Rotel RX-404 Repair -- IC dead?

[The Eggman]

Hi all--
FIRST POST beyond the Intro thread so... please be gentle. I apologize in advance for... whatever

My favorite project right now is this old Rotel... wood grain, silver face... she's a beautiful example of Japanese (!) audio.
Quickie on stats for those not familiar:
30wpc@8ohm
8Hz to 45Khz
0.1% THD
Damping factor of 40

Born in 1979 at 8kg, she came to me broken, yet I would love to make her sing again! I've replaced fuses and cleaned her up (pots, fuse points, rca's) but there's this static-y warbling in both channels. Almost like it wants to amplify, but gives out and keeps trying at a high rate. The IC is separated from the heatsink (yet has never felt hot while running.) Could this be just because it was burned out at some point? This is from the'bay so no knowledge of its history. Interesting note that just as it fades to black after I power it off, the sound seems to clear up for that last split second...

Hope someone here has seen this behavior. Looks like these IC's are available still... opinions on the likelihood of this being the culprit?

Also, best way to adhere that sucker on there?? Full disclosure, I've only recently turned my sights on audio repair... I'm from the computer world.

Thanks for any input!

2016-05-11 22-00-29 - IMG_8579

• The Eggman
• May 17, 2016

Rotel RX-404 IC free from heatsink

 

[EchoWars]

Need to look at the output of the preamp with a scope. If that looks OK, then the amp is at fault, unless there's some wiring problem of some sort.

 

[stillspin'n]

Can you post a pic of the IC, i am guessing its the output IC, like a STK*** type chip. Have you checked the DC voltage at the output with no load? Was any of the fuses blown or did you just replace them, if any were blown what did they lead to? Since the IC is loose from the heatsink, are there any pins that has been unsoldered, sometimes techs will unsolder power supply pins from the chip to see if its shorted, if any are loose check for shorts before reconnecting them.

 

[ZOOM]

The IC is still available? Unlikely. Most sellers only offer fakes that will blow up right away. Genuine amplifier ICs from the 70's are extremely rare.

 

[grabread]

I agree Rotels are Beautiful but they are Japanese not British.

 

[petehall347]

manual is saying si-1135hd . a quick look at prices says its very expensive .
it is not a good idea to power up without it attached to the heat-sink .

 

[The Eggman]

Wow, so many replies... so quickly. Thank you all.

I have checked it over well for broken solder joints, burnt components, blown caps and the like.

Echowars: A scope would be the next step in my technical evolution I believe, good succinct answer.

Stillspin'n: Just realized I didn't put public permission on the pic. My bad. Maybe it can be seen now?

ZOOM: Not surprised! I can hope... not much invested yet, may be worth the risk.

grabread: LOL isn't everything "made in XYZ"?

petehall347: Might be worth it, she's so purty! I agree about heatsinks, but I'm a trained engineer. There are levels of heat, and when I say that guy doesn't get warm I mean ZERO HEAT from that baby at full volume. Suspicious?

Sounds like the next step is either to invest in a scope or chance replacing "the large black chip in the picture which I'm calling the IC" and hoping. Good info... lots of projects 'round here but I hope to attend to this within a couple weeks. In the meantime, while I search down one'r them chips, I hope to hear more opinions and input.

Best...


BTW here's her face...

2016-05-19 01-36-20 - IMG_8778

• The Eggman
• May 19, 2016

Rotel RX-404

 

[stillspin'n]

If you can tap into the audio input to the chip{preamp level} should be able to run that to another amp and you would be able to see if you had a good audio signal, that would let you know if you have good singal into the chip, and if so would lead one to think the chip is the problem.