Hi, I am an electronics novice wanting to repair a much loved 80's boombox. It was working perfectly for a few weeks after I cleaned the pots and switches, then it suddenly stopped working, no audio but tape goes round rewinds etc...lights light up, slight hum from speakers and speakers make the usual popping sound when it's switched on. Anyway I suspect one of the larger capacitors on the audio amp board may be the problem. I saw a video on YouTube demonstrating a MESR100 in circuit capacitor tester. I have a few questions about the tester before I buy one.
1 Could this tester damage other components in the boombox by testing capacitors in circuit?
2 Do all the capacitors in the circuit need discharging before testing or just the one that's being tested?
3 Can I discharge all the capacitors simply by switching boombox on with no power going to it?
4 If I do decide to buy a MESR100 tester do I go for the older model or V2 (version 2)?
1 Could this tester damage other components in the boombox by testing capacitors in circuit?
2 Do all the capacitors in the circuit need discharging before testing or just the one that's being tested?
3 Can I discharge all the capacitors simply by switching boombox on with no power going to it?
4 If I do decide to buy a MESR100 tester do I go for the older model or V2 (version 2)?
You can't test capacitors in circuit. You'll get a reading but it will be a reading of the capacitor plus the attached circuit.
Is there absolutely NO output? The only capacitor failure I can think of to cause that would be an open circuit output coupling cap.
As you said you cleaned all switches and pots, one of these going bad would be a pretty good explanation for no sound...
Is there absolutely NO output? The only capacitor failure I can think of to cause that would be an open circuit output coupling cap.
As you said you cleaned all switches and pots, one of these going bad would be a pretty good explanation for no sound...
I have personally used a Huntron Tracker before. They work just like the adds stake but are expensive for what they are. You can build one for yourself if you have access to an O'scope to use with this circuit.
How to build Oscilloscope testing module (huntron circuit) - circuit diagram
How to build Oscilloscope testing module (huntron circuit) - circuit diagram
There is a hum from the speakers, I bought this boombox not working a few years ago on eBay with exactly the same symptoms it is having now and I had it repaired by a professional who unfortunately no longer does repairs. He replaced a cap on the audio board though I do not know if he did anything else to get it working, regarding testing caps in circuit here is the link to the YouTube video for the in circuit cap tester
YouTube
There are a lot of caps in this boombox and it really would be a time consuming job to remove them to test them.
My partner was listening to the boombox when it failed. She told me it was working fine and suddenly there was a very loud buzz (which lasted about 1 second) that made her jump and then nothing (I would say there was still the hum that there always had been in the background though). Also my 9 year old son was fiddling about with the end of the jack lead that we use to play our tablets through via the aux input on the boombox. He was putting his fingers on the end of the lead causing a humming sound on the boombox. It was a few days after my son did this that the boombox failed, could this have done any damage?
While, the youtuber is not wrong exactly, most of the methods he mentions only work when the caps have been dead a LONG LONG time, most circuits start messing up much earlier than that.
As for testing in-circuit, the issues are as follows:
1) Most modern ESR meters will not turn on most traditional silicon devices. From the cheap multi-component testers on eBay, to the toys and low-end pieces like the MESR-1000 and Peak ESR reader, to the real LCRs like the Der EE DE-5000 (also sold by IET labs as the DE-5000 and 6000), unless you hit very specific capacitors (usually VERY VERY large), you will not trigger silicon devices.
2) Any circuit which has capacitors in parallel absolutely CANNOT be read, even by the in-circuit devices. It is physically impossible to measure them in circuit, you can only measure the parallel properties of all of the capacitors in that node of the circuit.
Keep in mind, a little trick that I know some do. You don't have to remove the capacitor entirely, just disconnect/lift one leg. That is good enough to break the circuit and get the measurement you need. Even better, you can do it with ANY ESR reader on the market period (for the most budget conscious, I advise the multi-component testers on eBay, for those wanting a real LCR I advise the DE-5000).
Thanks for your very comprehensive answer. I assume it does not matter if the positive OR negative leg is lifted when testing a capacitor? Would a
Jingyan M6013 Digital Auto Ranging Capacitance Meter be good enough for a novice such as myself? I already have a multimeter but it does not test capacitors.
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That sounds more like a power output chip popping its clogs. Very loud buzz and then nothing... the buzz you still hear could be from the transformer.
Correct, it doesn't matter which leg you lift, as long as you don't complete the circuit with the board.
No, that does not test ESR, it simply reads capacitance. Even at that, it does not do ti at a specific relevant frequency, making it useless for comparing to datashsets (capacitance for electrolytic is strongly frequency dependent). If you are just a simple hobbyist, then I would recommend something like this:
Amazon.com: Quimat Multifunctional Transistor Tester with LCD GM328，DIY Diode Capacitance ESR Voltage Frequency Meter PWM Square Wave Signal Generator: Industrial & Scientific
It gets you a much more full featured device which will complement your multimeter very nicely. Allowing you to not only test, qualify, and identify components such as resistors, capacitors, diodes, BJTs, Mosfets, etc., but also have a simple function gen and other features.
You can find various versions of this based off of the Mega328 or GM328. Just search for GM328 or Mega328 component tester on Amazon or eBay. You will see older versions with less features and newer ones (like this one, with the graphic LCD and extra srew terminals for other features). These designs are based off an open-source project of which the english language thread is here:
https://www.eevblog.com/forum/testgear/$20-lcr-esr-transistor-checker-project/
One of the main devs is a frequent poster in that thread. So you can get help, flash new firmware which add additional features, etc.
Could be, but I would think it is something in the input stage, be it a DC-blocking capacitor which now passes loads of DC or a filter cap on the input which now shorts the entire signal to ground, etc. In my experience, the buzz is usually conducted 60Hz or 120Hz (post-full-wave recitified) mains being injected into a working amplifier stage. If the power suppyl was fully dead, wouldn't you expect the amplifer stage to stop working?
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I think I may upload some photos of the insides of the boombox this weekend so maybe someone could point me in the right direction, I did try running the boombox from 12volts DC just in case it was the ac power supply that was the problem but the boombox still did not work. One other thing, previously when I took the boombox from the kitchen (where it had been on for a few hours) to clean the switches and pots, I noticed the ac transformer was very hot, almost too hot to touch, not sure if this is normal, bad ventilation? (it was quite close to the wall) or something else, the radio was crackling quite a lot before I cleaned the pots (and worked perfectly after cleaning). I will post some pics of the innards hopefully this weekend. As I say I,m only a novice so I will only be able to fix this thing if it is not too technical.
If the PSU is fully dead then the whole thing stops working. A zapped audio power chip is still most likely at this point (typically they also show physical damage from the overload with a split or crack in the epoxy).
The latest revelation that the PSU is now frying adds weight to the theory.
How about the Yosoo GM328 from Amazon?, There is also a Kindle instruction book available for a couple of £'s ' Using the Yosoo GM328: a guide for radio and electronics experimenters Kindle Edition ' gets good reviews?
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