Change diodes in a NAD C326BEE audio amplifier

Question

After reading this question, I have additional questions:

I have a NAD CEE326BEE amplifier which is +10 year old. Since 1 week, when I try to switch it on, there is a click and nothing else happens.

According to what I've found in different sources, this might be related to the protection circuit which is switching it off at startup.

I've inspected the board visually and I have found 3 diodes which were not soldered properly, it looks like they are producing heat which has damaged theirs soldering points:

According to the electronic shema these are the following:

The diodes described in the NAD C326BEE Service Manual are the following:

• Diode 1N4007 DO-41 1A/1000V (RoHS) D136
• DIODE ZENER 5.1V 3W 5% DO-41 1N5918B (RoHS) D134 / D135

I'm not able to find replacement components which the exact same specifications.

Questions:

• What would be a good replacements (I've read that higher wattage Zeners could be a good fit?)
• What could be improved to prevent future overheating?

Answer 1

What could be improved to prevent future overheating ?

Not much, they need to dissipate the energy so they get hot. What you can do is give the zeners enough "breathing space" so keep the leads somewhat longer and bend the leads such that air can freely pass the zeners.

...zeners with a power dissipation of 1.25W and 1.5W

Indeed, the 1N5918B is a 1.5 W zener and that the "3W" in the schematic is probably wrong. Also comparing the size of the 1N5918B to the 1N4007, the 1N5918B isn't much larger and the 1N4007 also has a maximum dissipation of around 1 W. If the 1N5918B really was rated for 3 W it would have to be much larger and/or need a (small) heatsink!

3 W zeners do exist, the 1N5919 but there is no 5.1 V version of that. You could just use two 5.6 V (instead of 5.1 V), there is still plenty of "headroom" for the 7805.

So your solutions:

1. buy 2x 1N5918, 5.1 V, 1.5 W and assume that the "3W" isn't true.

or

2. buy 2x 1N5919, 5.6 V, 3W, that will slightly decrease the voltage to the 7805 but that should be no issue. In any case: mount the zeners with long leads so that they can "breathe".

I would choose solution 2) btw, it is a bit more robust.

Answer 2

If you want to better cool those 3 diodes, then remove the white sleeves on the leads.

Then install a tiny fan. Or drill holes in bottom of chassis, for air flow.