"Easy" first project turning into nightmare..Nikko integrated amp w/ pics

soma89

Well-Known Member
A very cool looking tiny little integrated amp.
Nikko TRM-210
So i got this thing strictly to fix up (there was distortion on one of the channels.)


WHAT I HAVE DONE..
I first recapped the whole thing. One cap was in the wrong way and it blew, damaging a resistor in the process that ive since replaced.
I have also replaced 2 of the power trannies that had shorts.... ALL 4 now measure fine when out of the circuit.
Power resistors were also replaced (four 0.47ohm 2w resistors)
I have tested the transistors and bridge rectifier diodes and theyre all just fine BUT..

When i turn the darn thing on it it hums for a second then takes out one of the circuit breakers(there are three in total..one on the back labelled 1.25a...& two paired up inside, one is 1.25a(this is the one that always blows) and a 0.8a).
In one of the pics you can see 2 pairs of red and yellow wires leading up to the 2 breakers. The one that gets the red wires is the affected circuit breaker.


I cant really find anything on the schematic but its a pretty simple circuit thats proving to be a headache.

Im thinking of replacing all the resistors..i have noticed that one of the 20k trimpots looks a little dirty but could be some sort of damage? So far i have not tampered with the trimpots.

What other steps can i take to see what the deal is?

Im handy with a multimeter and i have an esr meter as well. Thanks!
 

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Could you post the schematic? Does removing the outputs stop the overload? If the recap made it worse, going from just some distortion to poping the breaker you should prove that all the caps are in the right way. Which cap was backwards and what resistor did it kill. The problem with going straight to the shotgun is sometimes you create more problems unfortunately.
 
Also replaced R677 right now as it looked kinda damaged as well.

I replaced the two c1061's with 41C's
 
Are Q655 and Q657 happy? Id be tempted to remove them and see if it can power up. Do you have a dim bulb tester or variac?
 
With the disclaimer that I've never worked on audio equipment, and I haven't had to trouble shoot down to a component in almost 20 years . . .
I think it might also be worth looking at R691. If it is shorted or far less resistance than it is supposed to be, maybe it is drawing too much current through the C669?
 
By the way the 2sc945s are known for intermittent failure and being noisy. Also those exposed pots crap out often. I would replace both the 500 ohm and 20k pots for good measure and the 945s. And yes cold bias. If the pot wiper is intermittently open bias will be going nuts. You can jump the pot legs so that if the wiper sucks its still all the way down. Jump wiper to the pin going to R683. However if the carbon is busted that wont help.
 
Ok so heres the scoop...
Q655 is measuring funky( 2sC853)
Q657 broke when i took it out (2sA545)

Anybody know of replacements for these? And 945s?

Ive looked some up but their main replacements are the BC639 and BC640 seem to be rare.
More headache but in the meantime i gotta get me a lightbulb tester
 
Pictures aren't too clear but I think I see BP on some of those caps in your photos. C601 and C610 for example. And I DON'T see the marking for NEG(-) on a lot of those black caps.

Are you sure these are polarized electrolytics?

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Second using a DBT. If your breaker throws this suggests a dead short to me - either outputs or the bridge rectifier. Thos two large caps having he negatives facing together seems a bit odd to me - or this could be correct - are they supposed to have this configuration?
 
Some educated guesses - first the red lead looks like it is B+ coming off the filter cap and going to the amplifiers. The yellow lead appear to be on the secondary winding going to the lamps only. Make sure that mica insulators are intact, and that the screws going to the tabs on the output transistors (41Cs) have insulated washers on them. If that is all okay, I would replace all the trimmers. The 500 ohm ones are the ones that set the bias, and they get intermittent with age, causing the outputs to short. DBT or variac with current meter would be very helpful. My 2 cents...
Also - you should be able to power up the amp without output transistors to test other components. (or you could unsolder one lead of each emitter resistors. I'd start with the original bad channel and if it doesn't pop the Circuit breaker, start checking voltages, Good luck...
 
Pictures aren't too clear but I think I see BP on some of those caps in your photos. C601 and C610 for example. And I DON'T see the marking for NEG(-) on a lot of those black caps.

Are you sure these are polarized electrolytics?

View attachment 905905
Yes some are indeed BP! Thanks for noticing. I was under the impressions that if its under a certain value it doesnt matter tho? If somebody could clarify that would be great
 
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