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Optimus STA-300 Crackle Sound

E

Eafnb

Member
I recently bought an Optimus STA-300 from goodwill. For a compact 15 watt receiver, I was surprised that it can generate such volumes than what it's rated for (in a good way of course). The problem is that when I first plugged it in, there seems to be some strange crackling sounds. This would continue for a good minute or two and then it would go away. Any suggestions on what I should do?
 
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Eafnb said:
I recently bought an Optimus STA-300 from goodwill. For a compact 15 watt receiver, I was surprised that it can generate such volumes than what it's rated for (in a good way of course). The problem is that when I first plugged it in, there seems to be some strange crackling sounds. This would continue for a good minute or two and then it would go away. Any suggestions on what I should do?
Click to expand...
Power switch maybe arcing. If the crackle is coming from the unit. If the crackle is in the speakers--- solder connection or transistor leakage.(noisy transistor)
 
avionic said:
Power switch maybe arcing. If the crackle is coming from the unit. If the crackle is in the speakers--- solder connection or transistor leakage.(noisy transistor)
Click to expand...
It's likely coming from the unit itself, tested my speakers with a different amp and they sound fine. How would I go about fixing a possible power switch arc or find out which transistor is leaking?
 
Eafnb said:
Both speakers and the crackling sound changes with the volume.
Click to expand...
Check the solder connection of the regulated power supplies. "pass transistors"..

Nix that idea... Check the solder connections on the two voltage regulator chips
IC701 and IC702. These regulators would be the prime suspects.
 
There seems to be this weird waxy stuff, should I be worried? I scraped some off but the problem was still there even if I didn't touch it.
 

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Eafnb said:
There seems to be this weird waxy stuff, should I be worried? I scraped some off but the problem was still there even if I didn't touch it.
Click to expand...
Don't worry about it. Its just wax.Thats the tuner circuit. Not the power supply.
Down load the service manual on Hifiengine.com.
 
One of my rules of caution for a novice.. Don't mess with the tuner unless your prepared to re- align it ....;)
 
avionic said:
Check the solder connection of the regulated power supplies. "pass transistors"..

Nix that idea... Check the solder connections on the two voltage regulator chips
IC701 and IC702. These regulators would be the prime suspects.
Click to expand...
I managed to disassemble it, and the solder connection seem...fine? I put some photos of what they look like and their solder connections to verify with you.

20190410_001139.jpg Both chips

20190410_000843.jpg IC702

20190410_000819.jpg IC701
 
avionic said:
Check the solder connection of the regulated power supplies. "pass transistors"..

Nix that idea... Check the solder connections on the two voltage regulator chips
IC701 and IC702. These regulators would be the prime suspects.
Click to expand...

I noticed that when I took both of them out, they have similar numbers and lettering to their replacements, for example:

IC701
From the Manual: NJM7812FA

Taken from the board: 7812A
JRC
M1063U

The link you sent: MC7812CTG

IC702
From Manual: NJM7912FA

Taken from the board: 7912A
JRC
M2055F

The link you sent: MC7912CTG

Why is the labeling different?
 
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