Boombox with occasional no speaker/s output - Page 1
Forum section: electronics / repair

author: lpc32
author_rank: Frequent Contributor
timestamp: December 02, 2014, 09:49:34 pm
content: I have an old boombox which sometimes, after turning it on, doesn't output to one or both speakers. It can sometimes be remedied by turning it off then back on, possibly a few times. Another temporary fix, if I recall, was to quickly turn the volume up then back down.The speakers are normally muted when the headphones out is in use, but I'm not sure that's the source of the problem.Any suggestions on how to find the cause?By the way, turning the device back on without it being hooked to the wall still leaves the power supply cap at about 1V. Is it a bad idea to then short it with a screwdriver?

author: Zucca
author_rank: Unknown
timestamp: December 03, 2014, 02:30:02 am
content: I suspect a bad joint somewhere, open it up and tap with an non conductive object the boards/components trying to reproduce the problem. Try to clean the volume pot also.With 1 Volt it should be not a problem to short it with a screw driver, but I have no idea why you want to do it.Generally you should provide pictures and more details (i.e. Speaker brand and type) because helping you now is like dealing with black magic.

author: particleman
author_rank: Regular Contributor
timestamp: December 03, 2014, 07:30:35 am
content: I would start with cleaning switches and pots.

author: Shock
author_rank: Super Contributor
timestamp: December 03, 2014, 10:21:27 am
content: Manipulate the volume and balance controls (potentiometers) with the unit turned off many times (dozens) then retest it by powering it back up. If the symptom goes away or comes back as scratchy audio then it's likely those potentiometers. Then do the same for EQ and tone controls etc to rule them out as well.Failing that being tested properly it's typically a characteristic of the amplifier ICs failing which is a very common problem.

author: lpc32
author_rank: Frequent Contributor
timestamp: December 03, 2014, 02:25:03 pm
content: I don't think it's a solder joint problem because once you get it working it continues being okay as long as it's in use. Also, when it fixes itself on turn off, there's a "pop" right before it fully turns off. If I recall, the pop is louder than the more muffled one you usually get on turn off.I'm pretty certain it happens with all inputs. Either way, the headphones out always works. But it's been a while since I used it, and currently the problem doesn't happen (I opened it up for scratchy pots).It's not a potentiometer problem. The scratchy pots do randomly decrease each speaker's volume, but you can always get some volume or noise by moving the pots. This doesn't happen when a speaker gets no output at all. By saying it can be fixed by quickly turning the volume up, I didn't mean just the physical slider. I vaguely recall it worked also when the input signal was turned up. Maybe I'm misinterpreting it, but I think the higher voltage may have helped.I'm guessing the source of the problem is either an electrolytic capacitor (none of them are visibly bad), or the speaker mute mechanism that engages with headphones connected.zucca: The speakers are part of the boombox, though they are removable. What pictures would be useful? Any specific areas on the boards?Yes, it's an 80s boombox. Here's a fancier relative from the same manufacturer:

author: David_AVD
author_rank: Super Contributor
timestamp: December 03, 2014, 09:18:13 pm
content: An internal connector with oxidised contacts could also be the cause. I'd unplug, spray both sides with deoxit or similar (very sparingly) and reseat each set.

author: Shock
author_rank: Super Contributor
timestamp: December 03, 2014, 09:31:06 pm
content: I've repaired that model before. While it's not the same as in the picture do as I say in the previous post and if the pots and controls don't change the fault then I suggest turning the balance (if you have one) fully to one speaker then to see if it's one side of the circuit. Check if it occurs when the stereo is cold or only warmed up or only and high or low volume etc. You can use an alternate input, headphones, and batteries to rule out other causes. These things probably use about $10 of batteries now though.As I mentioned it's commonly the amplifier IC's, they are almost a consumable and they won't last forever. But sure, test all the caps and replace any sets that have a single cap out of tolerance or bad esr.

author: lpc32
author_rank: Frequent Contributor
timestamp: December 07, 2014, 01:18:19 am
content: For some reason I didn't expect the headphones speakers-mute mechanism to be what it is: just a switched stereo socket. 11 pins, no less. The problem is either the internals or the solder joint that looks like it was barely soldered:The construction of this device is annoying. Although they did use a few connectors between boards, mostly it's just flimsy wires soldered on both ends; between the main board and the power supply, the motors, etc. There's also the above headphones mini-board that's soldered to another board to hold it upright.

author: lpc32
author_rank: Frequent Contributor
timestamp: December 08, 2014, 11:33:21 pm
content: After putting the whole thing back together the transformer hums, which I don't recall it doing before. Would it be just vibration against something it touches?By the way, what's that shield around the transformer?

author: Zucca
author_rank: Unknown
timestamp: December 15, 2014, 01:10:46 pm
content: Quote from: lpc32 on December 08, 2014, 11:33:21 pmJust a guess, maybe it will prevent the 50Hz main to find his way to the input signal circuits.