Intermittent protection issue with NAD 7155

[jamesmafyew]

I've got an NAD 7155 that I got second hand a few years ago. It has worked flawlessly since I got it, but recently started going into protection after about 10-15 minutes turned on. Sometimes it does it, sometimes it doesn't. When it happens, there's a loud pop from the speakers and then you have to wait a while before it will come out of protection.

I'm assuming the pop means that there's DC on one or both outputs and that's causing the output relay to turn off. I haven't been able to confirm this because I can't get the problem to happen when the amp is open on my bench, only when it's closed up and playing music on the shelf. I thought it was overheating at first, but it does it at a variety of temperatures, quite unpredictably. Sometimes if I let it warm up for 20 minutes before playing music through it, it will do fine and not click out.

To attempt to fix this, I've replaced all four alignment trimpots (known issue with this amp), and replaced every single electrolytic cap in the pre and power amp, including most recently the big 10,000uF supply caps. No change. I even replaced the relay, confirming that the contacts were good. Relay's diode and resistor have been replaced too. All the components around the protection circuit have been replaced. No change.

When powered up with the chassis open, all voltages read normally as on the schematic.

Could this be leaky drivers? Looking at the schematic, I'm having a hard time parsing what does what in the driver stage. The heatsink-mounted small transistors (1 per channel) have apparently been replaced... the schem says 2SD414, but the amp has 2SC2824.

The other important piece of info is that when I power the amp off, the negative supply rail drains normally over time from -44V to around -200mV. The positive rail drains normally from +44V to +18V, then stops. No matter how long the amp sits, that rail doesn't get any lower than +18V. I have to drain the cap myself.

Could this be some kind of ground return issue that causes a stuck positive rail after the amp has been on for some time?

I've looked all over for bad solder joints... and well, they all look bad. The amp's obviously had work done before... some cut ground traces, replaced with wires. Some ceramic caps added in places. But nothing that's moving or obviously messed up. I've reflowed probably 60% of the joints in the amp with no change.

Any help would be appreciated.

Service manual I'm working from:
http://local.anchorstates.net/nad7155sm.pdf

[palo]

Hi, i have the same amplifier, the smalls transistors in my NAD are D415 i don't know if replaced, is probabily , don't match with the schem.
My system work, but two chanels sound have a little diferent, one have more "punch".
The zone of R907 and R908 get dark and the capacitors arround this fails and i changeed. The C917,C916 and C909.
I think this warming in this zone is common in this model.

I need the value of C917 , don't have the original and in the schem this part is unreadable. I think this component is connected with the excessive warming.

If can provide this , i appreciated.

Thanks.
And sorry for my english........