NAIM NAIT blowing fuses

[jqsnait]

I have had this Nait for as long as I can remember, bought new in the '80s. It has been performing perfectly for the last 17 years since I moved to the states. 2 days ago I un-plugged and then plugged it in again while the power switch was on. I blew the 2A fuse - unfortunately, replacing the fuse does not fix it. Anytime I turn the power switch back on it immediately blows the fuse again. Any ideas please ??

[martin clark]

It might just be worth trying the next fuse size up (say 3A) - or a different type. The fuse needs to be 'T' rated (delayed-action) type.

IF you're sure the amp has developed a fault the first thing I would try is using a DMM to see if one of the transformer windings has failed 'short'. You'll have to de-solder the wires from the bridge rectifier to do this. Next thing I'd try is replacing the main rectifier bridge itself - when bridge rects fail, one of teh diodes goes 'short' with exactly the results you describe.

[jqsnait]

Thanks for the tip. I tried 3A slow blow fuses, same issue. However I disconnected the three wires in turn that push onto spade connectors on the main board and discovered it is the grey wire when plugged in causing the short. Removing the board from the amp and turning it over reveals just 2 connections from that grey wire spade terminal. A resistor and a black, square component with 2 ~ symbols and a + and - symbol. As you can tell ,although I can replace components I'm not exactly an audio engineer. I'd appreciate knowing what that square black component is as I assume it is causing the short. Thanks again.

[martin clark]

OK.

IIRC the three wires out of the Naits transformer are grey, green, and yellow - the green is longest, plugs in between the two main PSU caps and is the transformer centre-tap. The othe rtwo represent teh ends of the sseondary windings. Between them, when unplugged from the board, you should see something like 30-32VAC with the amp switched on at the front panel.

The fact the fuse doesn't blow when you power up without the grey wire engaged tells you that the problem isn't the transformer - if one of the secondaries had failed short, the fuse would blow even with the transformer unplugged from the main board. So I'm pretty sure it's the rectifier that has failed, the black square part you describe.

All you need to do is to find a generic replacement in a similar package (e.g. from mouser, digikey etc). I'd recommend a minimum 100PIV, 6A amp rating; larger is better (the Nait only runs on +/-22VDC rails, at maybe 1A DC under load, but a hefty margin is recommended). Just take care to replace the new bridge in the same orientation of markings relative to the PCB connections as currently fitted.

HTH

[martin clark]

Example of suitable replacement:
http://uk.farnell.com/jsp/Semiconduc...sp?sku=9381309

- just take care to line up that chamfered corner same as existing when you swap over.

[jqsnait]

Thanks a ton. Replaced the bridge rectifier and it works. I really appreciate the advice. One more small thing... the light bars (to light up the Nain symbol) have not worked for years aside for the occassional flash. Is there anything suitable to replace those with. They look like a pain in the butt to unsolder and replace.

[martin clark]

Hey that's great news, well done.

About the LEDS - they dont fail, but the series resistor from the power supply gets cooked over the years. If you follow the pair of wires back to the main board , in series with one of them you'll find a small resistor. Replace it with new, almost any value will be fine though 4700ohms (bright!) to10Kohm is about right. HTH.