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Old 29th February 2008, 02:23 AM   #1
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Default trying to fault find a Nikko amp

Hello

I have a Nikko IA 600, quite a giant beast, but very flexible for my needs and I use it a lot, it was second hand purchased about 7 years ago and has finally quit working. The lights come on but there is no main output and it doesn't heat up like it used to.
I have bought a wiring diagram for it.

I haven't done any fault finding for several years and I was using the wrong type of probe and I grounded the 60V internal supply a couple of times while probing about, (big bang but lights still come on and no obvious changes ) before thinking of wrapping masking tape around the exposed metal of the probe, yes....very careless.

Anyway, there is no power to the preamp stage (that's not the one I grounded) and in the relevant power supply section there is a kind of double rectifier circuit after the supply from the transformer, (which is at 60V a/c) and in the supply line on each polarity there is a resistor coded brown black gold gold, which I reckon to mean 1 ohm, marked on the diagram as 1/4 watt, and also marked on the diagram the letter "F", (fuse resistor?) and "replace only with manufacturer recommended parts"
My mini meter reads across them as infinity ohms and I reckon they have blown, but external appearance is OK, (not brown or black or burnt looking).

I only put the story above for interest and a feasibility check, and maybe advice in these circumstances about anything else I should check and replace while i am about it? I have found a LOT of dry joints.
My main question is am I supposed to buy replacement resistors from Nikko or if anyone can recommend the best replacement, or indeed, if this is the best part of the forum to post, or even the most appropriate forum.
With thanks
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Old 29th February 2008, 02:40 AM   #2
Leolabs is offline Leolabs  Malaysia
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Fix those dry joints first.
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Old 29th February 2008, 02:51 AM   #3
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replace the fusible resistors with standard resistors. Maintain the value though.
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Old 29th February 2008, 07:13 AM   #4
Bigred is offline Bigred  Canada
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Quote:
Originally posted by The Donk!
replace the fusible resistors with standard resistors. Maintain the value though.
Ummm not sure what you mean by "standard" but I'd use metal film flameproof for replacing fusible resistors
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Old 29th February 2008, 01:43 PM   #5
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Oh, I fixed the joints first thing, found about 50 bad ones so far including the power transistors.

Donk, wouldn't that be dangerous? Like putting silver paper across the fuse in an (English type) plug? I mean if something caused them to blow and needs fixing first? Or do they just blow because they are old?
But thanks for the replies.

Something that puzzles me is that the power transistors aren't heating up and the transformer isn't humming like it used to, even though there is power to the main amp circuit . Is that because the pre amp puts a bias on the power transistors and "turns them on"?
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Old 29th February 2008, 01:44 PM   #6
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Well, so far I have found two sources for 1/4w 1r "fusible resistor"
Manufacturer data says they fuse in 30 seconds at 16 times rated load
30 seconds sounds a long time but I guess i will have to go for that unless anyone advises against.
I wonder what caused them to blow without damaging anything else, guess I will have to fit tham first and see what happpens or has already happened ! to the next components in the circuit.
I have to get a minimum of 10 so plenty to play around with.
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Old 29th February 2008, 07:34 PM   #7
Bigred is offline Bigred  Canada
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If you can find the proper fusibles then all the better but metal film flameproof will work just fine
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