Nikko A-300D Hybridization! Sacrilegious?

[Ronj]

I know I have been posting too much about my little Amp, a Nikko A-300D.


As mentioned here
Nikoo A-300D, recapping dilemma...




It's because; there is something about it I like so much. Today, I have spent my whole Saturday trying to rectify the problem as mentioned in the thread below.


Nikko A-300D My bedroom amp developed distortion


My problem is; I don't have an oscilloscope to now to further troubleshoot. Also I can see for me to change stuff like transistors, I need buy them from UK or USA ebay and ship them which doesn't make much economical sense to further pump more money(like $30 to $50) in to this cute little amp. And worst I can't get of the schematics paid or otherwise.


I also realized that the PCB on the Nikko has become very fragile and giving away even when touched with a 25W soldering iron. It's already very messy as seen here after a recap and also some repair done by it's previous owner. It also has these god-knows-what-value biasing diodes which I am afraid if stops working, I have no way to get another.




So as I was thinking, I hit the the cross-roads of making an important decision. I just found that my Class-T TA2024 board nicely fits inside this little amp, so thinking, what if I just plonk my small little Class-T in there for the power section?


I have seen that the distortion problem I am facing doesn't appear on the headphones or Tape REC out. That means the pre-amp section in my Nikko A-300D is working just fine. So how about I salvage the pre-amp and just replace the power stage with this Class-T? At least specs on paper is a close match 16W+16W(80hm) for the original Nikko out put stage against 10W+10W at 8Ohms or 15W+15W @4Ohms.


The rail voltages on Nikko are +/- 15V which I have components to regulate down to a 12V to run my small T-Amp.


Here are some more pics..


With my Class-T kept on the heat-sink to show you the size


What I am not sure is, should I spent more time and may be more money in fixing this as this is a 70s amp from a classical era. Will you consider sacrilegious to butcher it with a class-T? I am in two minds... really..


And finally, if at all I decide to go ahead the Class-T route, where to tap the signal optimally? Should I try to tap it at some driver transistor? I think it uses two NEC 2SC853 are drivers? The other non-power transistors on board are 2SC1684 and a 2SC945 per channel. Or tapping from the Volume pot is another option? I hope volume pot center line is the input to the power-stage after the tone board?

[RJM1]

You might want to check the big brown resistors on the transformer side or the heat sinks
(from your pictures). If it does not distort on headphones it means that the drivers are OK and that you have an open output transistor, an open output transistor emitter resistor (big brown resistors) or a shorted output transistor and an open emitter resistor.
From your pictures I think they they are .41 ohm 5W resistors (all I can see is .41 KOA).

[destroyer X]

YES!

regards,

Carlos

[picowallspeaker]

Ohhh !!! I hadn't seen this !!!
I just posted on the Dilemma Thread

[Ronj]

Originally Posted by RJM1

You might want to check the big brown resistors on the transformer side or the heat sinks
(from your pictures). If it does not distort on headphones it means that the drivers are OK and that you have an open output transistor, an open output transistor emitter resistor (big brown resistors) or a shorted output transistor and an open emitter resistor.
From your pictures I think they they are .41 ohm 5W resistors (all I can see is .41 KOA).

The distortion is not continuous. It starts after few seconds to half a minute after the amp is switched on also another clue is the 2SC853 on the faulty channel gets very hot. I swapped it with the working channel and even that one started getting very hot as well. While the working channel 2SC853 doesnt get that hot at all...

So can I still suspect a faulty power transistor with the above info?

[Ronj]

Originally Posted by destroyer X

YES!

regards,

Carlos

Now that's making me reconsider the class-T route!!

[Coconuts 500]

WARNING!

If the sound is fine with headphones - the amp is fully working!

The only difference between speaker and headphone output is that the headphone output has a resistor in series with it.

Your problem lies elsewhere. Clean the speaker selector switch thoroughly - that is the most likely culprit. Also try to use different speakers (as they may be the problem).

[Ronj]

Originally Posted by Coconuts 500

WARNING!

If the sound is fine with headphones - the amp is fully working!

The only difference between speaker and headphone output is that the headphone output has a resistor in series with it.

Your problem lies elsewhere. Clean the speaker selector switch thoroughly - that is the most likely culprit. Also try to use different speakers (as they may be the problem).

I have gone beyond checking terminals etc many many days ago... I have seen designs, where headphone outputs are main outputs with series resistors...

But I don't think it's like that on this amp as both the outputs are separately wired from the main board. Yes I haven't traced the PCB further, so even if it is, with head-phone plugged-in at listenable volumes there is no distortion... Could it be because of some intermittent failure of driver or power transistors? I don't know!

[Ronj]

Originally Posted by Ronj

I have gone beyond checking terminals etc many many days ago... I have seen designs, where headphone outputs are main outputs with series resistors...

But I don't think it's like that on this amp as both the outputs are separately wired from the main board. Yes I haven't traced the PCB further, so even if it is, with head-phone plugged-in at listenable volumes there is no distortion... Could it be because of some intermittent failure of driver or power transistors? I don't know!

Just checked again. There is slight distortion even on headphone terminals when the volume is very loud. But not very noticeable, like when I use speaker...

Checked all the output transistors and emitter resistors... All the pairs read similar values like the working channel. Just learned that this works on a single +47V positive rail voltage. Power transistors A670 Collector reads 0V while c1060 collector reads 47V. I think I am gonna take the easy Class T route out...

[picowallspeaker]

Yes , those old amps used to have a supply voltage at around 50 V ,
for a max Po of 15 W , because of the scarce availability of higher power transistors ,not to talk about complimentary pairs.
I (used to ) have a Philips ...RH 590 with the famous germanium type AD 149
or a Prinzsound !!!! with SB255 , the first with a wonderful sound , the latter terrible . A famous Philips of the '70s , the RH 522 , was equipped with more modern SB 822 and the classic BD 137 ...as driver , putting out 30 W with +44V supply.
Nowadays that supply voltage is quite unusable , at least not with modern projects that ask for more current .
So Ronj , don't think to use the original trafo , because a 30 V step down to
feed a T-amp is too much for a regulator .
Then , what's the sound of a T-amp ????