Help with Xtant x603

[shawnk]

Allrighty Perry. Thanks a bunch for your help so far. I think I've got a situation here that if you can't help me with then I think I'm doomed. I have basically removed all of the drivers and all of the output fets in the sub/mono channel. I have also removed several of the small diodes that reside directly in front of the fets as they were shorted. I essentially wanted eliminated any possibility for this channel to cause the protection to kick in. My next step was to get the left and right channels to work properly. As the right channel seems to be in good condition I could focus on the left. I basically removed all of the drivers/fets and protection diodes in this channel as well. I fired up the amp and bingo.. no problems, at least no protection indicators nor was the amp drawing excessive current. At this time I measured the low current power supply at the same points that were provided in the tutorial. All measured spot on. My next step was to remove several components from my donor amp for replacements. I did so and then measured each component individually for shorts 'and' leakage. I then replaced all of the previously removed components with good ones. Fired up the amp and again everything went well.
Now this is where I'm bit concerned. After having the amp powered up for a couple of minutes I wanted to check for anything that might be overheating. As I was checking some components in a particular part of the board I could actually cause the 'overcurrent' led to trip. After a lot of prodding around I can now duplicate the problem regularly. So here's the deal. I have the amp is sitting horizontally on my bench. The screw terminals are closest to me and the fets are in the back. If the amp rests normally the led trips. If I place my hand under the board and put an upward pressure onto it, the amp is fine. I can't see a cracked trace anywhere, although I'm sure it could be easily overlooked. Now I'm not too sure how to go about fixing this. Like I said Perry. If anyone can help it's you! Thanks

[Perry Babin]

Can you tell which channel is causing the over-current to trip?

If not, the simplest way may be to remove D64, D70 and D76 (I think those are the right diodes), one at a time to see which one prevents it from going into protect. You must be VERY careful doing this because the defective channel will likely cause excessive current draw when the diode is removed. You must insert a 10 amp fuse in the B+ line and MUST have all outputs clamped to the heatsink before you try this.

[shawnk]

Ok thanks Perry. I will try this tomorrow/later today. I see the diodes you are referring to. Can you tell me their function/purpose?

Would you also be willing to take a look at this scope and give me your input? It's within my price point and I thought it looked good. Thanks

Oscilloscope...BK2120 2Ch 20 Mhz Calibrated w/Warranty - eBay (item 320482511303 end time Feb-07-10 20:43:37 PST)

[Perry Babin]

Those diodes connect the individual protection circuits for each of the channels to the power supply protection circuit.

It's probably a good scope. B&K makes good quality equipment. This one appears to be working properly.

[shawnk]

Ok. Thanks to you Perry I think I'm right on track with this amp

I removed the diodes as you had suggested and I could determine that it was the left channel causing the problem. At this point I essentially decided to measure voltage across all of the components in this channel as I could use the good right channel as a reference. After a lot of tedious measuring I noticed R25 had significant voltage across it's terminals ~9v compared to ~1.7 on it's twin resistor in the right channel. After probing R25 a couple of times 'bingo' I could actually cause the protection to cut in and out. I pulled the resistor, checked it's value and resoldered it back in. Now the amp fires up fine with no issues. I'm not sure if this was the best way to diagnose this, but I guess it worked. Any other tips for better efficiency would be much appreciated.
My next step is to replace/reinstall all of the components in the sub channel and hope that will be the last of it.

[Perry Babin]

If I'm not mistaken, R25 is part of the bias circuit and if it had a broken solder connection, it could have been causing the amp to draw excessive current.

It appears that you found the problem quickly. Many times, using other channels as references makes it easy to find the fault.

[shawnk]

Ok. So this mono channel still wants to give me a hard time. As I had stated before, the amp will power up fine now that I have repaired the left channel. However, now that I have replaced all of the previously removed components in the mone channel, the amp blows a 10 amp fuse upon power up. Unfortunately it won't even trip the protection circuit. I can power up the amp with the 3ohm resistor inline with B+ and there seems not to be any overheating components, which is slightly better than before when r183 & r184 would heat up instantly.
Any thoughts? What's my best plan of action now? Thanks!

[Perry Babin]

Is the bias possibly set too high in that channel?

[shawnk]

No sir. The pot is set fully counterclockwise.

BTW: I did order that scope, and it should be here in a couple of days

[Perry Babin]

Did you pull the drivers and check them?

If so, are you sure you reinstalled all of them in the correct locations?