Diagnosing Dead Channel...

mmcc

AK Subscriber
Subscriber
Hi - I just got a Realistic STA-14 with a channel out. This thing was cheap, and not worth a lot, so it seems like a good victim for learning a few things.

I did the usual pot and switch cleaning but one of the channels is very very faint. Here's what I know...

- The bad channel is noisy even with the volume all the way down.
- The channel is bad through the headphone jack as well as the speaker outputs.
- There's no crunchiness when turning the volume knob, balance knob, or tone knobs
- The bad channel is out when the tuner is selected as well.

I'm hoping for some guidance in troubleshooting this. I have a soldering iron, a solder sucker, and I'm not afraid to ruin this thing.

In examining the board, I saw some blue/green corrosion around a couple of resistors - see the pic, especially below the yellow wire.

Would replacing these resistors be a good place to start?

IMG_1216 (1).JPG
 
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Really need the service manual or schematic, did a quick search, no luck, don't suppose you have a paper copy or maybe
a sm of a similiar model?

Assume you don't have an oscilloscope. An audio probe is handy, do an AK search, there are "plans". It's just an old MM probe and
a small dc blocking capacitor connected to a cheap/disposable speaker. The idea being to probe the audio path starting at the RCA's
until the audio drops out. The probe channels the signal through the cheap speaker, it will be low volume. You can buy an amp from
china for like $3- that you can connect to the probe to up the volume. Something like,
https://www.ebay.com.au/itm/LM386-200-Gain-Audio ...

Certainly any corroded/damaged parts should be replaced
 
Really need the service manual or schematic, did a quick search, no luck, don't suppose you have a paper copy or maybe
a sm of a similiar model?

Assume you don't have an oscilloscope. An audio probe is handy, do an AK search, there are "plans". It's just an old MM probe and
a small dc blocking capacitor connected to a cheap/disposable speaker. The idea being to probe the audio path starting at the RCA's
until the audio drops out. The probe channels the signal through the cheap speaker, it will be low volume. You can buy an amp from
china for like $3- that you can connect to the probe to up the volume. Something like,
https://www.ebay.com.au/itm/LM386-200-Gain-Audio ...

Certainly any corroded/damaged parts should be replaced

Thanks for the advice. I actually do have an oscilloscope - it is a Heathkit one from the 60s that I bought ~15 years ago. The only time I ever used that scope - or any scope - was when I hooked it up to the signal generator I got at the same time to watch it go. It has since been moved from Arizona to Ohio and sat in my basement. Maybe (if it still works) now is its time to shine.

I do not have the service manual. There's one online for the STA-16, but that has 4 speaker outputs and a pseudo surround sound effect. I'll see if maybe the amp circuits are the same.

Here's a photo of the whole chassis - as you can seem, it is not a terribly complicated beast.

IMG_1218.JPG
 
You have one good channel to compare with the bad channel, I'm not a tech, just a hobbyist, I've had good luck testing each component "in circuit" and compare your readings with the bad channel one at a time until you find a discrepancy. You may not get accurate readings testing in circuit but they should be similar between channels.
 
isolating the problem is the first process

bad channel seems to have 3 symptoms, faint, noisy, bad - all 3 consistent, etc

is the other channel fully working - able to go loud with no distortion? if so not the power supply.
then
are all the switches working (after you've worked the back-and-forth for about 50x)
asking - could this switch be the cause of a signal not being passed?

using a good input signal (CDP on batteries):
1. using tape monitor (outputs) to another known good unit - both signals still good?
no - switches
2. move good signal to bad channel input, flip balance over to good channel - same or not
3. mono switch? try it
4. take good input signals and connect to tape monitor input, select such, still bad.

this can (there are more tests - so this is a start and if NOT totally 100% complete
and accurate is meant to give you the mental diagnostic steps) isolate it
down to preamp/line side or the power amp side.

then if you can follow a schematic. take a probe, following the signal path, jumper
from good side to bad side at same point. this uses a good signal to isolate where
the problem(s) begin. follow @mbz instructions for detecting this.

being thorough helps, IOW, since you are starting out, develop all the good habits
first. make them automatic - like washing a car before troubleshooting.

1. blow out the dust and wipe down the insides (forget the outsides for 10 years)
2. remove bottom cover, flip upside down, re-flow all joints - use flux - takes minutes
use magnifier - mark joints that look "bad" - note where corrosion occurs - note any
missing/broken/components/leads/traces, etc
3. resolder speaker/headphone connections.

in parallel, start thinking about replacing caps if the unit is more than 20 YO,
pulling transistors, testing them, re-doing the heatsink compounds and cleaning
up in general. only if you have experience and understanding of this - otherwise
you'll need to spend quality time and effort researching and learning this.

do document everything you've done. this way we can ask what you did and you
will be able to post a couple of paragraphs on what you did versus from memory
and taking a few dozen threads asking about missing steps, etc.

take pictures. better for you when re-assembling. do not use flash - use table lamps,
daylight through open windows, hold a sheet of white paper, etc.

good luck - at the end of all this. you will be saving another unit from the landfill
and you can then enjoy the music.
 
... (lots of good stuff redacted) ...

Thanks for the advice!

- The good channel can indeed go loud without distortion
- What's a CDP? I have a tone generator on my phone that I can use as a signal source.
- I've not tried the tape outputs. I'll give that a try.
- The only switch on this thing is the AFC switch for the tuner. Currently there isn't even a power switch.
- By "re-flow all joints", do you mean melting all of the solder joints to fill in any gaps, and touching up where necessary?

Just for reference, here's a picture of the front of the thing that I found online: STA14.jpg
 
If the STA-14 doesn't have speaker switches, and instead the speakers are turned off when the headphones are plugged in, then there is another switch for you to clean -- the one inside of the headphone jack. If it is dirty or corroded in might not be opening or closing properly. I've seen this almost completely take out one channel, so that it was barely present in the speaker output or in the headphone output (the latter when they were plugged in). I half pulled out, half flushed out a bug nest from a headphone jack once that was causing the same problem, but for some reason only screwing up the operation of the speaker switch in the headphone jack on one channel. There are other things that could be causing your channel problem, but if has that switch in the headphone jack you want to rule it out before tearing harder stuff apart.
 
If the STA-14 doesn't have speaker switches, and instead the speakers are turned off when the headphones are plugged in, then there is another switch for you to clean -- the one inside of the headphone jack.

Great idea - I'll try that tonight.
 
I know this is a old thread but I am having the exact same issue with the same model receiver that I just brought home from a flea market. Haven’t had time to open it up yet but will do that shortly.

EDIT: Actually mine is an STA-14A.
 
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