help me troubleshoot this old hi-fi

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cordura21

cordura21

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Hey guys. An old guy gave me an old hi-fi, those integrated with radios, etc... It has some nice, big speakers so I'd like to keep it.
It has a problem: the sound sometimes comes and goes, and it sounds like a bad tuned radio station, even if it is the cd player the one working. Sometimes I giveI turn the volume up fast and it starts to work fine. Sometimes I switched the A-B speakers back and forth and it also sounds good but eventually it breaks, specially at low volumes.
I think that the speakers are fine, since I guess when they are broken they sound bad all the time and not in intervales.
What do you think is the problem? Any method to troubleshoot them?
TIA, Andrés
 
If it's old, the pots and switches are probably covered in crud. Turn the unit off and work the pots back and forth 100 times. Same with the switches. Avoid "contact cleaner" - it can really gum up the pots. If it's just age, that should work. Good luck.

Bob
 
Like Bob mentioned, with something that old it's usually the mechanical things that wear out, pots, knobs, sliders, etc.

You might also try cleaning the speaker terminals. If they are even slightly corroded you could get the kind of problems you mention.

One other thing that often goes wrong with older equipment is the output stages, they carry the majority of the current used to drive the speakers and thus generate a lot of heat that can make solder joints weak and cause the stage to operate intermittently.

If you are comfortable with a soldering iron you can replace them yourself pretty easily. Just open the case, get the part number off the stage and have an electronics store find a replacement part. They are usually quite cheap. The output stage in most home stereos are pretty easy to identify because they are located near the back of the unit near the speaker terminals and are most often mounted to a heat sink.
 
I made some new discoverings. It has 3 line level ins: Aux, Tape 1 and Tape 2.
I am using Tape 2 now since it's the only one that doesn't make noise. Tape 1 was working fine but then it broke, and that made me fear that I was blowing them up although I am connection a Sony combo MD out (I guess that's for Mini Disc).

Another thing I noticed is that sometimes it gets worse when I move the thing, so somebody with more knowledge could get a hint from that.

Regards, Andrés
 
"Another thing I noticed is that sometimes it gets worse when I move the thing, "


Move what thing? The entire stereo, or just a connection? If it's the entire stereo, I'd open it up and check for broken/damaged connections - it may have been dropped, or had something spilled (liquid) into it.
And when you say the the Tape 1 input "broke", is the signal intermittant, or no signal at all? Did you work the RCA jack into and out of the female? If you can, try to run some fine sandpaper around in the input jacks.

Plan B:
Salvage the good speakers and trash the stereo.
 
I moved the entire stereo and it started to sound intermittent. The sound is "broken", like a badly tuned radio. Tape 1 and Aux do this al the time, Tape 2 come and goes but I shake the amp a round and then starts to sound fine. Both speakers and headphones do the same.
The only thing I did in the inside is to put some cleaning fluid in the volume pot, which is a dual pot for left and right.
I'm gonna sand the connectors but given that it changes when I shake it there must be something else.

Thanks for the tips, Andrés
 
How old are we talking here? Does it have tubes, or is it solid state?
 
it's a solid state, I think it has to be from the late 70's, the tuner uses a needle.
I want to save it cause it sounds good, and plus it has A B and A+B speaker options. That's cool to check mixes.
 
Cold loose solder joints..

Do the RCA jacks connect directly to a
PC board inside the chassis?
If so, you may want to resolder (with electronics solder, and proper flux) those connections.
What can happen is that years of plugging/unplugging to those connectors has make those solder connections loose, then they corrode, and become partially conductive. If you plug into the offending input and wiggle them, does the noise come and go?
 
hey, today I lost Tape 2 so I have no inputs.
The inputs go to a PC board, and if I wiggle them the sound comes and goes but very little. When it really changes is when I shake the whole device. So I fear that there's a big component loose, like those big capacitors it has.

I also noticed that sometimes when I turn it off and back on it works fine for a while, like the current shock helps.

Andrés
 
I use an old '70's class A reciever in the studio for everything and it gets some of the symtoms you describe from time to time. For me it's the switches and contacts and I just run a line from the air compressor in the garage and give it a blast at 40psi and it works great again.
 
Do you have a voltmeter? It's a laborious process, but with a meter you can start at the AC input, through the transformers, and through the input stages until you find either a break in a circuit or a component that isn't functioning. BE VERY CAREFUL when checking the AC circuit! You can get a shock that will send you into the ozone. You can check for continuity with the unit unplugged.
The fact that the inputs went out one after the other would suggest a component that is dying as the unit warms up. That could be several things, most of which aren't worth replacing. I have bought quite a few 70's era amps, some fine ones, at garage sales. I usually get them for 10-15 bucks.
Is this the only amp you have to mix with? If not, I wouldn't waste much time trying to fix it.

Bob
 
hey Bob. I have a voltmeter and I'm gonna try that but first I'll go for the cleaning thing.

I have a Sony amp - one of those stupid spaceship things -. I wanted to use this one since I guess it will be flatter. The Ohms on both are the same, so I don't have problem to connect the speakers. My question is if this Sony amp won't bring me a too colored sound.
 
Every amp colors the sound. This is especially true with consumer-grade amps. They are designed to favor certain freqs, and they all sound different. As for Sony, I've had several, and they are mostly junk. Some of their older gear is great, but not in the last 20 years. If it's all you have, use it. You'll just have to "learn" which freqs the thing is boosting/cutting/muddying(!) by listening to mixes on a variety of sources.
I see you're in Argentina. If you ever visit Miami, I'll set you up with an amp.


Bob
 
I found what it is. It's the source selector. It's a knob that lets you choose Aux, Tapes, etc. Inside there is a long plastic piece with a small sheet of metal inside, which transfers the selection you make on the front to the back, to the pcb where the inpouts are located. I don't know how to troubleshoot it. In the front I suppose there's no electrical device, it's just mechanical. I think the electric selector is a t the back on the pcb. I don't know if I should put cleaning fluid on any of them. On the pcb end there's a long plastic piece which I guess is the switching device.

Andrés
 
I'd try compressed air first and move to contact cleaner if I had to. Don't use WD 40.
 
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