Kenwood audio system problem

[cyberdyne]

Hi all, I have a KENWOOD M-A100 SERIES 21 system which has served me well for many years. Regrettably, the AMP has stopped outputting to one of the main speakers.

I have tested the non-working speaker on the working cable and it is fine. I have also tested the almost-brand new cable and that works. I've also checked the balance settings on the system and this is not the cause. So, I'm guessing something has died inside the AMP.

As I have the whole Kenwood separates system, what if anything can I do to try and work out what has gone and possibly fix this please or should I just give up before trying due to its' age/ cost of parts, etc.,?

Thank you in advance

 

[ChrisFrost]

It's either dead capacitors or a blown transistor. There's possibly some other issues but that's where I'd start. However, if you had the electronics knowledge that you'll need to actually fix the problem then you'd have probably done so by now... kind of a Catch 22 situation.

Have a look on Ebay for a replacement amp. I have seen the same model come up a few times, and since everyone seems to want piddly sized systems with no balls then they overlook better quality gear because it is too big. That's good for you because prices are low on bigger older gear.

 

[cyberdyne]

Thanks Chris. I am considering replacing it but was also hoping to fault find myself if possible. Presumably this would be a case of using a amp meter and testing each individual component. Luckily the layout has distinctive left / right sections so I at least have a starting point. I'm comfortable with a soldering iron and would be happy to have a go at replacing anything necessary myself if I could just find the offending part(s).

Would the blown/dead parts be visibly damaged or not necessarily? I assume testing them all individually is the way to go?
Thanks again.

 

[ChrisFrost]

Faulty caps are easy to spot. The tops or bottoms bulge. Adding gentle heat will sometimes coax a cap back to life temporarily and is a reasonable method for an intermittent component.

Transistors are a little trickier to test if not obviously blown. I use a 'scope so I can see the profile. Have a look online to see what might be possible with your type of meter. Chilling with freeze spray will sometimes coax a bit of silicon back to life temporarily if intermittent.

 

[cyberdyne]

Many thanks again, I'll have a look this evening.

 

[Sam Gangee]

Faulty caps are easy to spot. The tops or bottoms bulge.

Not always. There is often no visible sign of a problem. In fact even a capacitor meter won't always show a low value. Many years ago, my pal Roy, who ran a TV repair shop in Hartford, had a VCR that he couldn't fix. It was obviously a PSU fault but the capacitors looked perfect. I suspected the two 1000uF electrolytics so he desoldered them and measured the capacitance. Both measured over 950uF, which he thought was fine.

However, when I took my ESR meter the next day, it read more than 20 Ohms for each! (The ESR should have been less than 0.05 Ohms.)

Needless to say, replacing them fixed the fault and he never trusted his eyes (or his capacitance meter) again.

So a bulging capacitor is definitely faulty but a non-bulging capacitor is unknown.