guys,

I have a mint one of these here that didn't seem to work when I bought it.  Upon further inspection, it appeared as if the -18v rail and the 5v rail was dead.  I looked at the regulators, and the -18v was fried.  Replaced it, tested and noticed I still don't have 5v - I'm assuming this is for the solo/logic control, but can't figure out why it's not working.  It seems to come from the 7805 regulator, which I understand is a 5v reg?  It appears to be working when I test the resistance of the pins, but I suppose it could also be fried?  I made an order with mouser but didn't include that reg as I thought it was initially fine...

Any help much appreciated.
Thanks!
Sig

 

[Siegfried Meier]

So it turns out I did have a few extra regulators here...but they're 7805A, and the one in the PSU is a 7805CV.  I swapped it out anyway, just to see and still no 5V...

Is an A and a CV that different that it wouldn't work at all?  Or does my problem lie elsewhere?

 

[synthiaks]

Without a schematic its hard to tell. If there is any filtercaps that filter the 5V i would check if they are still good.

The7805s should be fine A and CV probably won't make a big difference. The CV might even have slightly better specs. But to be 100% sure you have to check the datasheet.

 

[synthiaks]

So i have had a look at the schematic.

I would do:

1: check if the PSU have 5V, if not,  check C7 and C8.

2: If you have 5v on the PSU unloaded try and connect the parts to it one by one. When the PSU stop giving out 5V you have the right part to search through. Now the fun starts! Where could the rail have a potential short circuit? Caps are always a good start. Old dry elcos or fried tantals shorting to ground. A fried OP amp?

 

[Siegfried Meier]

PSU has no 5v at all, anywhere.  I did give a quick test of the caps, and they don't appear to be bad...but I could replace them anyway if you think that will fix it.  Usually a multimeter will tell you whether a cap is bad or not.

 

[synthiaks]

Well then it can't be that hard.

If the transformer gives AC then it can only be caps, Diodes or the 7805

Caps can look ok but just crap out under load.

 

[JohnRoberts]

only a few possibilities... Is 5V regulator seeing  8V or more DC at it's input? Is output shorted?

JR