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NAD 705 pops and cracks

M

Mark Dallmer

New Member
Hi, beginner here, so please bear with me! :)

I have a NAD 705 receiver which when switched on makes fairly loud "pop" noises on the left channel, and the right channel most of the time only comes on after shortly cranking up the volume knob, then crackles quite a bit. After playing for about 10 minutes both those symptoms go away and it sounds really nice. But even at that stage I can reproduce the pops on the left channel by tapping the device anywhere, but most pronounced at the front.

Opening it up I tapped inside with a plastic tool and I suspect the culprits are the transistors mounted to those C-shaped black heatsinks (there is to of them side-by-side in the left lower area of the board, looking form above). Also one of those C-shaped heatsinks gets much hotter than the other.

Now the questions:

1. Am I right in assuming the problems must be bad soldier points, as the symptoms go away when the receiver warms up?
2. How likely the transistors are damaged, considering that it plays fine when warmed up, IOW is this fixable by just re-soldering or would I need new parts?
3. The right channel not switching on at startup would likely be a relay being defective? Would that also explain the crackles (which also go away after time) or is that related to something else?

Thanks for any suggestions!

Regards - Mark
 
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Yes, remove bottomplate, use good light and magnifier, you'll see cracks in solder almost everywhere. If not improve light and magnifier glass, keep looking, change angle till you see them.

When you remove topcover you probably see a lot of turned black glue and corrosion on leads from parts touched by that glue, scratch away and check if some parts have completely lost leads. Around the heatsinked regulators and transistors and around mostly elevated larger resistors you might see discoloured pcb (sign of heat), if small electrolytic capacitors are nearby they most certainly really need to be replaced, if you can measure capacitors you'll get confirmation. The areas you found most bad soldering bottomside will also be areas you need to replace electrolytic capacitors, the small ones at least. I think relais problems are in older Nads mostly.
 
Uhh, that sounds like a lot more work than I had hoped for or a probably really capable of doing... Thanks anyway, I think I'll start scratching away the glue and then looking for bad soldier points. Will post back how it's gone!
 
Hello Mark Dallmer. Welcome to AK. You can find the owners and Service Manual on HiFi Engine.com. Just register and download the manuals for free. Also check the volume control, and the other pots. Deoxit is used to clean the pots. How to use deoxit is found on the General Audio Discussion forum in case you need it. Bad solder joints can be reworked by applying heat with a soldering iron to remelt the joint then adding a little solder to reflow the solder then remove the soldering iron. Been working as a electronic tech for 34 yrs.
 
Oh Mark check all the electrolytic caps for leakage. If you see the plastic looking shrunk on the cap, it's gotten too hot. If you need to post some pictures to show what you find. Great people here on AK.
 
Just resoldering all bottompcb solderpoints might take 40 minutes ? That will most likely remove the problems you mentioned. If you solder in quiet room you can also often hear if electrolytic leaked soldering the contacts. You will smell it too :)
 
Thanks gain for the welcome an the useful hints! Tough I fear you have no idea how new all this is to me... my first and only soldiering experience dates back 2 months when I replaced the display lights on a NAD CD player with LEDs. That went well enough, but just by looking at the cramped PCB I can tell this won't be nearly as easy. Anyway I hope to give it a try over the weekend and will post back.

Regards - Mark
 
You have a small soldering iron ? I mean the "cramped" made me wonder. In new devices you'll mostly find surface mount, those parts can almost be invisible. Open your smartphone :) 705 only has giant parts and giant distances in comparison :)
 
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