another input cap question for SI t-amp

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schumpe

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Joined 2006
#1
I bridged the C3 and C4 on my T-amp, with a wire leaving the original capacitors in place.
Like so:

http://forum.stereophile.com/photop...5/password//sort/1/size/medium/cat/505/page/2

I was thinking of leaving all the resistors in place as well, "VERSION 2 -- "JUST THE CAPS, MA'AM"" from Michael Mardis' website:

http://www.michael.mardis.com/sonic/start.html

Now I'm confused about the polarity of the new input caps. I'll be using polarized electrolytics for now and was thinking of wiring them to the input wire from the board connector. Even if I had the skills I couldn't wire them to the pads right now, since the old caps are still there.
I saw some contradictory info about the polarity, so I'm just double checking.

The positive side has to go towards the board and the negative towards the pot, right?
 
#2
I just checked the instructions for the Amp-32 kit (same Tripath chip) I'm about to put together, and negative towards the pot, positive to the board is how he shows the input electrolytics.

Best to wire them off-board; the pads on the SI boards are prone to lifting with any stress on them. Easier to hook up some better caps when you get them, too.

--Buckapound
 
#3
So I bridged the capacitors and installed the new ones.
However, I don't get any sound. I heard a slight thump on the speakers when I turned it on the first time. I thought that was due to the larger value of the new caps, but no sound. The next couple times I turned it on, no thump anymore.

I removed the bridge on the old caps and also removed the new caps from the signal path.
Still no sound, but now I heard some noise from the speaker when I turn the amp off.

The amp gets power and the LED lights up. I checked all my input and output wiring with a multimeter and it's fine.
I also check the speaker outputs with the multimeter and it reads no DC or AC (when the amp's playing).

I'm kind of clueless what to check for. I have no idea what could've gone wrong.

Any help for troubleshooting is much appreciated.
 
#4
schumpe said:
So I bridged the capacitors and installed the new ones.
However, I don't get any sound. I heard a slight thump on the speakers when I turned it on the first time. I thought that was due to the larger value of the new caps, but no sound. The next couple times I turned it on, no thump anymore.

I removed the bridge on the old caps and also removed the new caps from the signal path.
Still no sound, but now I heard some noise from the speaker when I turn the amp off.

The amp gets power and the LED lights up. I checked all my input and output wiring with a multimeter and it's fine.
I also check the speaker outputs with the multimeter and it reads no DC or AC (when the amp's playing).

I'm kind of clueless what to check for. I have no idea what could've gone wrong.

Any help for troubleshooting is much appreciated.
Click to expand...

Did you remove the resistors when you bridged the caps?... and if you did , where did you connect the new caps...

Or... did you bridge the caps and then do the 'just the caps' mod?

Can you give us more detail and maybe some more pictures of your wiring?
 
#5
I just bridged the caps, no resistors were removed.
I reversed all the modding steps I performed.
The amp is now in the same state it was before - or rather not, since it doesn't play anymore.
I might be able to take some pictures later. It all looks like before to me....
 
#6
schumpe said:
I just bridged the caps, no resistors were removed.
I reversed all the modding steps I performed.
The amp is now in the same state it was before - or rather not, since it doesn't play anymore.
I might be able to take some pictures later. It all looks like before to me....
Click to expand...

When you bridged the caps, you needed to remove the resistors. This gave your bias voltage a direct path to ground...not good.

Check the caps are still OK by testing their resistance. You should get no reading or a steadily increasing value to infinity.
If you get a small resistance then the caps may have been fried or there is still a bridge under the caps.
Try taking the caps out completely and wiring in a new set as per 'just the caps' on MM's site.
 
#7
Thanks for the quick response. I'll check it when I have a chance.
However, I am confused. I thought from Michael Mardis website, I could bridge and replace the caps without removing any resistors.

Just to clarify: I did not run my amp just with the bridged caps in place. I added new input caps just before the connector to the board after I bridged the caps.
 
#9
schumpe said:
Thanks for the quick response. I'll check it when I have a chance.
However, I am confused. I thought from Michael Mardis website, I could bridge and replace the caps without removing any resistors.

Just to clarify: I did not run my amp just with the bridged caps in place. I added new input caps just before the connector to the board after I bridged the caps.
Click to expand...

OK just to clarify the methods:
1. If you just replace the caps then you remove the ones on the board and connect the off-board caps to the same 2 pads.... so that the signal goes through the new ones the same way as it did through the old... make sense?
2. If you bridge the caps on the board then you should do the 'stealth' mod and remove the ground resistors (not the input resistors).

Basically the signal path should be chip - input resistor - cap - source.......... when you look at it in reverse. The ground resistors do not always need to be there but if they are then they must come up-stream of the caps i.e Chip - input resistor - cap - ground resistor - source.
 
#10
I see.
So what I did is I left the original input caps on the board.
I bridged them with a wire as in this picture:

http://forum.stereophile.com/photop...5/password//sort/1/size/medium/cat/505/page/2

Then I hooked up the new caps the same way as in the stealth mod.

Unfortunately, I did not review the schematic, I thought the caps would be in the same place in the signal path, just in a different physical place.

Is there a chance I killed my amp that way? How would I know.
 
#11
Not sure what the damage will be but the excesive DC could have fried the chip. It doesn't always but......
To see if it's dead or not you need to get some caps and do either the stealth or caps only mod. Don't spend a fortune on these caps just for testing get some cheapies from maplin.
 
#16
Well, I started working on this before your last post.
I removed the two resistors, bridged the caps and wired the electrolytics as instructed for the stealth mod.
Still no sound.
However, I left the original caps on board and just bridged them, as buckapound told me.

http://www.diyaudio.com/forums/showthread.php?s=&postid=1249488#post1249488

post#515

So unless I actually have to remove the original caps, my amp may be dead. Or it could be something else that's wrong....
 
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