How to open an Infinity Kappa 202a amp? - diyAudio
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Old 14th July 2006, 11:24 AM   #1
diyAudio Member
Join Date: Jul 2006
Question How to open an Infinity Kappa 202a amp?

Well, in a failed attempt to fix my Thump amp, I decided I would purchase my friends infinity. It works but the potentimeters have lots of dead spots. Tuner cleaner did not correct it. It will simply just cut out and if I touch the pots, it will work again.

There is no cover and if I remove the two screws on the bottom I cannot slide out the inner chassis. On the outer edges there are lots of allen screws that I am leary of removing. I am assuming these hold the mosfets to the case, if I remove these I am afraid there may be nuts or something behind them that I may never get back in. Any idea's?

BTW, in terms of quality and durability, how are these amps?
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Old 14th July 2006, 01:27 PM   #2
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Location: Louisiana
In my opinion, you should not open the amp. The outer shell is one piece. You have to slide the amp out.

The biggest problem is that they use sil pads for insulators. The transistors stick to the pads and can be VERY difficult to un-stick. If an insulator is damaged, it can be difficult to replace (especially if your hands are too large to fit inside the heatsink).

There are no nuts. There are clamps with threaded holes.
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Old 15th July 2006, 02:11 AM   #3
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Explain the sil-pads, do you mean the clear things that look like sheets of mica that go in between the mosfets and the case? How should I gain access to the internals? This amp has two power supplies and really is awome, if it does not cut out that is.
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Old 15th July 2006, 06:53 PM   #4
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Sil-pads are silicon rubber insulators that are used in place of mica and grease.

Often the transistors stick to the sil pads. For most amplifiers, it's not a big deal because you can individually break the transistors from the insulator. In the Infinity amps with the one piece heatsink/enclosure, it can be very difficult to unstick them (if they stick). If you damage an insulator, it can be difficult to replace unless your hands can reach to the damaged area.

Your amp may come apart and go back together with no problems. Before you disassemble it, I thought you should be aware of the possible problems. If there are problems, the amp may be damaged further.
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Old 15th July 2006, 11:44 PM   #5
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Ok, I think i understand now. I will give it a shot, at this point I used CRC brand QD electronic cleaner on the pots, and then used radio shack tuner spray with no success. I guess it is just sealed too well.
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Old 20th July 2006, 08:17 AM   #6
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Location: Northern California
The pots are sealed plastic type, and you replace them, as cleaning is pertty much a waste of time. Also you must remove the allen screws, you will note that the endplates use short ones keep them seperate.

All the rest must come out next, save them to a different bag.

YES the inside assembly will stick, as the sil-pads do melt and stick like they have become welded to either the case of the transistors.

Them you must gently tap the entire enclosure on a soft mat till you get the insides to break free. *Tap in a motion that directs the eq side out first towards the bottom direction.!* I tap the entire amp length wise on my spare static mat, but have had to actually drop the amp on end about 4 inch to the floor on a few repairs.
I know this sounds bad, but i fix these all day long, and this is how I fixed my last two personal 202A's that i own. Just be careful not to dent or damage the amps soft aluminum surfaces.
Enough said, lets move on.

You must also help all the pot shafts to clear the case, they can be pushed in a bit, not much, just enough to clear the case, no more.

YES the inside assembly will stick, as the sil-pads do melt and stick like they have become welded to either the case of the transistors. I find they stick to the case and you must remove this residue before reassembly. I use Acetone. it works well and completely degreases the internal enclosure but you must final wipe with IPA alcohol to remove any residue left by the Acetone. I also use a stiff plastic bristle bruch with a long handle to reach the nether regions inside. I soak some paper towels and wrap the brush, then clem till all the biege sil-pads material is removed. A chore at best, but it must be done.

After your finished repairing the intermittem pots, or bad solder joints as I have found. Then you can reassemle the cleaned and repaired amp.
I use Kaptom tape as a new insulator.
It sticks in place on its on, its rated for 500 degree's (way more than sil-pads) and if its good enough for satelittes it good enough for your car amp. I have never had a failure using it, but its pricey, and available on e-bay, listed under what else kapton.

I also use a fine coating of silicone paste on all the semi's on reassemble. if it ever has to come open again, you will appreciate this final touch, plus it aids in a void free seal between the semi's and the case-sink. * You must remove all the old sil-pads on the entire set of semi's and clean the backs of all the semi's till they shine like a new nickle, then apply the silicone paste sparingly, as a thin layer is all that is needed.

Then your ready to gently guide the amp back together, paying very close attention to the entire process, as to make sure you don't breach the Kapton insulator material, and inadvertantly short out all your hard work. A simple ohm-meter test from the case to either of the power terminals will be enough to check this out, a short and you might as well start over on the reassembly.

I also use a pointed probe as a alignment dowel to gently finese the amp board back into perfect alignment, by placing the probe in one of the hex cap screw holes and gently walking it all back in place by using a side to side prying motion. Works like a charm everytime.

With all that done your ready to re-install all those cap screws, starting in the middle first and working your way out one at a time one each side till they are all started. then you can follow even tightening from right of center to left of center one at a time. then go back and wrist check them all to a given wrist tightness only as they strip out if you get Gung-ho. try to set a uniform tightness.

remember to test with your ohm-meter for shorts before final tightening and then again after final tightening has been completed.

Well there you go, at least 2 hours of the most fun you have ever had in your life, next to time your relative put that badger in your pants
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Old 20th July 2006, 10:18 PM   #7
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Thanks for the write-up, I do have a question regarding the kapton tape. How is its thermal transfer properties? If I understand this correctly, the kapton tape goes between the transistors and the aluminum case. What transfers the heat to the heatsink? The few amps that I have worked on have either a dry type thermal tape, or mica with the white paste. Do i need to apply the grease to this kapton tape to maintain thermal transfer?

Also you seem to know these amps inside and out, are they a good amp that is worth keeping? What are the problem area's that you have experianced? The dual power supply makes it look like it was probably top of the line back then.
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Old 20th July 2006, 11:41 PM   #8
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Location: Northern California
The Infinity line of amps are OK in my mind. As I stated I own 2 - 202A's, and I also own 4 102a's, and 6 Beta 50's.

They sell for cheap on e-bay and internally they have just as good parts and design as any other amp from that time period. The Uni-sink chassis is a bit strange for most but it also has its merits engineering wise.
I also rebuild mine with upgraded parts from this era of Mosfet engineering. it adds a bit of durability to them as the parts available today are a far cry better than back in the day.

As for the Kapton insulators: I use kapton tape 1 inch wide. It has a not hardening glue that fills the voids in between the surfaces, on one side.

Its not cheap, but if you search the auctions it will cost you less than retail, which is astronomical to say the least.

As a insulator its rated at 500 degress the sil-pads would be dust at that point. They (NASA) use this stuff to coat wiring that is exposed to -250 F to +250 F in the void of space, so its not a week link. It has excellent dielectric properties as a insulator. Being rated much higher than sil-pads for electrical breakdown.

Your rebuilding a car amp, and although its a hi-tech item, its not the shuttle. So it is a bit of over kill technonlogy wise. The non-hardening glue on the back of the tape will flow to fill the voids between the transistor and the case. This is exactly what Silicone grease does.

The sil-pads have a lot of advantages, if your never going to open up the amp again. They hot flow to fill voids like silicone, but never cook away with heat. But you get welded transistors to the case and destroyed sil-pads upon teardown of the amp. So they become a throw away item and fairly painfull thing to completely remove from all the surfaces of this one style of amplifier heatsink design.
If it were any other amp, no big deal. On this one its a giant pain in the ars to remove and clean up completely. You must restore a flat heatsink surface or you will fail the semi's due to uneven seating and poor heat transfer.

So I replace them with Kapton tape. You can even reverse the tape and use the sticky side to the transistors as the glue will not vaporise and it is silicone based, it will transfer heat. Then you just paint a thin coating on the dry side of the tape to prevent scuff damage and slide the amp in its uni-sink without holding your breath, and praying it did not cut the surface. plus Kapton is fairly tough to cut, sil-pads are easily destroyed my mis-handling as they are very soft thermo plastic.

Well there you have it all my best advise. Oh please don't even consider trying to place mica pads in this style heatsink chassis. Even infinity did not do that. Plus how did you think you would get them to stay in place as you slide the entire amp chassis back into the sink ???
The chassis is fairly close tolerance and the slightest wiggle will put you out of alignment and risk damage to the semi's or the heat sensors located in the middle of the outputs (very delicate, please be carefull with this little bead sensor, as they can shear off easily and parts might be a pain to find)

One other thing, these Infinty amps were intended for 4 ohm stereo loads. If you run them at lower loads like less than 4 ohms mono, they will overheat and cause you grief. They were built in the day that 4ohms was the standard, and 4 ohms mono was the extreme condition they were ever meant to endure. So consider you usage well in advance of hooking it up and you will be alot happier down the road

Also in use you will note the output side of the amp seems to be the main source of heat for the entire amp while its running, especially if you dare venture below its rated output impedance, like say 1 ohm and 2 ohms mono. The amp will heat up very quikly and shutdown, if not fail itself.
Just some sound advice from a fellow owner
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Old 21st July 2006, 11:27 AM   #9
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What about 4 ohms mono? Will that hurt it? Also you failed to mention what else fails in these, like what failed in the ones that you have worked on?

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Old 21st July 2006, 03:44 PM   #10
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Location: Northern California
4 ohms mono is the lowest you would want to go, as the amp will be able to cook your lunch for you if you go lower than 4 ohms mono.

My test bench is capable of 90 amps continious at 13 volts and my test loads are rated at over 2 kw, and the overtemp condition was reached fairly quickly at any load lower than 4 ohms mono. It has to do with the high internal rail voltage that this amp has. Low ohm class AB amps rarely venture above + - 35 volt rails, this amp has + - 50 volt rails inside, So low ohms below 4 is a No-No

The repeat failures are in the power supplies. As I stated in my earlier post I rebuild the amps with better grade power fets from todays mosfet technology level. I replace the IRFZ-44's a 50 ampere rated fet, with Intersil 75336 or any 75 ampere rated mosfet. This adds durability to the power supply nothing more.
When the Mosfets fail they take out one of the SMD driver transistors on the power supply drive card that stands upright vertical at the power plug end of the amp. The sot-23 transistor is located on the back side iof the card as it is both thru-hole and SMD technologies combined.
I also replace the main power caps on the power supplies 4 in total, as they have most likely taken a beating over the years of use the amp saw before my bench.
Plus cap technology has also improved over the years, so you can actually shove a better garde larger rated device in the same footprint space.

Hope this helps, and get you where you want to be
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