Blew an LM4780 chip last night!How?

[Dominick22]

So last night, I was fooling around with my completed mono amps trying different speakers. I actually had the cd input signal on pause when there where a few bright flashes.

When I looked closely, The 1, 3, and possibly 2 pins were fried!

So any ideas as to what caused it?
I have used the amp for about 5 hours (easy listening) intermittently so far before this happened and the other mono amp is fine, although it is wired the same way???

Thanks,
Dominick in New Jersey

 

[peranders]

What did the "fooling" consist of really? How does the amp and the wiring look like? Some short, loose wire etc?

A LM4780 doesn't pop just like that.

 

[Dominick22]

Well, I decided to try them on my Martin Logan Clarity's and listened for 1 song, about 3 minutes and then hit pause on the cd player. About 1 minute later, while on pause still, Bada Boom Bada Bing! An indoor lightning show!LOL

I am really clueless. Is it possible that there was a delayed failure reaction??

Dominick

 

[Narcisse91]

Are any other components damaged?

 

[peranders]

Did your amp had any output filter? Martin Logan's are a bit capacitive by nature.

Does your amp have any input filter? Maybe HF garbage went through?

 

[Dominick22]

The compenents were put together exactly the way they come from audiosector.com!

I did not add any caps or anything else.

So did the ML's burn it?

Dominick

 

[Dominick22]

From what I can SEE, nothing else was damaged.

 

[tiltedhalo]

LM4780

The ML are a very inductive load, the impeadence at high frequencys can drop as low as .5ohms!!! Chip are prone to high frequency oscillations also. You should always use an input coupling cap with parelleled solutions, also an input network 1k and 22k to ground to set input gain, you need a Ci cap or servo.

 

[Bazukaz]

It isn't easy to burn these chips , even with hard capacitive/inductive loads.Check power supply , rectifiers , insulation to heat sink.

 

[Dominick22]

My heatsink for the chip was touching the mounting plate for the 330 va transformer...but the mounting plate is isolated and it doesn't make actuall contact with the transformer itslef.

Could this be the cause? I wouldn't think so.

Dominick

 

[Russ White]

Isolate the chip

Your best bet with the 4780 is to always use something like a mica isolator/insultor directly between the chip and the heatsink. Your chip fairlure sounds like a short to the chip case. I have seen this more than a few times.

 

[Dominick22]

Russ, Any link to where I could see/purchase this mica type insert???

I have never heard of this!

Thanks for the input,

Dominick

 

[elementx]

they don't look like much... just flat pieces of plastic like material cut into various shapes. you've probably seen them on to3 transistors... just google mica insulator.

 

[traw]

I had the same thing happen, zap and dissintegrated the pins. I touched my tuner's chassis to the heatsink. Damaged my tuner in the process as well.

Can one just get mica sheets and cut out shape? Damn, I was just at electronic surplus store during lunch.

Any alternate material? Well, imagine other material could impede thermal conductance to heat sink.

 

[davidsrsb]

The use of mica is rare these days. What you want is something caled Sil-Pad from Bergquist, which is an elastomeric material.

If you read the chip data sheet, it is protected against output short to ground, but says nothing about short to a supply. The tab is at -ve supply I think.

 

[Dominick22]

So I am trying to take you reccomendation on the sil-pad...but which one do you use. These are what is available from the company.

http://www.sil-pad.com/order_samples.cfm

Thanks,
Dominick

 

[Bearman]

If you have an old hair blow dryer around that doesn't work, they use mica insulators around the heating coils. They are kinda thick but it's easy to shave a thin slice off.