Power Section on a mixer

[villain154]

Hello!

So i have hit a brick wall in getting this giant mixer fixed. It seems that every time i try to turn this thing on the fuses blow (1204 and 1205). I recently replaced a capacitor(C1227) that went bad (before this it was working flawlessly). Not sure what the exact component that went bad that would cause it to just blow fuses on the reg (hoping that its not the tranny). I do have the service manual and all of the necessary specs. I know a little bit about basic electronic construction/repair/theory, but this is a little bit over my head. If anyone knows anything about this ANY help would be greatly appreciated.

Thanks in advance,
Anthony

This is just the schematics for the power section. If you feel the whole thing or the service manual would be helpful just let me know!

 

[Nigel Goodwin]

Well 1204 and 1205 are on completely different supplies, so shouldn't both blow - unless there are two independent faults?. If it didn't start doing this until you replaced C1227, then it seems likely it's something you've done wrong.

1205 and C1227 look to only be on the phantom power side, so the mixer should probably work even if 1205 is blown.

Common faults to blow secondary fuses would be the bridge rectifiers - it's VERY unlikley to be the transformer..

 

[Reloadron]

Is AN1201 a connector that can be disconnected? Also if it is easy to do you could think about removing the full wave bridge D1201 and check it.

Ron

 

[villain154]

Wow!
Thanks guys for responding so quicky!

I thought it was strange that both 1204 and 1205 seem to blow as well. I noticed that even after both of the fuses blew, the phantom power led and switch still seem to work. I did a test to see if the board was outputting anything at all and nothing was passing through, no hum or anything.

I can remove D1201, it looks like it may be a bit of a turd to remove (decent hunk of glue on it). You'll have to forgive i'm still new to a lot of things, but what would be the simplest way of testing D1201 with a multimeter.

An1201 can be disconnected, it routes to the HP board. I tried disconnecting all of the pin connections but as soon as i plug in the connector from the transformer it blows the fuses. I'm trying to cut down on my fuse consumption seeing as I've been through 12 so far (stupid i know).

Thanks again, all this is greatly appreciated.

 

[Reloadron]

Disconnecting AN1201 would isolate everything downstream that section of the power supply drives. Since disconnecting it really makes no change in that the fuses still blow you can pretty much figure the problem is in the actual PSU.

Pins 1 & 3 provide Xfmr power to the bridge through the fuses that blow. Pin 5 of the Xfmr connector is the transformer center tap. Now if you have an Ohmeter you want to measure between pins 1 & 3 of the transformer connector (with Xfmr disconnected) Then measure between pin 5 and 3 and 5 and 1. We are looking for a very low resistance like a short.

Ron

 

[villain154]

Okay so i did a pin test and it appears that it is ∞ on all combinations that you suggested. I guess that means that is the bridge rectifier that is faulty? I found a place that sells the one with the specific model number, but they only sell it in quantities of 500. I came to the conclusion that the model number is propitiatory to the company that supplied it to Yamaha (PBU804). So after some research i found the part on mouser sold by fairchild that has the same exact specs that Diodes inc had (8A 400v). I hope that this is a safe assumption, as i have never had to replace a bridge rectifier and do not know the variables that would be detrimental to its function.

 

[gots2no]

Most likely you have some output transistors that are shorted and the power supply is OK.