Repairing Yamaha Delay Part 2

So... after a few weeks off I finally have some more time to finish continue working on this Yamaha delay unit...

Why U no work?

So... we left off with me going to buy fuses...

Alright, get this... I go to home depot to buy fuses, and I tell the guy I need some 3/4 amp fuses.  He doesn't  have any in stock, so he hands me a box of 1 amp fuses and says they'll work just fine...  seriously...  he doesn't even know what I'm doing with it.  That 3/4 amp fuse might be protecting the whole unit from exploding in a giant fireball of epic proportions, raining death and radiation to the entire United States and for some reason, parts of Brazil.

Thanks a lot, Home Depot guy

Or maybe I'm just being over dramatic... whatever.  I bought 1/2 amp and 1 amp fuses.  Go figure, the 1/2 amp fuses blew right away.

Turns out the 1 amp fuses didn't cause an explosion (you win this round, Home Depot guy), but a heatsink was getting hot enough to fry an egg, and that tends to indicate a problem.  Sure enough, I found some nifty voltage test points on the board... there are 8, +15, and -15 volt rails, and the 15V rail was dead.

Awww yeah!

I'm no expert on power supplies, so I tried to find a schematic to see what I'm dealing with.  I guess those dudes at Yamaha don't want a "non Yamaha certified" person to repair their stuff... go figure...

I guess I'm on my own...

Well, after an hour mapping out the power supply and testing voltages, I still had nothing.  Of course I measure the one that was working, and the one that wasn't (which is a mirror circuit with PNP transistors).  No dice... the voltage was sagging all the way up the power supply.

Yup... BJT fo life

So... lets cut the rest of the circuit out... find out what's failing.

What's a jumper or 2 between friends?

I guess that did the trick... the power supply is actually at -15 volts, and the other end of the circuit which it should be hooked up to is shorted to ground...which means my problem is the rest of the circuit.  Luckily for me, every other part of the circuit had it's own jumper connecting it so I just kept cutting until I found the offender.  A tantalum capacitor!  My mortal enemy.

You see, unlike electrolytic or ceramic capacitors, which fail open, tantalum caps fail shorted.  I just pulled it out and reconnected that section to the -15 rail and voila!  

And then... fail...

A different tantalum cap blew.  And another...

Oh well, I wasn't sure what they did, so I just pulled them out.  I tried to replace all 3 with ones that had a higher voltage rating, but for whatever reason, that circuit really doesn't like those caps...  Every time I cut one off, the circuit worked, and as soon as I tried to replace them, it would fail... so screw it, I took them all off.

Oddly enough, when hooked up a guitar, it worked again... hehehe. It turns out they're just some bypass caps to smooth out ripples on the power line, so hopefully there isn't any noticeable hum, or maybe I just accidentally created the greatest rack mounted analog delay of all time... we'll just have to find out.  

I'll have to get this back to Chout at the Fortress of Desire and see how he likes it!

Questions? Comments?  Want to tell me how great I am?  

get a hold of me at soundular-electronics@gmail.com

P.S.

For all you aspiring electronics people out there... even pros make mistakes.

Don't see it yet?  Let's look closer

You have brought shame to Yamaha

You see those three grooves cut into the board?  Someone screwed up, and after this PC board was made, someone (or a machine, it is Japan, after all) had to go through and make 3 cuts in EVERY SINGLE one of these.

I only recognize this because that's exactly what I had to do to fix my drum light boards... derp...