R.I.P Soundcraft 400B Power Supply

I was helping my friend recap his 400b console. We got all the input strips and the master section completed. I replaced all the 47uf 25v electrolytics with nice Panasonic Low ESR 220uf 25v caps. Anyway, we plugged them all in and there was some problems; all the meters quickly went to max and we smelt something burning.... (oh shit turn off the console!) Anyway, turns out one of us wired a couple capacitors on the input strips backwards. No biggie...replaced them with new caps in the right direction. We then just tested the master section and one channel we knew worked and planned to go channel by channel to make sure that if there was a problem we knew what strip it was coming from. So with the master channel and just one input strip plugged in....POP a capacitor in the power supply blew. Was this caused by the board being extremely short loaded? Too much current maybe? I feel like it's a stupid mistake. Any help would be great. I can build stuff fine, but I am pretty awful at troubleshooting. Thanks.

According to your description, most likely explanation is that, because the console is short loaded, the unreg voltage was higher than normal, and probably the cap didn't like it. Not very surprising, I bet they must be 20 years old! But you must double check the regulators, one may not have liked being shorted by the reversed caps.
Then there is a possibility that one of the regs being duff, the opposite voltage would go to it through the load, and reverse-voltage the cap.  This can happen because the 400B PSU has no reverse-voltage protection diodes. Which cap has gone? Pre or post reg?

I disagree with the short-loading theory; it is incorrect for Soundcraft consoles up to 1992. Post-1992 is out of my Soundcraft experience as I moved on to another mixer-maker at that time.

When testing consoles at Soundcraft in '82 & '83, the methods were pretty much the same for the full range of consoles (200B, 400B, 800B, 1600, 2400, Series 4):

Plug PSU into empty frame & test voltages in the frame.
Power off.
Plug in meters & master module. Test voltages & functionality.
Power off.
Plug in Groups. Test voltages & functionality of Master & Group system.
Power off.
Plug in channels. Test console as a complete system.

The Soundcraft PSUs were designed to give out their rated audio-rail voltage (usually +/-17.5V) no matter if the PSU was plugged into a load or not. The PSU output voltages would drop if a fault load was connected to the PSU or if the PSU itself developed a fault.

Hope this helps.

you may have put a lot of ripple current on the correctly wired caps when you first fired it up,

the mis wired caps represented a short, so the pwr trans kept fueling the fire,

this bad trama on the sec side made the good caps go bad,

when you plugged them back in, they got mad at you, did you pay your taxes?

Gareth Connor said:

I disagree with the short-loading theory; it is incorrect for Soundcraft consoles up to 1992. Post-1992 is out of my Soundcraft experience as I moved on to another mixer-maker at that time.
The Soundcraft PSUs were designed to give out their rated audio-rail voltage (usually +/-17.5V) no matter if the PSU was plugged into a load or not.

Click to expand...

You haven't properly read my post. I was suggesting that the non-regulated (pre regulator) voltage may exceed the limits of a 20 years+ old capacitor. When the console is short-loaded, there is little current drain (just the meters), the unreg voltage is at its highest.

CJ said:

you may have put a lot of ripple current on the correctly wired caps when you first fired it up,

the mis wired caps represented a short, so the pwr trans kept fueling the fire,

this bad trama on the sec side made the good caps go bad,

when you plugged them back in, they got mad at you, did you pay your taxes?

Click to expand...

I don't follow you, CJ. Unless you mean the PSU has been damaged to the point some AC or reverse-voltage went into the mixer...

have you ever heard a big tube amp with a shorted tube or something?

the power transformer modulates a nice bass note until the plug is pulled,

yes, you really need to over fuse the amp,

so it does this rather than quit making noise completely,

I don't think it can happen there. There's 3pin regs in the 400B PSU, with current limiting. And there are 22R protection resistors on each board.
It would be ointeresting to know if the PSU is still ok, if fuses have blown, ...

So was the cap that blew on the input or output side of the regulators?

A schematic would help.

If the output were pulled low then wouldn't the input of the Vreg be pulled low(er) too? 

I don't see how a cap can pop from this scenario unless it was on the output of the Vregs and the regulator went bad from being shorted and then the cap sees the unregulated voltage through the bad regulator..

Thanks everyone for your help and advice, it is much appreciated. I am not sure if the cap that blew was on the input or output side of the regulators. It is a 470uf 100v axial electrolytic cap. There is a schematic here with the blown cap circled in red. http://i19.photobucket.com/albums/b162/ian1101/400BPowerSupply-1.jpg

One of my concerns is that the PCB on the master section of the console may have been labeled incorrectly, causing me to wire one of the electrolytics on the master section backwards. The only reason I'm worried about this is because the input strip PCB had one of the electrolytic cap traces labeled backwards. Would a backwards cap in the master section cause the power supply to blow?

Although I can't read the values, I believe that section is the phantom voltage source.

The cap is technically on the "input" side of the regulation.

honestly the thing might have just dried up and popped on it's own.

Stick another one in there and turn on the power supply. 

If it pops again then there is something wrong..  ;D

But really, replace it and try the powersupply by itself.

But to answer your question, unless the phantom voltage goes through the master section, I doubt the backwards cap would make it fail.

Usually backwards lytic caps will boil and pop, not dead short the powersupply.  do the "backwards" caps look bubbled out on top?

Another note, how do you know they are backwards?  I'm not sure that I would think soundcraft would send out channels with bad silk on them.

if it has a plus and minus pwr supply, then the caps on the minus rail will stuffed  positive to ground...

so watch out for that eventiality,