Novice interested in electronics

[tvissoc]

I pretty much a novice when it comes to really understanding or testing electronics. I've built some circuits (board etching, soldering, etc) but never designed circuits. I have a basic understanding of electronics, but transistors and their magic smoke always trip me up. :-) I'm basically here to gain some knowledge on testing circuits, especially audio amplifiers. I have some vintage electronics (is '80s really vintage?) that I'd like to resurrect and/or maintain. I still have the receiver and speakers that I bought with my first tax return back in the '80s. A Realistic STA-785 receiver and a pair of Realistic Mach Two speakers that need re-foamed.

[Mooly]

Welcome to the forums tvissoc

Some of the older gear can be well worth saving and a quick once over checking for poor soldering and replacing old electrolytic caps can work wonders.

[tvissoc]

Originally Posted by Mooly

Welcome to the forums tvissoc

Some of the older gear can be well worth saving and a quick once over checking for poor soldering and replacing old electrolytic caps can work wonders.

Thanks. I'm suspecting the Sanyo audio amp in the receiver. Just don't know how to test it.

[gootee]

You will need at least a multimeter.

One cool thing about stereo amplifiers is that they have two identical channels. The circuitry for the two channels should be identical to each other. So you can take measurements from both channels, at the same point in each channel. If there's a problem, then the measurements will differ, where a problem is affecting only one channel's circuit. Or, the problem could be elsewhere. A typical first type of measurement to try would be the resistance between each test point and a convenient ground point, somewhere, with the unit unplugged.

If it's close to 30 years old, I would just start by replacing every electrolytic capacitor, large and small. You can buy new ones from places like mouser.com or digikey.com . Just make sure you use a cap with an equal or higher voltage rating, and roughly the same capacitance. Certainly +/- 20% would be good enough, for the capacitance value, and even more of a tolerance range would almost always be close enough. Usually, erring on the high side would be the best way to go, if you had to choose one way or the other. If you feel the need, values can be added together by wiring the caps in parallel. Watch out for any non-polarized ones. And make sure that you try to get caps that have similar lead spacings, and can dimensions that will fit where the old ones were.

[Mooly]

Originally Posted by tvissoc

Thanks. I'm suspecting the Sanyo audio amp in the receiver. Just don't know how to test it.

That sounds as if you mean an STK type module. They can suffer with poor soldering externally on the pins.

Depends exactly what the fault is really. Speaker relays (if it has them) can be a source of problems (tarnished contacts).

[JMFahey]

Just what is it you are suspecting?
Meaning: do you think "this stuff is 30 y.o. , surely something must be worn/old" or you have some specific symptom?
The approachn is not the same.
If preventive maintenance, yes, do what was suggested above.
If you have some specific problem, you'll have to troubleshoot it.
Anyway, start googling around, maybe you can get the schematic.
You might even have an original manual, if so scan and post it.
Good luck.

[tvissoc]

Originally Posted by JMFahey

Just what is it you are suspecting?
Meaning: do you think "this stuff is 30 y.o. , surely something must be worn/old" or you have some specific symptom?
The approachn is not the same.
If preventive maintenance, yes, do what was suggested above.
If you have some specific problem, you'll have to troubleshoot it.
Anyway, start googling around, maybe you can get the schematic.
You might even have an original manual, if so scan and post it.
Good luck.

Realized I didn't specify in this thread what my problem was... Realistic STA-785 receiver. Just recently producing almost no audio. Crank the volume all the way and I can barely hear sound from speakers... Speakers are fine hooked to another receiver. Unit has an STK4182II.

On eBay right now, there is a schematic and manual. I'll probably buy it. I already have a manual that I can scan in.

Thanks,

Tony

[Mooly]

If both channels are the same, very faint sound... wild guess but worth checking... make sure no preamp out to poweramp in "links" are missing on the back (if it has that) or, that the correct input is selected and not looped through a tape monitor or something.

[tvissoc]

Originally Posted by Mooly

If both channels are the same, very faint sound... wild guess but worth checking... make sure no preamp out to poweramp in "links" are missing on the back (if it has that) or, that the correct input is selected and not looped through a tape monitor or something.

Yeah, I'm going to go back and check some of the "obvious" things. I'll try another audio source too.

Tony.