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Old 15th August 2007, 07:44 PM   #1
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Default Carver TFM-15 - Need Help!

I have been using my Carver TFM-15 amp for a couple of years for deejaying without a hiccup. Recently, I had the amp unplugged for about a month while I was on vacation, and since returning I am having the following problems:

The amp will turn on normally and remain on for about 10 minutes. After this time (regardless of whether speakers are connected or if music is playing), the relay will switch off for about a minute. It will then switch itself back on, and the amp will continue to run fine indefinitely.

Note this happens only when the amp is turned on when it's cold. If it has been on and warmed up, this issue does not repeat.

I purchased the TFM-15 defective a little over a year ago. At that time I believe there were cold solder joints on the output transistors. After de-soldering, testing, and re-soldering them, the amp worked flawlessly. I have a write-up of the procedure here on my website:

http://www.lehigh.edu/~evb209/projec...er/carver.html

What could possibly be wrong with my amp? I have already performed the following tests:

desoldered and checked the relay
checked the relay diode (it tested fine, replaced it anyways)
refreshed output transistor solder joints
tested all PCB resistors, diodes, and transistors (in place, with multimeter)

It seems like this is a issue related to how the amp warms up. Maybe a bad solder joint somewhere? Any and all information would help me out a great deal! Note I began this post at the Carver Audio forum here:

http://www.carveraudio.com/phpBB2/vi...er=asc&start=0

I will work on refreshing all solder joints in the meantime. Please help!

Thanks,
Angelo Brisimitzakis
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Old 16th August 2007, 06:07 AM   #2
anatech is offline anatech  Canada
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Hi Angelo,
Look for over heated solder joints near the rear of the main circuit board.

I must say that I am not comfortable with people with no servicing training running around in these units. This is a complicated amplifier and I like them.

So a word of caution here. The power caps stay charged up. Discharge them through a 10 ~ 33 ohm power resistor. Do not discharge by shorting the terminals! These amplifiers can be very dangerous. Also consider that the more energy there is, the faster and more complete the damage is when a mistake is made.

-Chris
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Old 16th August 2007, 07:04 AM   #3
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Thanks for the input Chris. I've made it a habit to discharge all large filter caps with a 10ohm, 10W resistor.

I know I'm no expert, but I have had good luck troubleshooting these amps before. I hope your expertise can help me fix this one! Any other recommendations besides re-soldering all joints? Does this really sound like a thermal issue, or is there something more sinister at work here?
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Old 16th August 2007, 09:45 PM   #4
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Hi Angelo,
Always look at stuff that runs or ran hot.

-Chris
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Old 16th August 2007, 10:31 PM   #5
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After all the time I spent testing the board, its seems like there was just another bad solder joint after all!

I re-soldered all the traces on the main PCB, and the amp is no longer exhibiting any of the previous problems! I will continue listening tests, and if all goes well I will change this thread to "resolved".

Thank you again for all your advise! I've got another project coming up (involving two broken TFM-25's), and I'm definiately going to need your help on it!

-Angelo
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Old 17th August 2007, 01:54 AM   #6
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Hi Angelo,
Good work. I'm glad you solved it. I use a little TFM-6CB on my bench.

My strong advise to is is a little labour intensive (sorry), but this is the right way to do these things.

When you find connections that have been hot and the solder joint has gone grainy, add some fresh solder then use a sucker followed by wick to remove it all. Clean the component lead, scrape it or use sandpaper. A little solder flux (for electronics) and resolder using only enough for a good joint. You must clean the area after removing the solder so you can see cracks in the traces, and after soldering to check for solder bridges or whiskers. Use a solvent, do not scrape the PCB - ever! The board should look better than new.

You will find that your solder joints will outlast the wave soldered ones.

I do this on almost every single repair I do. I can tell instantly if someone else has been in there. So can people who know me.

-Chris
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Old 17th August 2007, 01:37 PM   #7
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That is definitely the right way do it, but my methods tend to be a little more quick-and-dirty. I re-heated all the joints to melt the solder, and topped them off with a little more. The ones that bulged excessively were fixed with some solder wick. All joints were inspected, and the space between traces close to each other (transistor pins, etc) were scraped with an exacto knife (sorry!). I made sure not to cut deep into the board, but I probably could use a less abrasive method in the future. Do you have any flux and solvent recommendations in particular?

Also, since you are a moderator, would you mind changing the name of this thread to "RESOLVED Carver TFM-15 - Need Help!"
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Old 18th August 2007, 04:41 AM   #8
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Hi Angelo,
Quote:
Also, since you are a moderator, would you mind changing the name of this thread to "RESOLVED Carver TFM-15 - Need Help!"
I could, but I don't think that is the correct thing to do. You had a problem, you worked it through and ended up with a running amplifier. I would therefore decline the request as it serves no purpose than to confuse people who may look for your thread under the original heading.

Quote:
my methods tend to be a little more quick-and-dirty. I re-heated all the joints to melt the solder, and topped them off with a little more.
The problem with doing that is this type of repair does not reliably fix poor connections. There are times when a component lead has a coating or oxide on it before it's soldered at the factory. They can open up again and you will have one devil of a time finding them the next time around.

So there is only one way to do a repair, and that is the correct way. I would urge you to redo it properly, but this is up to you. If you ever repair something for someone else, make darn sure your work is neat and correct. The worst jobs I see are "I took it to a friend that knows something about electronics", or the ever popular "my neighbor does electronics as a hobby, so he fixed it for me".

-Chris
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