Seventh Circle Audio

Hi there,

Last night, two of wires more than likely crossed, either +/-0 and +/-27 or 48. I think I shorted the whole thing. Some day...I will be a genius. Or not. In any case, can anybody think of where to start?

Does it make sense to just build a new one? Ultimately, I know some components can take voltage and some cannot. It seems a shame to ditch the power transformer and the big caps, the resistors...

Should I start by replacing the electrolytics? By tracing back from the molex connector, as per the schematic?

Tim, I thought maybe you might have an idea about what might immediately fry if you shorted the wires.

Thanks,
kelly

First, disconnect the supply from the preamps and apply power. Do you have voltage at any of the outputs? If not, check the fuse. It's hidden behind the transformer and tough to get to with the supply in the chassis. Don't forget to disconnect the AC before probing with your ohmmeter. Let us know what you find.

Hey Tim,

I checked the molex connector (the fuse is still fine, if I'm looking at it correctly...it's not broken), and don't get any voltage on the tiny Molex metal "windows".

kelly

You need to check the fuse with an ohmmeter to be sure. You also need to disconnect the supply from the preamps to find out what's really happening. There may be no voltage because there's a dead short across the outputs. Measure the outputs with no load to be sure it's the supply and not the load.

I measured without connection to pres and got nothing from the supply.

So you've tested the fuse with your meter and measured more or less 0 ohms? There's not a lot that could fail that would take out all three outputs completely. Are you sure you haven't tugged a switch lead loose or something? If the fuse is OK, you'll have to remove the supply from the chassis to continue troubleshooting. Be very careful, since there are uninisulated AC line connections on the bottom.

Thanks, Tim.

I'll check the switch leads when I get home. I guess that could have happenned when reseating the face plate. I'll check.

thanks...

It WAS the fuse. Thanks for suggesting that I measure it. It looked fine, until I compared it to another fuse I had. Comparing appearances, the good one was gold/copper-like; the burnt one dark and rust-like. Strange.

Thanks. I'm stopping by the distributors today for new fuses. And, I'm doing all my molex connectors again.

kelly

In my opinion, building the power harness is the worst part of the whole project, but the appropriate crimping tool makes it almost tolerable.

Yes...it very officially SUCKS!

Thanks for your help everybody.

kb

Hey folks,

I replaced the fuse, but I'm getting exactly +/- 32 (instead of 27) and 58.5 on the phantom on the little molex windows. It seems highly conicidental that I'm getting exactly 5v extra on the B+/- and 10v on the phantom. Does that ring any bells?

I'm wondering if anyone can think of where to start. I'm also wondering if anyone knows how to test capacitors.

Thanks a bunch. This is kind of fun in a weird way. I spent all last night studying diodes. Forward biasing, reverse biasing, avalanche...really interesting stuff. My girlfriend is really nice!!

kb

Did you build that power supply?
Are you absolutely certain you're measuring those correctly, referenced to ground?

Pin 2 GROUND - put the black or ground lead there
Pin 1 + 27
Pin 3 -27
Pin 5 +48

What does your meter read if you measure a 9V battery? Seriously. Once you're sure your meter isn't lying to you, measure the supply with no loads connected and tell us what you find.

Hey...thanks for helping.

A nine volt battery reads 8.65v. But, I'll change the battery in the multimeter to be absolutely sure.

I'm still getting:

pin1 33.5
pin2 0
pin3 -33.6
pin4 0
pin5 59.4

I dunno. I guess I'll just start hashing through the schematic. I just thought the presence of extra voltage might indicate a starting point for looking for component problems. It seems like the rectification is ok, so the bridge rectifier isn't the immediate suspect? Could it be a resistance issue? Won't resistance bring the voltage of a circuit down (i.e. up if it's blown)? It seems like a single diode failure would mess up one of the outputs, not all them by a factor of 5 or 10. Maybe it could. I'll keep reading.

Again, thanks. And if you think of some good starting point to test, I'd be really, really grateful.

Kelly

Can you very carefully measure the voltage at U1, pin 2? This is the center pin of the regulator that's on top when the supply is in the chassis. Let us know what you find.