Nikoo A-300D, recapping dilemma...

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Hello All,
I have this little Nikko Amp(A-300D) which I am in love with.. It’s my bedroom system and I acquired it from a gentleman here in Singapore few years ago… It still works flawlessly... Today I thought may be I should recap it, and use with a pair of Altec Lansing Corona 893B.

So I opened it up again to take a closer look… Lo and behold... It seemed like someone has already changed few caps… Or at least I can see wires with soldering iron burn-marks… Seems like someone as dexterous as me was at work…

Things like power supply filter caps seem to have been replaced.
Currently it has Elna 2200µF/35V Electrolytic caps… And I can also see Korean brand Samwha electrolytic caps as well… I dont know how good they are but that looks like a replacement job too…

The main arguments for the recapping are; there are less than 24 caps on the board in total and the PCB is a simple layout single layered design with abundant use of wire jumpers to complete the routes. All the electrolytic caps are nicely indicated with polarity/orientation on the PCB(with the symbol as well as ‘+’ marks)… And All the values can be read.

But considering my dexterity when it comes to soldering, I am still a little hesitant to embark upon a “change’em all” mission… Only if it is worth the risk :) I would also like to go audio-PP way for Caps on the signal line… But I don’t have a schematic…

So is there any easier way to identify the coupling caps, than to manually track the lines? Better if someone has the schematic (which I am really not very hopeful of)
 
Attached are the pics of the board… Your suggestions please…

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.



Info in Japanese…
NIKKO ƒvƒŠƒ�ƒCƒ“ƒAƒ“ƒvA-300D‚ÌŽd—l ƒjƒbƒR�[/“ú�K“dŽq
 
The main arguments for the recapping are; there are less than 24 caps on the board in total and the PCB is a simple layout single layered design with abundant use of wire jumpers to complete the routes. All the electrolytic caps are nicely indicated with polarity/orientation on the PCB(with the symbol as well as ‘+’ marks)… And All the values can be read.

So, to understand you correctly: The main reason for fidling with the amp is: "because it's possible".
Now, I don't see anything wrong with that, but only if you like the smell of resin. The best technical result I can think of is that it still works as good as it does now after you'r finished (on offence)
 
So, to understand you correctly: The main reason for fidling with the amp is: "because it's possible".
Now, I don't see anything wrong with that, but only if you like the smell of resin. The best technical result I can think of is that it still works as good as it does now after you'r finished (on offence)

Well.. The main reason I thought was, this being a 1970s amp, theoretically the electrolytic caps on the amp should be either at their end of life or drifted considerably from their specs...20years is what people say electrolytic caps really last… I am not even sure how this amp is actually supposed to sound in fact... Of course being a vintage Nikko, it may not be as detailed or fast like some of my newer amps ( Pioneer A400, Marantz PM6003 etc). But I suspect the desired sound from the amp’s designer would have been tighter and more detailed presentation. And I would like to use it for years to come…
 
I went ahead and ordered all caps.. All 32(yeah I missed counting few earlier) of them! :) And element14 delivered them to me this afternoon! Yet to open the package though...

An unusual find... Spend 20mts earlier today and I found that there is a 3300µF/50V cap as well... I tried to trace the tracks to see where it is powering, it seems to be supplying a Single power supply which is marked as B+ and those 1N4007 type diodes you see in the below picture( see in half-bridge configuration) seems to be the rectifier section for this cap...

Having said that I didn't get time to find the rectifier section for the dual rail supply with twin 2200µF/35 caps. the bigger diodes in the picture, I am not sure are part of oa full wave bridge setup, but then i couldnt find the remaining two diodes... neither I could continuity trace them to any of the 2200µF caps... May be I need more time those 20mts were not good enough... thats what happens when there is no schematic :)
 
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Today I started changing the caps and I am half-way through... I did find a few dry solder joints also along the way....

Then I saw that two diodes are kind of glued to the heatsink per channel... and one of them got unstuck... I don't have any idea what kind of glue it is.. I have never seen something like this before... Can any one here please help me out what compound is used to glue diodes to heat-sink? It looks like rust colored now, as there is some rust also on the heat-sink where it was stuck... below are the pictures... please look at the red oval's i marked to identify them... Any suggestion on what to use to stuck them back on to heat-sink is really really appreciated!
 
Mission complete with out any causalities so far...
I changed'm all!!! All power supply caps I increased by 1000µF or so... so used 4700µF and 3300µF instead of 3300µF and 2200µF respectively...

I used Panasonic caps for all power supply filtering... All small electrolytics I used Nichicon... And all caps 3.3µF and below, I used Panasonic Metalized Polypropylene including for what seemed to be two sets of signal coupling caps at 3.3µF...

Here is the picture of the board after change...


Oh yeah it still works! Took 2-3hours for the high-end to come out though... I don't know why... I was never a believer in electronic component break-in...

To deal with the "sticky situation" of those diodes... I have used some silicon selant for the time being,,, I know it is not supposed to be used... so still waiting for someone to help me find the right glue...

Cheers...
 
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Hi RonJ
Those thermal sensing diodes will be for the bias control and the main requirement is close contact with the heatsink. Clamp the diodes to the sink while any adhesive sets to make certain. Funny that they became detached - why?

So long as the leads remain electrically isolated from the sink, any high temp. adhesive will work (i.e. not 5 min. epoxies). It is surprising that a metal flag type was not just screwed to the sink - this was common practice with thermistor/diode sensors in the same period. There is no glue requirement there, perhaps just a dab of heat conductive paste.
 
Hi RonJ
Clamp the diodes to the sink while any adhesive sets to make certain. Funny that they became detached - why?

They were never clamed to the heat-sink at least no traces of any clamping... They were covered with something that look like soldering flux... and they were stuck on to the heat sink with that... then I saw two the diodes were no longer in contact with the heat-sink... thats how I noticed it...May be I will clean then throughly and re-attach using some epoxy which can take 100DegC or so...
 
I am in love with this amplifier.....lovely one!:)

regards,

Carlos
Me too ...!!
I recapped totally a Nikko TRM 600 three months ago
And I'm thinking of eliminating all the knobs ....:rolleyes::mad:
well , that would be a new amplifier .
Because it has a separation between pre and amplifier stages , in the
end I much preferred to use a separated and specialized preamplifier.
The preamplifier is the real bottleneck of the project , though it does its
duty decently ( you wouldn't believe it after seeing the schematics : transistor>coupling cap;transistor>coupling cap;etc ) Also , after having substituted the supply caps from
6m8 to 10 mF , I had encountered another big bottleneck which was the wiring to the speaker selector , then the switches , then the 'surge' protection
and eventually the binding posts . I removed them and put big wires and binding posts ( the wires, I used them for my mid-tweeter )
Indeed , many clever people say ' do not mess with old amps and don't touch or change anything' ....:p:):confused:
 
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