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Old 30th November 2008, 08:53 PM   #1
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Default Nad T752 Repair - help!

Hello All,

I have a NAD T752 A/V Receiver which I'm busy repairing.

I have found various parts blown in the PSU, and have repaired it and got it up and running, however...

Please see the attached diagram which is part of the PSU.

U702 (7812 regulator) and U703 (7912 regulator) run really, really hot.

I see that the Nad designer has tried to alleviate dissapation in these regulators, by inserting two 5.1V zeners in series with the voltage rails. These zeners went s/c which is what caused the original faults.

My feeling is, that this is a poor design. So... I replaced the zener diodes with two 15v pre regulators, on a large heatsink, but sadly I still have a lot of heat in all the regulators. The input voltage as measured, is 23v DC, so 23v is being stepped down to 15v, which is then being fed to the 12v regulators U702 / U703.

Can anybody offer any advice to reduce the dissapation in this circuit (apart from adding a lower voltage transformer - no space!!).

By the way, I have also replaced nearly all the capacitors in both the +-12v PSU and the 5v PSU.

Thanks,
Tony.
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Old 30th November 2008, 09:09 PM   #2
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It is oK for power supplies to run hot.

It is worth measuring the temp of the heatsink.

Those regs will upto 150 degrees C before dying.

Without changing to a SMPS then the only way to reduce heat is to increase the size of the heatsink.
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Old 30th November 2008, 09:13 PM   #3
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I can't keep my finger on the heatsinks for more than about 2 seconds, they are that hot... In fact they are uncomfortably hot...

But as you say, I should measure them - but I'll have to buy a temperature probe first!

Thanks.
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Old 30th November 2008, 09:15 PM   #4
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Those regs should take up to an amp.
You could measure the current to see if they are in spec.

If the heatsink is gettting hot, then is there any ventilation in the enclosure ?
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Old 30th November 2008, 09:38 PM   #5
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This is running with the cover off.

I have attached a pic of the heatsinks - so it can be seen how close together they are!

The second pic is my custom heatsink (just a plate of thich aluminium).
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Old 30th November 2008, 09:40 PM   #6
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Seems you can only post a single image at a time - so here is the second, of my custom heatsink.
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Old 30th November 2008, 09:43 PM   #7
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I would just run it and see how it goes.
The regulators will cut out if they get too hot nayway.
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Old 30th November 2008, 09:46 PM   #8
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Will do thanks for your help.

Just been looking at your website - interesting.

And reasonably priced too.

Don't know how you do it so cheap really..!
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Old 30th November 2008, 09:49 PM   #9
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Quote:
Originally posted by audio_tony
Will do thanks for your help.

Just been looking at your website - interesting.

And reasonably priced too.

Don't know how you do it so cheap really..!

I dont have any overheads.
I work full time in other work too.
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Old 1st December 2008, 05:06 PM   #10
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Just a thought. How about feeding the regs from a half wave supply from the tranny. It may be possible to reduce the excess voltage that way.
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