NAD 306 - few questions

Karl vd Berg

Super Member
Hi all,

I have now a NAD 306 (1993-94) that I'm planning to replace the capacitors, the 2 relays (?) and cleaning switches and potentiometers, but first I'd like to ask you a few things about some components under heat.

Turned it on this morning and it seems to work fine, although the sound is not as clean and "full" than my other 304, little brother. The bias were pretty high on the left channel, around 18mV (instead of 7mV, acc. to SM), and around 9mV on right channel.

This is the amp, still in good condition.

eam8me.jpg


:scratch2: Removingthe bottom cover, darker/burnt areas where a couple of NPN transistors - 2SC3467 (MP package) and a 22kΩ resistor (2W) get very HOT (see 3rd pic).

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R425 resistor (22kΩ / 2W) and the 2 transistors are the main source of heat!

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Some question are:

1. should I worry about these components, or is it better to replace them with higher wattage ones? And the other smaller resistors, too?

2. do they get hotter and hotter because of other aging components?

Another thing is that this amp has two relays. One is in the power amp board and another right next to the binding posts.

3. both of them have to go for new ones or just the one in the power amp board?

12308r9.jpg


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I really want to refurbish this unit into new one (inside). Any further advice and suggestions will be greatly appreciated. :)

Thank you. :thmbsp:
 
I have a very similar 314 amp that I'm about to do the same thing to (full restoration). One thing I've found with older equipment, and it's not just limited to NAD stuff, is that dry joints can affect a build up of heat. On the NAD 3020s I work on a lot, I see this board discoloration around the power supply. Using a desolder tool on the main regulator transistors, I often find that the transistor is attached to the solder pad by only a small area of solder. Less contact area = higher resistance = more heat.

See if you can find a schematic for your amp, and maybe consider replacing caps in the signal path with either film caps, or low leakage Nichicon KLs bypassed with films.

As for the relays, if they aren't too expensive, you might as well replace both. I would hazard a guess that one is for the protection, the other might be for a switched second speaker output.

Lee.
 
That board is scary discolored. Pull and test every component in that area to be safe, after finding a schematic. If there's something dead, that might have been the problem,or it could be somewhere along the line causing stress to this area. Again, schematic needed. For instance, if it's power regulation, it's probably something drawing excessive power.
 
I don't know the NAD 306 from a hands-on perspective, but from the schematic, R425 should be running somewhere around 1W, so a 2W part seems reasonable - I'd first check R426, the corresponding part in the other channel.

As far as I can see, they should be the same, so if R426 is not similarly burnt then it suggests you have a problem on the R425 channel that needs further investigation, be it dry joints or failed component(s).
 
See if you can find a schematic for your amp, and maybe consider replacing caps in the signal path with either film caps, or low leakage Nichicon KLs bypassed with films.

As for the relays, if they aren't too expensive, you might as well replace both. I would hazard a guess that one is for the protection, the other might be for a switched second
speaker output.

Lee.
Thank you, Lee. I got the schematics from HiFiEngine. I'll put bellow this, part of the main power board (left channel). I wrote in it the transistors for better following.
I'm not sure if it's class A/B power amp section. I think due to heat, some caps are not in specs any more. This happened with the NAD 304, too.

And as for the relays, if the 2nd relay is for the "Extra Speakers", then I'll replace only the main one as I use two speakers. Or I can swap them: it doesn't seem that
the previous owner used 2 pairs of speakers. Thanks again.

ivandezande said:
That board is scary discolored. Pull and test every component in that area to be safe, after finding a schematic. If there's something dead, that might have been the problem,
or it could be somewhere along the line causing stress to this area. Again, schematic needed. For instance, if it's power regulation, it's probably something drawing excessive power.
Goldie99 said:
I don't know the NAD 306 from a hands-on perspective, but from the schematic, R425 should be running somewhere around 1W, so a 2W part seems reasonable - I'd first check R426,
the corresponding part in the other channel.

As far as I can see, they should be the same, so if R426 is not similarly burnt then it suggests you have a problem on the R425 channel that needs further investigation, be it dry joints
or failed component(s).
Thank you, too! I surely will do that. Measuring bias in the left channel it showed something like 18mV, instead of 7mV. I pulled the Phono board and saw the same area
in the right channel: it looks less burnt than in the left channel. I'll measure all those components.

15ho2mw.jpg


Please note that on board, one or more electrolytic capacitors are not following the board's marks/polarity.

4kgp4j.jpg
 
On the relays, the parts list shows both to be the same part, so you should be able to just swap them, although not knowing their history (your photos seem to suggest one or other may have been changed before - they don't look to be identical ?), I'd be inclined to just replace both and be done with it.

On the caps - again, I'd compare the two channels first, e.g., are the same caps oriented the same in both channels ? and then check the orientation directly on the pcb traces / versus the schematic. It's not unheard of for pcb markings to be wrong...

Comparing the pcb layout in the manual to your pcb photo above indeed suggests there may be some differences.
 
Toshiba 2SC4793 for stock Sanyo 2SC3467 ?

Quick update and a question! :)

I'm gathering info before sourcing the components (new caps and better resistors, etc). So far, so good, but for those stock Sanyo 2SC3467 E - in the burnt PCB area above - I could find original Toshiba 2SC4793 as replacement. Any problems regarding fT and Cob for their purpose, highlighted below in the schematics? :scratch2:

Stock: Sanyo 2SC3467 - 200V / 0.1A / 1W / ft 150MHz / Cob 1.2pF

Replc: Toshiba 2SC4793 - 230V / 1A / 2W / ft 100MHz / Cob 20pF

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2hzpud.jpg
 
Sorry, I can't help on the transistor question, but I'll 'bump' you up, maybe someone else can...
 
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