Denon PMA-790 eats fuses (but not anymore)

[PE9ZZ]

I got this great amp in after an extensive repair/refurb. The refurb was valid only the amp returned with the complaint "blown fuses". Indeed, the fuses in the line for the power output (2x 6.3AT) had failed. Could not find anything amiss but upon returning the amp the fuses were found to be blown again. Egg on face ensued. New fuses were installed. Again the amp worked flawlessly until my house circuit breaker (16 A) tripped. The coin dropped. The big toroid has a massive inrush current and the new reservoir caps C47/48 (which were replaced by Kemet 33 mF/75 V units) presented a dead short on top of that at poweron. So I delved into my tray and found a P13 inrush current limiter aka NTC of 10 ohm. After installing it the inrush current was 15 A (vert: 5 A/div, hor: 5 ms/div):



Still substantial but not enough to blow any fuses. The NTC is mounted on the mounting strip next to the power cord entry:



The NTC nibbles off a tiny bit of headroom. As it heats up this becomes even less. Full power is 2x 172 W into 8R.

Now I am still not sure that this solves the issue. But I can't think of anything else. Any ideas?

 

[Karl vd Berg]

Hi, look for time-delay fuses (class T/TT?) that can tolerate or withstand the inrush longer without going open, avoiding the use of thermistors.
They have a higher degree of time delay which enables them to handle high inrush currents longer than common slow-blow fuses.
I can't remember now the exact type of these fuses that Amplimo provided when I need a 1.6A for a 300VA toroidal trafo, but they were slower than a regular slow 1.6A fuse.

https://www.swe-check.com.au/pages/learn_fuse_speed.php

 

[PE9ZZ]

Hi, look for time-delay fuses (class T/TT?) that can tolerate or withstand the inrush longer without going open, avoiding the use of thermistors.

The fuses installed were already slow (single T). TT fuses are generally not available, certainly not in my fuse tray. Besides, the inrush current of the toroid triggered my house circuit breaker. The NTC is a permanent solution. And sat in my NTC tray.

My question is whether this is the only issue this amp has. Or rather had as it behaved itself during testing.

 

[Hyperion]

Now I am still not sure that this solves the issue. But I can't think of anything else. Any ideas?

Soft start relay?

 

[PE9ZZ]

Soft start relay?

Same thing but much more complex. This is a single component.

 

[idf.bts]

Hi, look for time-delay fuses (class T/TT?) that can tolerate or withstand the inrush longer without going open, avoiding the use of thermistors.
They have a higher degree of time delay which enables them to handle high inrush currents longer than common slow-blow fuses.
I can't remember now the exact type of these fuses that Amplimo provided when I need a 1.6A for a 300VA toroidal trafo, but they were slower than a regular slow 1.6A fuse.

https://www.swe-check.com.au/pages/learn_fuse_speed.php

I was going to suggest slow blow, but he says they already have that. TT seems like a less intrusive approach.

In terms of its only issue, he asking now. I think it probably is. Usually its the last thing you touched, so he addressed that reasonably. I think it's probably fine.

I'd keep it running for a few weeks just to see if any other edge cases show up, but I would tell the owner its not something you can test until you are sure because its impossible to think of all the test cases.

 

[PE9ZZ]

I'd keep it running for a few weeks just to see if any other edge cases show up, but I would tell the owner its not something you can test until you are sure because its impossible to think of all the test cases.

It's been running without a hitch for months now so I'd say the issue has been resolved.