[fishmalt]

I'm near completion of my initial build of the 100W N+ and ran into a problem. I have tested my amp up to step 30.  However, when I attached the pcb/heatsink to the chassis and connected the RCA to the PSB, I started blowing fuses in the IEC power entry module.
 
questions:
 
1. - Do the pcb/heatsinks have to be (electrically) isolated from the chassis when mounted to the chassis?  I assumed this is not the case.
 
2. - I've since disconnected everything from the pcbs except RCA input wiring and kept blowing fuses.  Only after disconnecting the ground wire (from the IEC power entry module) where it attaches to the chassis did the fuses stop blowing.

I'm currently looking for guidance on how to move forward in a systematic way that doesn't require me to continue buying 5A 250V fuses by the truck load.

Thanks in advance for any help provided.

Mike
Anchorage, Alaska

[Seano]

Do you have an electrical connection between the PCB and the heatsink?  If so this ain't s'posed to be there.  Check the bolts that connect the PCB to the heatsink aren't contacting the heatsink on their way through.

[fishmalt]

Out of the 4 bolts that connect each pcb to the heatsink, T9 & T10 (on the right side) both show a connection with the heatsink acording to the DMM reading - the rest are fine.  I will fix this problem and see how it goes.

Thanks,

M-

[AKSA]

Michael,

Indications are that you have a short of one of the power output transistors with the heatsink.

The collector of these transistors is at rail voltage, and if the mica washers are not insulating the transistors from the heatsink ledge on which they are mounted there will be an overcurrent situation which blows fuses on the IEC input module.

With all power disconnected, check each device with a continuity tester between the center terminal of each device and the heatsink.  If you get a buzz from the DMM, loosen the securing nut on that device.  If the buzz then goes away, you know which one is guilty.  Bend it up, remove the mica washer, inspect it, clean away any dirt or swarf on the washer, ledge or device, and refit with more grease.  That should fix it.

Come back to us here if this does not work.

Getting colder now fall is approaching in Anchorage?

Cheers,

Hugh

[fishmalt]

I am getting more bemused with this problem.  I followed your advice and rectified the problem.  No longer blowing fuses I went to bed last night somewhat content thinking of my soon to be completed amp. . . except, this evening after testing the amp and everything checking out, I connected the RCAs from my preamp and small bookshelf test speakers and experienced an awful noise coming through the speakers, so I thought it might be some kind of ground loop problem and switched around how my components were "plugged in." After turning the amp off to change my system I am to blowing fuses again. 

Without anything attached to the amp I re-evaluated the original problem (mica washers not properly insulating the transistors from the heatsink) and everything checks out fine via the DMM readings. However, the fuses (IEC power input module) are back to blowing. . . any wisdom to share?

It is starting to get cold in Alaska so I need this amp up and running for the bloody long winter that's on its way. . .

Thanks for your assistance (obviously I could use some),
Mike

[AKSA]

Hi Michael,

Before moving ahead with more diagnostics, and given you are using a couple of large trafos on 115Vac mains, try upping the fuse to 7.5 or 8 amps and see if it continues to blow them.....

In the meantime I will think on the funny noises, which sound a bit peculiar.

Can you send me a few nice pictures of your setup, including the modules, sinks, wiring and RCAs?

Be sure to insulate both RCAs from the chassis of the enclosure, too;  the earth should not touch chassis.

Cheers,

Hugh

[fajimr]

mike

I had a similar problem on my 55N+ but was originally running a 3amp fuse.. changed to a 6 and had no further problems...

here is my thread- not sure if it applies to your situation though (I don't know much about the hoodoo that goes on under the hood.. just happy that my amp makes beautiful music  )

http://www.audiocircle.com/index.php?topic=28165.0

good luck
jim

[AKSA]

Mike,

After your photos and another email, I'm anxious to know if all is well.....

How are you travelling with the 100N+?

Hugh

[fishmalt]

Hugh,

After reading your e-mail I clearly realized I had a problem with the RCAs.  I took them out, drilled slightly larger holes, and used grommets to ensure there would be no incidental contact with the chassis on their way through the holes.  Also, the SIG GND to star earth measured 10R.  So far, no blown fuses . . .

This evening I have finished rechecking everything and all seems well.  As soon as I can I will conduct the audio test. 

I all goes well, I may actually be listening to music soon (which is just in time since we got our first snow fall today.)  I need something to calm my nerves as my lovely wife put the car in a ditch 1/2 mile from home due to the snow and ice covered roads.  I will post a follow-up if it goes smoothly and if not . . . well I will be posting for further assistance.

Thanks for your help,

M-

[fishmalt]

My amp is completed. I let it burn in for 5 days non-stop (hooked up to 8R 20W resistors) and it is now playing in my system.  Everything is as it should be. . . at least as far as the amp is concerned.

Thanks for the help Hugh (and others).

Here are a few pics of the finished product.

http://www.audiocircle.com/index.php?action=gallery;area=browse;album=864

Mike

[AKSA]

Mike,

This is fantastic news!!  That RCA input socket gets a few people in, believe me........  you ain't Robinson Crusoe!

Beautiful metal work, you are an artisan!

Thanks for letting us know.

Cheers,

Hugh