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Carver M-500 blows capacitor
Carver M-500 blows capacitor
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Old 8th June 2011, 05:55 PM   #1
ogopj is offline ogopj  United States
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Default Carver M-500 blows capacitor

Hello All!

I'm hoping somebody here can help me out with this. My dad has a Carver M-500 amp that is probably about 25 years old by now that is not working, and I would love to get it fixed for him for Father's Day.

I took a look at it, and all but one of the capacitors on the power board were bad/leaking. I looked around and found replacements for all of them. The only real problem was the big two (6800uF, 80W, +85C) on the front side. Those two had four posts, and all I could find were caps with two posts. No problem as two were only used for "support" and were blank anyway. (More on that in a bit)

I replaced all the caps, plugged it in, and beautiful music ensued. I ran it for about 2 hours, and sadly realized I needed to give it back. So I put the top on, and took it back to my dad. He plugged it in and the fuse blew, and he said there was smoke coming out of it. So I took it back, and sure enough, one of the two big caps had bulged and when I tested it, no longer had any resistance to it.

So I purchased another thinking maybe something came loose during transport to his house, or who knows, maybe just a bad one. I put the new one in, plugged it in, and again, beautiful music ensued. I put the top on, let it play, and after about 40 minutes, the fuse blew. I took the top off and thought maybe it was just getting too hot. I replaced the fuse, plugged it back in, and it worked for about 30 seconds until it blew the fuse again. I looked closer, and the same cap is again bulging....

About those big caps. As I said, the originals had 4 pins, 2 that were blank, and two active. The only replacements I could find only had two. No problem except that the holes didn't match up. To make them fit, I used a piece of metal (came from a cordless phone for the battery connection) to extend the pins so they could reach the holes.

Finally, on to my questions(s). Could what I did to extend the pins be causing the problem? If so, what should I do to make them fit? Should I go to radio shack, get some blank circuit boards, and make a proper fitting adapter board? Or, if that isn't the problem, what else could be causing it, and what should I test?

Any help or suggestions would greatly appreciated. Thanks in advance!
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Old 8th June 2011, 06:28 PM   #2
Mooly is offline Mooly  United Kingdom
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Carver M-500 blows capacitor
Have you measured the voltage across the caps when the amp is working ?

First thought is that the caps are not rated correctly or are fakes... apparently there are many around and pictures etc of them have been posted on here somewhere.

The power supply in the Carver is complex and uses a triac to control the power transformer... so check the voltage across the caps first. If thats OK then it looks like they could be fakes or not up to the job.

Manual here,
Carver M-500 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

Edit... see if the power supply adjusts correctly when following the service manual procedure.

Last edited by Mooly; 8th June 2011 at 06:32 PM.
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Old 8th June 2011, 06:52 PM   #3
BigE is offline BigE  Canada
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Are those "4 post" caps actually duals? ie, two caps in a single case? If so, you can't replace them as you are doing.
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Old 8th June 2011, 07:01 PM   #4
Andy5112405 is offline Andy5112405  United Kingdom
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Check the diodes in the power supply, especially the Bridge Diodes feeding the Large caps. If a cap has failed short circuit it could have failed its bridge short circuit. In such case you might be feeding AC (or VERY LUMPY DC) to the cap causing its early demise.
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Old 8th June 2011, 09:13 PM   #5
effebi is offline effebi  Italy
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Just a note. You quote an 80W capacitor. I hope you mean an 80 V (Volt) capacitor. If you got one with a lower voltage, then it is normal that it blows very early. A higher rated voltage, on contrary, will do better (safer) but will be probably bigger in size.
Check well the polarity of the connection, it is ez to get wrong with the four legged holes and two legged replacement.
Bending the legs is not a problem as long you do not make some unwanted contact with some other component or track on the PCB.
To improve mechanical stability you might simply redo the holes ( in a blank area) on the PCB and then bend the terminals under it to reach the proper connection pad. Be careful.
When you buy new ones , check if you can fit a 105C one, they are more rugged than the 85C. Don't need audiophile stuff, just good standard brand (i.e. Nichicon,Panasonic,Epcos etc.) Avoid cheap funny or no-brands stuff.
effebi

Last edited by effebi; 8th June 2011 at 09:22 PM. Reason: further info
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Old 9th June 2011, 03:10 AM   #6
Tajzmaj is offline Tajzmaj  Slovenia
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Ah Carver again.......I have 10 racks of those amps for touring and one m400 just for fun.....
So there is a lot of things which can go wrong. Someone mentioned that bridge rectifier and diodes.....must be checked. Second thing Mooly mentioned triac regulation at the primary....
Carver amps use triac regulation in the primary circuit ,,to charge,, psu caps at wanted voltage. This voltage can be adjusted via trimpot inside. so you must measure voltage on caps to ensure that it isn't higher than specified in the service manual. If there id too high voltage it can be caused by defective trimmer.
But there is one more possibility.
Those amplifiers have 3 different voltages for output stage and one +-12V to feed other circuits. Those other 3 are something like +-25 , +-50 and +-75V. You must confirm all those voltages, because all of them are monitored by psu. If there is defective cap which can't be charged any more than voltage is too low and psu try to charge it by increasing conductive angle of triac. Next thing which happens is that there is still too low voltage on that cap but on all others is way too high ......the rest you know. Such scenario cause only one side of psu to blow up caps.....
I have such case in one of my PT2400 amp......
Be careful with high voltages......
There was similar discussion few weeks ago. Carver tfm55........I cant remember
Best regards
Taj
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Old 9th June 2011, 03:27 AM   #7
Tajzmaj is offline Tajzmaj  Slovenia
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aha
http://www.diyaudio.com/forums/solid...r-problem.html
Bad Carver M1.5 Power Transformer??
and there must be some more....
Regards
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