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Clearnfc 17-07-2020 09:47 PM

Amp Teardown and Dangers of Buying old 2nd stuff....
 
Just want to share this and issues we all face when buying 2nd stuff online, esp when they are very old.

Just bought this old amp yesterday..... 20yrs old...lol... It still good but a hint of distortion at high volume so decide to take it apart. So share some teardown photos.

The preamp circuit board. The layout is high symmetrical since its a stereo amp.
https://i.imgur.com/LV2EaOL.jpg


These are OPA604 opamp. 3 of them per channel. Considered old these days and better ones are available. But not sure how it will affect the sound, feel like making a socket and start swapping opamps to play!! Sounds fun!!
https://i.imgur.com/NnWHNCY.jpg

The back of the board. Those nichicon caps needs to be replaced. Notice those long metal "poles" on both sides, they are connected to the power section. Hmm... feel like replace them with wires and sheild them.
https://i.imgur.com/mGFReNl.jpg

The power section. Black cylinders are power filtering caps, each is 6800uF. Looks like 0048, needs to be replaced. Again, its high symmetrical in design. Black transistors are Sanken 1860/4886. Its class AB amp. Each pair is rated for 80W and 14A. So, 6 pairs per channel will have lots of power for speakers. Lots of grey resistors, they are ballast resistors. Needed due to running transistors in parallel.
https://i.imgur.com/lYbg7gh.jpg

This is the danger of buying 2nd stuffs esp very old ones. Chances are, mods and other stuffs have been done to it. They are 2 vishay Dale 5ohm resistors connected in series soldered across the 10ohm SMD resistor. No idea why. But maybe its to reduce distortion or something. The board has been recapped (poorly done), but power filtering caps wasn't replaced.
https://i.imgur.com/xBXkxHu.jpg

The best thing is that you can't really kill these amps. They are just circuit board with bunch of components. So, they all can be replaced. Just take some time and effort. Best of all, components are usually very cheap. Now you know how much $$$ they make from these amps...lol...

wwenze 17-07-2020 10:20 PM

Can tell the SK transistors are replacement parts. One possibility is to let the newer transistor match the internal resistance of the older transistors. (Or bias, or gain, or whetever.)

Clearnfc 17-07-2020 10:27 PM

Quote:

Originally Posted by wwenze (Post 128541410)
Can tell the SK transistors are replacement parts. One possibility is to let the newer transistor match the internal resistance of the older transistors.

Oh ok, thanks for pointing out to me!! I didn't notice that. Cheers!!

Clearnfc 18-07-2020 02:05 PM

OK, now I know why those mods were there. Its to replace the SMD resistors. I removed the dale ones and realise the SMD resistors aren't working, they are showing infinite resistance. Guess they were fried. Would need to order new SMD ones to replace them.

twist_shout 20-07-2020 01:19 AM

care to share the model of this amp?

Clearnfc 20-07-2020 09:57 AM

Quote:

Originally Posted by twist_shout (Post 128582240)
care to share the model of this amp?

Its a very old KAV-300IL... But after hearing it, i think i still prefer the original KAV-300i.

Clearnfc 20-07-2020 10:00 AM

Made a mistake... I replace the red cables to silver plated ones. Gonna to put in some high quality copper ones. Now looking for 6N ones. If not i think i will get the qed original. Its still my fav cable

Clearnfc 20-07-2020 11:47 AM

Byw the speaker posts are crappy aluminum ones thats gold plated so you cant tell... Lol... I thought they will use something better.

benedium 20-07-2020 12:57 PM

Maybe better one will sound worse? Heheh

Clearnfc 27-07-2020 12:46 PM

Just finish replacing all the 8 power filtering capacitors on saturday!! I use KEMET ones bought from due to their ultra long life (15K hours)....

Run in a bit and whoo..... sound good even at high volumes....

Now starting work on the S-300i I bought.... haha.....The power caps are a b***h to remove..... The factory used silicone to secure the caps to the board. Nearly rip the traces out.... End up having to take the capacitor apart to remove it.....lol....

Clearnfc 27-07-2020 12:52 PM

I was also polishing the 4 fuses and contacts...Every bit counts..lol

Also remove the 2 speaker fuses. (Black tubes and bottom sides of the amp circuit board). Yes, i know i am risking my speakers but i feel the benefit is worth the risk.

Interesting to note that s-300i doesnt have it. Only 2 power fuses per channel. No speaker fuse.

3SG_NS_Wong 27-07-2020 01:03 PM

now i know who to contact should i ever want to buy old amps....hehehee

Clearnfc 27-07-2020 01:59 PM

S-300i Amp teardown
 
Yeah, need to repair it. But it sounds very good. Maybe it can be better after recap. I like the chassis most... ITs really solid compared to the 300IL/400XI. That one the top cover is flimsy. The problem with this amp is that you feel there is so much to improve...lol....


The pre-amp board looks good. Other than crappy capacitors which needs to be replaced. The op-amp is OP177G. Pple say it sucks but it seems to sound ok to me. I am not sure if I should replace the opamp with a newer model..OP177G is a 3rd gen opam and we are not in 6th gen with the 4077....

https://i.imgur.com/JgHVkfD.jpg

The power section. Interesting to note that this is designed like a dual mono amp with 2 mono modules. And the burnt marks at the right side. The SMD resistors gets very hot. Some were fried and needs to be replaced. ITs worse on the S-300i because they are smaller and so closed together. Unfortunately, there isn't enough space to put better ones.

Seriously, I don't understand what they have to cramp the parts together on such a tiny circuit board when the chassis has so much space. I really felt tempted to redo the whole thing from scratch but afraid I might just give up halfway...lol...

The while blocks are MPR ballast resistors from a korean company....Hmm.... I really feel like replacing it with discrete resistors.

This amp is rated 150W and we can see why. There are only 4 pairs of sanken bjt compared to 6 pairs for 300il. Interesting to note is that the output of the leftmost sanken pair is goes back into the input of the other 3 (believe is current feedback loop).

https://i.imgur.com/ST3GW3X.jpg

Several diodes near the power. They are not rectifiers. Got a feeling they are DC blocking filter but can't confirm.

https://i.imgur.com/3DPzdbR.jpg

YouHeypiCanLiao 27-07-2020 07:44 PM

Quote:

Originally Posted by wwenze (Post 128541410)
Can tell the SK transistors are replacement parts. One possibility is to let the newer transistor match the internal resistance of the older transistors. (Or bias, or gain, or whetever.)

If so cheap and symmetrical perhaps change both

Sent from Tehhan's iPhone using GAGT


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