That corroded R204 resistor - 3.3k (1/4W) is in the idling current adjustments circuit for the Right channel. When you replace it then you can replace the other R203 resistor of same value (in the Left channel) with new ones of 1% metal film. Not necessary for the left channel, but why not?
The A-520 is a nice amp. I had mine recapped but didn't replace the main 12,000uF / 56V ones, though.
But I replaced the relay... I think I also replaced the stock NJM4558 opamp with my fave OPA2604, and for me that was a nice replacement. I really like sound of the OPA2604 in these Yamaha amps. I installed a 8 pin socket and tried different opamps, including the NE5532 as well... Of course, you don't have to do all this, but I had fun in trying these IC's in the preamp section.
I am so happy to see this statement, OP Amp rolling is something the headphone guys swear by.
How noticeable are the effects of switching from op amp to op amp???
Did you rank the ones you like??
I have tried only 3 opamps in these amps: NE5532, OPA2134 and the OPA2604. I found the latter very open, 3-D, warmer, etc. I like the 2604.
For me: OPA2604 > OPA2134 > NE5532 >>>>> NJM4558
Very interesting stuff. Good advice about changing both resistors, I'll do it. The Finder relay I already have, got extras when I made my last order. Caps will get ordered in the next few weeks I hope.
The opamp socket is a very interesting thought. I've been wanting to buy some LME49720 and OPA2107 opamps for my pc soundcard (Asus Xonar Essence STX) as I've read a number of reccommendations for them, so why not for my amp. It would be fun.
The only thing that I find in the A-520 that looks like it could be the opamps are 2 inline 8-pin packages marked M5218L 4302V. And space is very tight. I would guess that there must be an adaptor socket to to go from inline 8 pin to a dual 4-pin chip, but I don't see how I'll find space. Or is the opamp tucked away somewhere that I'm missing?
A different question. I see that the A-520 has the same 2SA1306A and 2SC3298A transistors as the A-1020 has. With the A-1020 they are mounted on a card and clamped directly to the heat sink for cooling, whereas on the A-520 they are just standing all by theirselvs on the main pcb. Is there anything to be gained by clipping a small heat sink on them?
Cheers,
James