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Old 17th January 2012, 01:08 PM   #1
m4ick is offline m4ick  United States
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Default MTX Thunder81000D

Opened this one up, all the PS fets are burned and one of the rail caps is vented (3300uF 80WV), the top is cracked wide open.

Just curious what would cause the rail cap to fail so badly. Is it just age? I assume that ALL rail caps should be replaced, correct?

Also, D198 and D199 are in circuit. Should I remove D198 to prevent voltage being applied to the RCA shields?
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Old 17th January 2012, 02:23 PM   #2
m4ick is offline m4ick  United States
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Any chance you could help me find an exact replacement for the rail caps? I'm not sure about the sizes I'm finding. It's made by Chemi Con and is the SMH series, I found it on mouser but it claims to be 1 inch in diameter and 40mm in length. Seems tall.
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Old 17th January 2012, 07:35 PM   #3
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How tall are the capacitors in your amp?
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Old 17th January 2012, 08:16 PM   #4
m4ick is offline m4ick  United States
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The rail caps are 1 and 3/8" inches wide and 1" tall. Lead spacing is about 1 cm.

I also need the gate resistors for the PS. According to the photos in your tutorial they are 39.2 ohms, one per bank, but I am not good with SMD sizes and wattage handling. If you could point me in the right direction on those as well it would be greatly appreciated.

And what do you think about D198, should it be left alone or removed?
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Last edited by m4ick; 17th January 2012 at 08:20 PM.
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Old 18th January 2012, 03:39 PM   #5
m4ick is offline m4ick  United States
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These should fit, I've never had to replace rail caps.

380LX332M080K022 Cornell Dubilier Aluminum Electrolytic Capacitors - Snap In

Don't know much about ordering rail caps, are these ok for this amp? I don't know what specs I should be looking for other than capacitance and voltage handling.
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Old 18th January 2012, 04:13 PM   #6
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When ordering caps you also need to check the lead spacing to see if its the same ..
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Old 19th January 2012, 02:05 AM   #7
m4ick is offline m4ick  United States
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Sizes and lead spacing look good, the datasheet claims low ESR. Is that all I need to know?

Sorry for being so particular, first time ordering rail caps and don't want to see $40 go to waste.
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Old 19th January 2012, 03:09 AM   #8
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Those should work.

You'll need a good iron or low temperature solder (maybe both) to remove them. Item #17 on the Tech Tips 4 page.

For general information on selecting replacement capacitors, read item #8 on the Resistors and Capacitors page.
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Old 25th January 2012, 10:48 PM   #9
m4ick is offline m4ick  United States
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Well, I ordered all 8 capacitors and now the customer says he doesn't want to pay to replace ALL of the rail caps.

What are the chances this amp will fail relatively soon if I replace 1 rail cap instead of ALL rail caps? Or what if I just replace the 4 that share the same rail?
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Last edited by m4ick; 25th January 2012 at 11:02 PM.
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Old 26th January 2012, 12:41 AM   #10
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Replacing the 4 parallel caps should be OK but if one of the originals failed due to age or an internal problem with the cap (not caused by over-voltage, etc...), the others may be likely to fail.

If you repair the amp without replacing all of them, tell him that there is absolutely no warranty on the amp if the amp fails again due to one of the remaining caps failing.
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