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Old 30th September 2006, 05:13 AM   #1
Bgun is offline Bgun  United States
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Default mesha problems

i have a rockford fosgate 800.2 with the mesha heatsinks. one of the output transistors burned off the mesha board and peeled up part of the copper heat pad it sits on, is there anyway i can repair the heatpad or should i get a new mesha board? also it looks like a few transistors are missing from the mesha, even though there is gate resistors for them as well, is this normal or are they just missing? i got this amp broken and i am not sure if someone else has been in there.
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Old 30th September 2006, 12:56 PM   #2
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There's no way to repair it. It must be replaced. If you can't find a a donor/junk amp, you'll have to get it from Rockford. Get the number off of the insulator before you call.

If there is a 0.1 ohm source resistor, there should be a transistor. Use the other channel as a reference.

If the outputs failed, there is likely other damage. Use a high power lighted magnifying glass to inspect all of the surface mount resistors.If any have defects in the top surface, they need to be replaced.

You need to check all of the gate resistors with your meter while the transistors are out of the circuit. Also carefully check all of the source resistors before replacing the output transistors.

There are two sets of MPSA06/56 driver transistors (not the two near the center of the channel on the board). Replace them. Even if they seem to be OK, replace them. If any one is leaking (electrically), it will destroy the new outputs when the amp is powered up.

There are two 20 ohm resistors (marked 200) connected between the drivers and the outputs, they have likely failed.

There is a 1k ohm resistor near the center of that channel's group of components that fails. It's R227 on the 800a2. It may be the same in the 800.2.

Get a schematic when you order the replacement insulator from Rockford.

As always, when initially powering the amp after the repair, you should use a fuse of ~10 amps or a current limiter. On this amp, it only really serves to protect the power supply transistors. This amp has a huge bank of capacitors. It charges them then switches on the output biasing. If there is any defect that you missed, the output transistors will likely be damaged even if you're using a current limiter or small fuse in the power line. You must thoroughly check everything possible before applying power.
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Old 30th September 2006, 03:45 PM   #3
Eva is offline Eva  Spain
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You may also choose to discard all that MEHSA useless junk and reuse the original aluminium plate with longer screws to press the transistors directly to the heatsink with adequate mica or silicone insulators. That requires some labour time and new transistors, though.
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Old 30th September 2006, 04:52 PM   #4
poobah is offline poobah  United States
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Dumb question...

What the hell is MEHSA???????

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Old 30th September 2006, 06:00 PM   #5
Eva is offline Eva  Spain
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MEHSA is one among dozens of stupid useless ideas sold to the public as technological breaktroughs in car-audio. See the picture on first post. It consists in soldering the TO-220 transistors directly to very thin isolated copper-sheet islands stuck to an aluminium plate, which is then bolted to the actual the heatsink and coupled with thermal grease.
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Old 30th September 2006, 06:06 PM   #6
poobah is offline poobah  United States
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Hmmmm.....

I use 2 ounce plate islands for heatsinking little baby things... a watt or so. I been putting 220's and 247's on live copper sinks (busses) for years.

This is new? It's all marketing BS I guess...

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Old 30th September 2006, 07:40 PM   #7
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poobah:
They've used these for years. I think the design has merit when properly applied.

Eva:
Although I preferred the older anodized aluminum insulators and clamps, these are probably more efficient. They work well but I would have preferred to see them use clamps to apply pressure to the transistors. Using only the screws in the insulators, you have to double-check the seating of the insulator on the sink. If the amplifier is properly reassembled and reinstalled, the amp will be completely reliable.
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Old 30th September 2006, 07:55 PM   #8
poobah is offline poobah  United States
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Hey Perry,

I'm all for anything that cuts assy costs.

I recently studied another design using SMT stuff. All the components were on one side of the board, the 220's (d2PAK)were soldered flat on the board over a dense matrix of vias. On the opposite side of the board, heavily plated, were large bare areas of copper. The whole board was simply placed on a sheet of silicone over a flat heat sink with spring clamp bar for the 220's.

A thermal masterpiece... no, but probably not required. The devices are over-sized and run nice and cool anyway. But, an elegant solution for rapid production.

I guess this is similar to the MEHSA. I think any method that allows in increase in surface area at the isolation junction (silicone/mica) is a good thing to look at. If only to provoke some thought... so I can do it better.


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Old 1st October 2006, 07:34 AM   #9
Bgun is offline Bgun  United States
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is re attaching a new copper pad with thermal resin an option on this? if i take off all the transistors, put down thermal glue and bake this mesha board in my oven at 400f to cure it, would this work? or what if i scrape or sand off the area the pad is sitting on and use tape and thermal compound, jerry rig up a pressure bar for the two resistors? time isnt really a factor, but replacing the mesha board is 80 bucks all said and done, and im not sure this amp is worth that much.
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