TRC-931 dead channel help ???

Hey,

So my lovely Lasonic 931 that recently arrived has a problem. Only one channel works. Here's the deal:
When I got it I accidentally got stereo for a brief moment as the broken off transformer rattled around inside. So both speakers work.
Plugged in headphones and only got mono then as well.
I cleaned the speaker contacts, function switch and here and there where it looked dirty with plenty of contact cleaner.
The function switch is damaged and does not "switch" properly but turns very loosely.
All wires seem to be connected to the right place and not damaged.
Worked all switches and contact back and forth 5-6 times or so.
Still only mono .

A picture. The speaker in the picture is the one that doesn't work.


I don't know what else it could be, anyone got any ideas?

Peace

So it worked fine then you plugged in headphones and now only one channel works?

One problem I see is it looks like amp and heat sink are bent out of position possibly causing dry solder joints to amp.

baddboybill said:

One problem I see is it looks like amp and heat sink are bent out of position possibly causing dry solder joints to amp.

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Agree with bill... you can clearly see the amp chip is bent and no longer inline. Likely the result of the transformer striking the heatsink which in my opinion, is poorly mounted to begin with since it only has 1 screw in the center allowing it to pivot/rotate. However, I suspect the issue might be worse than a simple dry solder joint... the chip might have internally fractured connections from the stress.

Okay, do you mean the horizontal silver "radiator" at the bottom of the picture? Feel free to circle things on the picture. Is there anything I can do about it?
I also had a tip to work the headphone jack with deoxit and the contact fast and many times, and I can now get stereo in the headphones if I wiggle it a bit. The speaker also makes a clicking noise for a while, so it's partially alive .
Would buying a new circuit board help? I might know where to find one.

Peace

if you can , i would suggest reheating the solider joints at the pins of the amp chips and the head phone jack as well. this will melt the solider and make old/cold connections better.

But be cautious because when the amp chip gets bent over its also possible the traces were pulled up by solder causing them to rip which will cause even more issues

Bottom cooler indicates that board suffered mechanical impact, and it might have caused surge

The reflection on the photos can be deceiving, but I'd say you got at least one blown condenser (marked with arrow), and several more suspicious looking (blow at the top/popped out) marked with question mark.

It is easy and cheap to find replacement for them, but first you should let someone experienced check the other side of the board to see if all elements have solid soldered contacts. Most important of all, check if contacts of IC attached to that bent bottom cooler are in place, and re-solder them so that IC gets in proper position.

and belive me it doesnt take much to rip a trace on one of these....

monchito said:

and belive me it doesnt take much to rip a trace on one of these....

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I agree I just worked on Ira's old Bercha that same thing happened with amp chip bending over ripping over 6 traces so bad that I can't even figure out where they go

SpaceLobster said:

Okay, do you mean the horizontal silver "radiator" at the bottom of the picture? Feel free to circle things on the picture. Is there anything I can do about it?
I also had a tip to work the headphone jack with deoxit and the contact fast and many times, and I can now get stereo in the headphones if I wiggle it a bit. The speaker also makes a clicking noise for a while, so it's partially alive .
Would buying a new circuit board help? I might know where to find one.

Peace

Click to expand...

Not saying for sure that the chip is bad. But I wouldn't bet on that chip surviving for very long. You can desolder the chip, then loosen the screw below the heatsink, recenter it, tighten it back, then resolder the chip back into place. But unless you know what you are doing and have good enough skill and equipment to solder and desolder without introducing excessive heat, the soldering process could itself damage the chip if it isn't already. Each pin should only have low heat applied (280-350 deg. C) for 2 seconds or so. Then cool, then next pin.

i had a similiar problem with a philips when one channel doesnt work. The problem is the headphone jack like if you haved plugged a mono headphone (pin) in a stereo jack or maybe a headphone with a fatter pin . Basically u may have used a wrong size pin or a bad one.Maybe the socket is very old.
Inside the headphone socket there are two rings that are pressed that cancel the speakers as they get pressed(when we put the pin) and we hear sound from the headphones and vice versa.Sometimes one of the rings stay in pressed state even after removing headphone pin and you lose one channel. The solution is use a needle and move the ring in the socket see if you are hearing the channel again. if it helps and the channel works then Never use the socket again or change it with better ones(if possibe).
Hope this is related or maybe its another problem.

Thank you all very much for your very helpful information !
However I am going back to Sweden for Christmas and will not be working on this until January . But it sounds like I better get a soldering kit then and really step up my reparation skillz so I can have this badboy pumping for springtime :sin: .
I will let you all know how it goes.

Peace

SpaceLobster said:

Thank you all very much for your very helpful information !
However I am going back to Sweden for Christmas and will not be working on this until January . But it sounds like I better get a soldering kit then and really step up my reparation skillz so I can have this badboy pumping for springtime :sin: .
I will let you all know how it goes.

Peace

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May I also suggest when ever you get Boombox and suspect damage, never and I mean never plug in to test before checking it over