Nikko TRM-600 vintage stereo amplifier

[suiramix]

Hi ,
Got this little amplif and tried to find the schematics ...no luck, just TRM-500 model. According to other forums, there is no big difference between them! So basically the this Nikko amplif will power for 1 sec and the primary breaker will cut the power. Did a reset of this breaker same result ! Open the wood top cover and everything is amazing clean , no visible damage or burned components...and lots of wires (kind of scared) !!!
What will be the right approach in this case ... should I disconnect the power supply and measure the voltage?

[budm]

Re: Nikko TRM-600 vintage stereo amplifier

Start by providing good clear pictures of the inside.
What components did you test so far for shorted circuit?

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Hi Martin,
I will post some photos... for now, the amplif was plug-in without any speakers and flip the main switch. The red light turn on for one sec and after that is dead! Not been able to measure anything...see attached similar TRM-500 schematics.

[petehall347]

Re: Nikko TRM-600 vintage stereo amplifier

probably shorted output transistors or rectifier diodes ..

[budm]

Re: Nikko TRM-600 vintage stereo amplifier

Hi Martin,
I will post some photos... for now, the amplif was plug-in without any speakers and flip the main switch. The red light turn on for one sec and after that is dead! Not been able to measure anything...see attached similar TRM-500 schematics.

So you cannot test the components such as Transistors in the power amplifier sections, rectifiers in the power supply section?

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Yes guys...just 2 screws holding the power transistors , removed both and ...are dead ! What it will be the best replacement of 2SD188 and 2SA627 ?
Thanks again!

[petehall347]

Re: Nikko TRM-600 vintage stereo amplifier

Mj21194g
mj21193g

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Hi Pete,
Thanks for your help...this is the first vintage amplif I ever opened and kind of overwhelmed (even is not that complicated). However, I did notice another transistor marked NEC C959 with some overheat marks , removed and is fried.
What will be the best equivalent for this 2SC959 and possible 2SA606 ?

[petehall347]

Re: Nikko TRM-600 vintage stereo amplifier

2n3440

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Hi again,
I have the output transistors available are ready to replace the old one. I assume after that a biasing is need it. Did a lot of reading about DC voltage on the speaker output bindings (close to 0V) but still not clear about idle current through the output transistors ...can someone explain the process and what is the optimal value for this model. Thanks again!

[budm]

Re: Nikko TRM-600 vintage stereo amplifier

R659/660 are for DC Offset adjustment.
R675/676 are for bias adjustment.
Make sure not to connect speakers to the output because there is no protection circuit (I.E speaker protection relay) to detect large DC offset, so check the DC offset the out put to see how close it is to 0V, if it shows <100mV or so that will be good sign, if it shows in Volts then try to use the DC offset pot to see if you can set it to 0V (10 ~20mV reading is fine)
So you will set the offset Voltage to be as close to 0V as possible after the set is warm up for about 1/2 hour.
the typical bias current for the output Transistors is about 30mA, so you will set your meter to milliVolt range then measure the DCV across either R691 or R693 (for one of the channel) then adjust the bias pot to show 14mV (0.47 Ohms x 30mA = 14.1mV), do the same on the other channel on R692 or R694.
I would suggest you to use probes with some kind of clips to attach to the resistor legs so you will have less chance of slipped probes and short out the circuit.
If I were you I would put 75 Watt incandescent lamp in series with LINE (HOT/ PHASE) feeding the unit so if it lights up brightly and stay that way then you know you still have problem with the circuits, the lamp should come on bright for seconds during the main filte caps being charged and then the lamp should dim down a lot after that.. you will also notice that the lamp may change brightness when you make the bias adjustment. I usually do the adjustment with lamp in place and do it again without the lamp.

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Hi Martin,
Thanks again for your detailed tutorial! The DC offset went smooth, no too much adjustment, but for bias of power transistors one of the trimpot is newer (R675) than the other (R676) and I can not adjust it too much.
However I did connect the speakers and Right channel is super clean and amazing sound ...the Left channel with distortions. Also on this LH channel I did change Q660 and Q653 with 2N5415 and 2N3440. For now I will replace the trimpot (both channels) and report the findings.

[suiramix]

Re: Nikko TRM-600 vintage stereo amplifier

Finally, both channels are loud and clean...one emitter resistor (R694, 0.47R) was acting strange...replaced with similar for now and all the biasing adjustments are done nicely.
Last question: as per schematic diagram, there is no indication for the power of this resistor , it looks like 2-3W and is not wire-wound or ceramic. What will be best option for this case? Thanks again!

[budm]

Re: Nikko TRM-600 vintage stereo amplifier

3 ~ 5 Watts non-inductive ceramic resistor type.
You can use something like this but 0.47 Ohms:
http://handsontec.com/index.php/prod...ment-resistor/