Amp Teardown and Dangers of Buying old 2nd stuff....

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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.
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!!
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.
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.
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.
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...
 
Last edited:

wwenze

Great Supremacy Member
Joined
Dec 2, 2002
Messages
73,382
Reaction score
18,278
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.)
 
Last edited:

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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!!
 
Last edited:

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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.
 

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
Byw the speaker posts are crappy aluminum ones thats gold plated so you cant tell... Lol... I thought they will use something better.
 

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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.
 

Clearnfc

Banned
Joined
Apr 10, 2019
Messages
14,116
Reaction score
3,713
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....

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).

ST3GW3X.jpg


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

3DPzdbR.jpg
 
Last edited:
Joined
May 11, 2017
Messages
62,450
Reaction score
11,791
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
 
Joined
Feb 24, 2006
Messages
110
Reaction score
2
Got it from krell .. my kav-400xi using same power pbca as kav-300il with bussmann mdl-10 rated at 10A and 32vac but allow me to contribute for old time sake šŸ˜‚
 
Important Forum Advisory Note
This forum is moderated by volunteer moderators who will react only to members' feedback on posts. Moderators are not employees or representatives of HWZ. Forum members and moderators are responsible for their own posts.

Please refer to our Community Guidelines and Standards, Terms of Service and Member T&Cs for more information.
Top