90R9F Resistor blew when powered (test) [resolved]

[hbonilla]

I finished installing the boards on my second Kaiju. I installed the 5670's and applied power in order to perform the tests. Five seconds into the test I hear a loud pop and some smoke comes out of the A side board, the 90R9F resistor blew up and burned.

I double checked that all of my connections were correct and they are. Any suggestions on what to do?

Thanks

[Paul Birkeland]

I'd have a careful look at those black zener diodes to see if one is in backwards. Also look for solder bridges and/or improper jumpers on the bottom of the PCB. 

[hbonilla]

Thanks Paul.

I double checked jumpers at the bottom of the PCB and the seem ok. Diodes were correctly oriented (but they also fried!) Looked for solder bridges and cannot find any.

Is it possible that it has something to do with the 5670?

My next step is to walk backwards and unmount the boards and do the pre board mounting check again. Is that a good idea?

Thanks again Hector

[Paul Birkeland]

You need to inspect the black zener diodes that mount to the terminal strip under the board.  These are part of the circuit related to that C4S board, and issues with their installation will cause the exact problem you experienced.

-PB

[hbonilla]

Thanks Paul.

I think I found the culprit. When I checked the Zener diodes I realized that I did not trim the A6 end of the diode that is connecting A6 and 27L and the untrimmed tip of the diode was touching the center of the A socket!

So now how to fix this. I trimmed the diode. What should I do now, should I replace the entire board or just the parts that blew? Is it possible that something else is damaged?

Thanks again

[Paul Birkeland]

I would email replacement parts about getting a new board and parts. (that char won't ever go away!)

[hbonilla]

Will do thank you. H

[hbonilla]

Ok. Back to square one.

I built the new board,installed it and had the same results...burned board. Here is an image of the Zener diodes, all correctly oriented.

Any thoughts?

[Doc B.]

Did you replace the zeners? By your description that was your root problem.

[hbonilla]

No, I did not replace the Zeners.

The issue I thought was the culprit is that the Zener  connected to A6 and 27L was not trimmed and was making contact to the center of A. So I only trimmed it correctly and that was that.

Should I have replaced them?

In fact the blow this time was not so quick and it gave me time to check some voltages. IA exactly 530v OA -22v  ?? Is this a clue?

Thanks Hector

[Doc B.]

You said earlier in the thread that the the zeners were correctly oriented but that they also fried. Then you found that one was shorted to another terminal. I would say, yes, based upon that info the zeners should have been replaced when the board was replaced. The high voltage indicates that the zener string is now open and not regulating. You should replace those zener diodes before you power up the amp again.

[hbonilla]

Yes, my comment about the Zener diodes being fried was incorrect. I will change them anyhow, replace the board and carefully check all connections before plugging in again.

We'll see.

Thanks

Hector

[Paul Birkeland]

In your photo, it looks like a lead from one of the zeners is untrimmed and resting against the chassis plate.

Replacing the zener diodes is absolutely a mandatory part of repairing the issue you had. This was something that would come up from time to time with Paramounts, so we've been down this road before.  Not replacing the zeners may just lead to new boards blowing up repeatedly.  I even took the steps of contacting the folks at replacement parts to be sure that the zener diodes were included in the package.

[hbonilla]

Done!!!

It was definitely that trimming. I replaced all Zener Diodes, put a new board in and it works! So happy, thank you.

Hector