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Old 17th May 2012, 12:09 PM   #1
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Default need help - modded amp blowing fuses

Well, I finally got around to upgrading the caps and am having some issues. I am popping the rail fuses. One on each side. On the left channel, I am popping the positive rail fuse as circled in red in the picture. On the right channel, I am blowing the negative rail fuse. Both fuses blew after powering it on with heatsinks bolted in place. When I first powered it on the left channel positive fuse blew but I did not have a fuse in the right channel negative slot. I had taken it out and forgot to put it back in. So when I first powered it up I had both fuses in place on left channel and only the positive rail fuse on the right side so I thought I only had an issue with the left channel. Then I noticed that the negative rail fuse was not installed on the right channel. Before powering it on again, I checked around to see if I had a short on the right heatsink. I couldn't measure one, so I put a fuse in and it popped as soon as I flipped the power switch.

As you can see in the pics, I took the heatsink back off the left side. Putting both fuses in and powering the amp on nothing blows. So it would seem that there is a short caused by mounting the heatsink. I know that the transistors need to have the insulator installed and it was in place when the fuses blew. I have checked the areas where I soldered and I can not see any solder bridges. I only soldered 8 spots ( 4 caps). They are circled in white. Cap C7 is the large 220uf non polar electrolytic which was swapped with the 220uf Bi-polar Muse which is a direct replacement. Cap C6 was replaced with a .47uf Wima MKP4. This is the bypass cap for C7. I installed it with writing facing up, same as the Wima MKS4 which was there previously. C1 was removed all together and C2 was replaced with a 4.7uf Claritycap ESA MKP film cap upon recommendations from AndrewT and others in my other thread. I actually installed this in the location of C1 as the mounting tabs on the board were larger and easier to open up a bit for the larger lead diameter of the clarity cap. Since these were in parallel to begin with, I don't see that as being an issue. The only other cap I swapped was C12, a .1uf MKS4 which I swapped for a .1uf mkp4.

Now, this amp was working fine before changing caps. Like I mentioned above, the left channel stopped blowing the fuse when i removed the heatsink. It only blew the positive fuse (red lead). I have not yet reinstalled the heatsink to see if the problem reoccurs. I did check the center leads on the transistors on the other channel and got no continuity to ground with one lead grounded to the chassis. I thought this side should be good but as soon as I put a fuse in the empty slot. it popped that one as well. (negative/blue lead).

Not sure what is going on. I immediately suspected a short to chassis through the transistor but this does not seem to be the case. I have now pulled the right channel heatsink and this side is now ok as well. All fuses are in and the amp powers up and the fuses hold. This points to a short with the Heatsink but I can not find one. I also checked my solder points on this side and no issues. I am using Artic silver 5 for thermal paste and the insulators that were originally installed.

anybody have any suggestions as to what might be going on or what to test? i don't see an inherent issue with the replacement parts as being the issue and everything works with heatsinks off, but as mentioned, don't see a short either. really stumped.


Mike
Attached Images
File Type: jpg left channel layout.jpg (566.2 KB, 68 views)
File Type: jpg right channel layout.jpg (568.1 KB, 64 views)
File Type: jpg A200_0001.jpg (100.8 KB, 65 views)
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Old 17th May 2012, 12:12 PM   #2
AndrewT is offline AndrewT  Scotland
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How many threads have this same enquiry?
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Old 17th May 2012, 12:14 PM   #3
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Artic Silver 5 is a electrically conductive paste as it contains a metal powder. You should use the non conductive white electrical thermal compound.
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Old 17th May 2012, 12:40 PM   #4
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i googled and everywhere said artic silver 5 is non conductive?
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Old 17th May 2012, 12:46 PM   #5
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Well in order to get that silvery shine it has to contain a metal, and non conductive may be true at the kinda voltages you find in a PC but not at the 35+ volts you find in an amp.
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Old 17th May 2012, 12:49 PM   #6
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Quote:
Originally Posted by AndrewT View Post
How many threads have this same enquiry?
Enquiry about fuses popping in general or about Artic silver causing the problem? Obviously knowing of something that is a potential issue makes searching for it easier. Seeing as Artic Silver is said to be nonconductive, I would not have suspected it. If this is not what you suspect, could you point me in the right direction of what to search for please. You were very helpful in my original thread, and I spec'd my parts based on your recommendations.
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Old 17th May 2012, 12:54 PM   #7
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you know, I kept trying to understand how something that is supposed to be 99% silver can not be conductive, but before I used it, I searched google and everything came back nonconductive. I did not search this site. If I hadn't already been misled to believe it to be ok, I wouldn't been so perplexed. So pretty much the paste is the whole issue. It's the only thing that makes sense based on what I am seeing.

Mike
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Old 17th May 2012, 08:35 PM   #8
mjona is offline mjona  New Zealand
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Hello Mike,

Casting a quick eye over the photo you posted, can you clarify whether the Red Block WIMA cap at the extreme left near the amplifier output leads, is original or has been a modification at some stage.

The board seems to be a mirror image enabling correct orientation for left and right channels inside the chassis. Looking to the equivalent overlay on the pcb on the right I note the spaces provided for C15 and R34 differ from what I see at the left.

The resolution in my laptop is not good enough for me to spot if there is a resistor sitting below the Red Block WIMA cap on the board as mentioned above. This resistor would have to be around 5 watts or so in rating and it might have been more easily seen in the photo.

This combination of capacitor and resistor forms a stability network and if the resistor is missing or replaced by a component of smaller rating there could be stability issues that are the cause of blowing fuses.

It is a good idea when trouble shooting power amps to disable the channel that is not in focus. Blowing fuses is also inconvenient so a couple of 7 watt 1k wirewound resistors in parallel - you can twist the leads at either end and solder each end so formed to a small alligator clip and use these to bridge across your fuse holders. You can measure the voltage drop across the fuse holders with these in place to work out roughly the amount of current being drawn due to a fault condition.

Another sign of fault is heating of these bridge resistors - a moistened finger is a quick test I have used before using a meter. Keep the your free hand on the on/off button switch if these become too hot to touch.

Michael Jonassen

Last edited by mjona; 17th May 2012 at 08:59 PM.
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Old 17th May 2012, 08:40 PM   #9
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Default Arctic Silver 5

Actually, the sales talk is that only the silver part is 99%, like saying this amp. case is 100% steel etc. I note they
warned you might measure capacitance between pins if you contaminated connectors with the stuff.....Hmmm

I would guess you got 99% of 5% there or they would be asking a lot more than $5 for a few grams.
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Old 17th May 2012, 08:56 PM   #10
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Quote:
Originally Posted by mjona View Post
Hello Mike,

Casting a quick eye over the photo you posted, can you clarify whether the Red Block WIMA cap at the extreme left near the amplifier output leads, has been replaced at some stage. The board seems to be a mirror image enabling correct orientation for left and right channels inside the chassis. The spaces provided for C15 and R34 differ from what I see at the left.

I believe they are the original caps. WIMA mkp10. I'll check values when I get home. It is position C15. According to schematic, should be .22uf. There is a resistor (R34) uderneath.Left and right boards are mirror images and can be used on either side.
.
The resolution in my laptop is not good enough for me to spot if there is a resistor sitting below the Red Block WIMA cap on the board as mentioned above.

Yes, resistor under cap.

I would think this resistor would have to be around 5 watts or more in rating and it might have been more easily seen.

according to A200 schematic, R34 is 1.5ohm 2w. I'll double check and see if mine is same value.

This combination of capacitor and resistor forms a stability network and if the resistor is missing or replaced by a component of smaller rating there could be issues from that cause.

These should both be stock.

Michael Jonassen
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