RF Punch 75.2, help plz - diyAudio
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Old 15th September 2006, 12:23 AM   #1
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Default RF Punch 75.2, help plz

I am new to these forums and amp repair in general, but I have a Q or two. What is the part called that is square, has white powder of some sort underneath and they are located on both sides of the amp, 5 each side ?(FETs) I think,lol. This is a RF Punch 75.2 and has been fixed twice by an incompetant dillhole in town and both times the repirs were crap. Thus I am learning on my own how to fix this pile of crap.The described part X1 is blown to hell and needs replaced but I can't read the numbers on it. If some one could get me a schematics of this amp it would be greatly appreciated as Im kind winging it here. Thanks a bunch in advance. e-mail gaylord_725@msn.com
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Old 15th September 2006, 02:29 AM   #2
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here is a pic.



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Old 15th September 2006, 04:51 AM   #3
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Hope you have the correct tool to work on that MESHA board mount of the fets.
And yes it looks as if the device saw way too much heat when it was installed, also way too much solder

It was a mosfet, and with that much heat check the little SMD resistor tied to the gate lead, and possibly re-flow the solder on the resistor.

Check the opposite side for the same damage. it takes two fets to make a power supply.
Also look near the 494 IC near the transformer there might be some damge there also.
I usually see damaged fet gate driver transistors. If this one uses them.

as always I hope this is found helpful

P.S. any 50 amp mosfet will work like IRFZ-44, mtp50no6, etc..
I use Intersil HUF 75 series rated at 75 amperes, others on the forum here use IRF-3205's, no biggy, they all work, if they are installed properly back on the MESHA boards.

The MESHA boards are soldered in a reflow oven. Its a fairly automated process, like baking bread. Just hotter.
I would suggest special low temp solder alloy's, and thin electronic grade flux.

I take less than 7 seconds to remove the bad fet, and less than4 seconds to re-attach the new device. I also have a cooling plate and a cool air fan to help reduce any excessive temperature that may have been induced into the board and the semi's.

I have been also told that RF will sell you new MESHA boards, you might want to call them and check...

Good Luck !
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Old 15th September 2006, 10:00 AM   #4
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You also need to check the driver transistors for the power supply. They will be small surface mount devices near the TL494/TL594 IC. They will have a markng like 1A/2A, 1G/2G, or 1T/2T. The transistor with the '2' is most likely to have failed. In amps with more power supply FETs, the device with the '2' almost always fails. In your amp, it may have survived. Good replacements for the part 2A, 2G and 2T devices are the MMBTA56 (2G) or the MMBT4403 (2T).

If the gate resistor is marked 101 or 680 (100 and 68 ohms respectively), I'd suggest using the IRFZ44N as a replacement. If the resistor is 51 ohms (510) or lower, the others mentioned would be OK. I generally use IRF3205s. The transistor directly opposite this one may have the markings intact if you want to use the original component.

Check the gate resistors after you've removed the FETs.

The extra solder doesn't look good but it doesn't mean the tech know what he was doing. I'd prefer to see more solder than necessary than too little. If you use too little solder, the transfer of heat from the FET to the insulator may be insufficient. If solder flowed from under the transistor to the tab of the transistor when it was soldered in place, that's fine. If a large quantity of solder was simply applied to the tab and insufficent heat/solder was used to get good solder flow under the transistor, that could cause the amp to fail prematurely.

If the power supply has failed twice and there is no other damage (no shorted output transistors...), it's possible that he missed the defective driver transistors or the gate resistors are out of tolerance (sometimes their resistance increases significantly when they are stressed by failed FETs). It's very common to have resistors that are completely open.

To remove/replace the transistors, I use a small butane torch. Radio Shack sells them. You can get it done with a soldering iron alone but it's somewhat difficult (not impossible) to do it and get a reliable repair. In either case, you should buy extra FETs in case some are damaged when replacing them.

Justonemoreamp, can you recommend a specific low temp solder alloy for him? I use regular solder and don't have any problems but if you can recommend a specific manufacturer and part number, it may make it easier for him.

If you want to repair the amp so you can learn something new, the people on this board can likely get you through it. If you simply want to get the amp repaired and are not doing it as a learning experience, find a new/different tech to repair it.
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Old 15th September 2006, 08:10 PM   #5
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Thank you for your help guys, very helpfull and if I have more Qs I know where to go Thanks again.
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Old 17th September 2006, 10:20 AM   #6
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Sorry I been at the shop all day,

You need Indium alloy solder. and it comes pre-cut in TO-220 pad size.
Its expensive when purchased retail, so don't get freaked when you see unrealistic prices for this stuff (Indium prices like gold in some places).
But they know you need it to, so catch-22, ya know ?

There are way too many different alloy types to go into here, but generally speaking any of the Indium based products will melt and flow lower in temp than Antimony/Tin , and lower than 60/40 lead tin.

You can use a torch, But the temp control sucks, and you must be very careful with that much heat.

Realize that your not really in control of the temp and go wisely when appling heat from a open flame source.

Oh and always apply the higher heat source from the bottom of the MESHA board, as the Mesha side must be hot enough to allow the solder to melt before the device will release, and it must be hot enough to reflow the solder after the new device in placed.

I generally remove all bad devices, and even out any excess solder at the same time.
Then I apply a thin and sparing amount of liquid flux with a applicator brush. Then a solder pad if needed.

Too much solder is not good. It only needs enough to provide a bond, nothing more. Anymore will require excessive heat and my damage the device
Then I place the pre-bent device in place, start the bottom heating.
I also will use a solder pencil on the tab of the device if I am in a real hurry.

When the device pulls down on its on, stop appling heat and allow the solder time to harden up abit, then move on.

Any thin liquid flux will do, try not to use paste type flux's as they apply too thickly and cause a big mess later.You don't need the extra clean up involved.

Working with MESHA takes abit of getting use to, so don't hesitate to consider buying new boards from Rockford.
They will save you all the issues and time of doing it yourself.
And the job will be simplified to re-soldering in the rail of devices

Hope this helps
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