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Old 15th February 2006, 04:27 AM   #1
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Default repairing a/d/s/ 640 amp

Hello,
Trying to sort out what these chips do... I'm trying to put theoretical knowledge down to my first amp I'm repairing.

The output transistors are easy enough, but there are 2 sections of transistors I don't know what they do...

There is a pair of PNP/NPN transistors right next to the rectifier diodes (center tapped x2) that I think drive the pulses from the TL494 ? They are 2N6490/2N6488 here is the link to their data sheets if necessary:

http://www.st.com/stonline/products/...re/ds/5237.pdf

And then there are 8 NDP7060 N-Channel Enhanced Mosfets (can I call them just Mosfets?) which are all the same... here is the data sheet:

http://www.fairchildsemi.com/ds/ND/NDP7060.pdf

These are blown or at least the first 3 were... so I bought 10 new ones... was that good to replace all of them? also bought the pair of 2N6490/2N6488 as well to be replaced. Could I have used cheaper IRFZ44 or something similar, or should I stick with the real deal ones that the amp came with....

Click the image to open in full size.
Click the image to open in full size.

If the complimentary pair 2N6490/2N6488 are driving the regular and inverted outputs of the TL494, what are the 9 NDP7060 mosfets doing? stabilizing the rail? they're all the same. Sorry if this is a dumb post, but I'm really really just getting started...

Also I heard it was a good idea to isolate the audio section from the power supply section with a bi-polar power supply, how would one go about doing that, and testing the audio section before getting to the power supply? Taking out all the mosfets?

Thanks in advance for any help. Bill
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Old 15th February 2006, 07:22 PM   #2
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Join Date: Nov 2003
Location: Louisiana
These are blown or at least the first 3 were... so I bought 10 new ones... was that good to replace all of them?

**** It's best to replace all at the same time using components that have the same date code (insures that the parts are relatively closely matched).


Could I have used cheaper IRFZ44 or something similar, or should I stick with the real deal ones that the amp came with....

**** Unless the original design was weak (high failure rate), it's generally best to use the original parts (especially if you're just getting started). As you get more experienced, you can use different replacement parts but sometimes it requires that you change other components to get the circuit to work properly.


If the complimentary pair 2N6490/2N6488 are driving the regular and inverted outputs of the TL494

**** It's unlikely that the 88/91 pair are driving anything on the 494. It's more likely that the pair are regulators that are connected to the rail voltage. The rail voltage may be going back to the 494 (via a voltage divider) to regulate the rail voltage.

, what are the 9 NDP7060 mosfets doing?

**** The FETs are driving the transformer.


Also I heard it was a good idea to isolate the audio section from the power supply section with a bi-polar power supply, how would one go about doing that, and testing the audio section before getting to the power supply? Taking out all the mosfets?

**** It's typically easier to repair the power supply than to try something like that. Some car amplifiers have multiple power supplies that require that the power supply transformer be driven with an AC/pulsed DC signal to generate all of the voltages. Connecting a bi-polar supply to the rails of the audio section may work for some amplifiers but not all.
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Old 16th February 2006, 02:35 AM   #3
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Join Date: Aug 2005
Thanks much, that makes alot more sense now. I'm getting there, slowly As always, your posts are greatly appreciated. I follow all the threads in here about amp repair, so even if it seems like it may be for just 1 person, many more are listening intently.
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