Sony STR-3800L Stereo Receiver Help

[MrOneShotAway]

Hope every one is doing good .
Any way, I got my hands on an old Sony AMP, that's not super impressive with stats, but I would really like to get it back into working condition.
When I've connected it to power, the bulb lid up and it seemed to work. As soon as I connected it to a speaker, it started producing very large amounts of noise, and with a kind of loop that makes its amplification larger over time (matter of secondes).

I'm a really big noob, so I've tried Googling some things. Some posts suggested checking the transistors, so I did. The NPN once read around 0.53V B-C, B-E and the PNP's read around 0.6V too. The resistance for all transistors from E-C was okay, well at least it gave a large resistance and not something that is striving for a zero value. I've tried checking the speakers connections, and I've found out that the black connection, the minus one, at the B speakers isn't connected together. Maybe it's just an old wire, but what it's really that, how do I diagnose the problems?

Therse a dual C filter cap, that means it's a cap based on two capacitors that have the same minus lead (Parallel?). Do I need to change that big cap? and can I change it with two of the same value capacitor? I've tried searching for the dual c cap, but no luck...

 

[MrOneShotAway]

Come on, guys, some one at least has an idea

 

[Bud_J]

I'm wondering why you think the cap might be bad? In any case, yes, you can swap it out for the same size/type/voltage.

If you can't find the same voltage, you can go higher. For example if it's a 16V cap, you can replace with 25V.

If it's polarized, then you need to get a polarized cap and match the polarity on the board.

Where was the volume knob (turned up or down) when the speaker produced feedback?

Peace,
Bud

 

[Tony Stewart]

All the electrolytes of this old unit might be suspect.

 

[USER REDACTED]

The whole thing looks good, but some really crusty. Try first to cleanup and get rid of the rust, then inspect leads - perhaps there is some short-cut somewhere or eaten trace by rust. Reflow with fresh solder the joins looks really bad. Then I would change all electrolytic caps and also check voltage from regulators.

 

[MrOneShotAway]

The whole thing looks good, but some really crusty. Try first to cleanup and get rid of the rust, then inspect leads - perhaps there is some short-cut somewhere or eaten trace by rust. Reflow with fresh solder the joins looks really bad. Then I would change all electrolytic caps and also check voltage from regulators.

By rust eaten trace, you're speaking of a the PCB copper traces? How can I spot witch of the solder joints need some worn love? Can you maybe mark a few with a paint program that really pop to you? By checking the voltage from the regulators, you mean checking the voltage produced from the transformer, or the little regulators that look like power transistors?

All the electrolytes of this old unit might be suspect.

There are not leaks what so every, so how would I go on and check if I need to change them?

Edit: The connections in the inside of the amplifier that's connected to the speakers, have a ton of corrosion, any way to deal with that?

 

[USER REDACTED]

The device is more than 35 years old and that is the problem. If it was stored in some humidity place as basement or attic, advanced corrosion cannot be avoided. Dealing with rust depend on magnitude of it, which cannot be seen quite well on the provided pictures. If connectors are corroded, you have to clean it up as well or replace.

Healthy joins is shiny cup. Otherwise than that is problematic spot. I have one 20 years old FM/AM SAMSUNG radio which had a similar problem with sound and just few reflowing spots solved the issue.

When do reflowing, keep the temperature of solder station around 260-275C and if cannot melt in 1-2s, avoid longer time. Otherwise you may damage the component or the pad (may easily be pulled up). Just wait a bit to cool, put fresh melted solder over, remove it with vacuum pump or desoldering wick and make fresh soldering. Follow the temperature rule, otherwise significant damage could be made.

You can see under solder mask if corrosion started to "eat" trace. That can go from already corroded join and continue under mask.

Yes, check the voltage output from voltage regulator. Service manual and schematic would help during repair. However, service manual is usually not easy to find and if exists, usually cost $15 or more. All upper are simply general suggestions.

If electrolytic caps need to be replaced as well, use low ESR quality brands, not cheap (few cents) ones.

Pictures are too small to conclude anything else. And also top side of PCB is not shown.