Hey all, my old ka went out and I wasted my last $45 on a pa amp that sounds horrible and has no power, I cant find anything comparable local and im out of money and have 5 15" cabinets in my living room taking up space. Fixing this amp is my best option at this point.
I'm ase certified in automotive basic electric, don't know much about amps though. I have the skills to repair it, but don't know anything about diagnosing an amp. When it went out I was playing the amp at moderate volume and out of nowhere my receiver went up in smoke, afterwards the kenwood stopped working. It powers up, no output power though. I took the hood off and see nothing obviously burned up. surely the reciever going out must have some significance, could a power surge have done this?
anyways, I have a service manual, if anyone can point me in the right direction for diagnostic procedures and parts it would be greatly appreciated.
I'm ase certified in automotive basic electric, don't know much about amps though. I have the skills to repair it, but don't know anything about diagnosing an amp. When it went out I was playing the amp at moderate volume and out of nowhere my receiver went up in smoke, afterwards the kenwood stopped working. It powers up, no output power though. I took the hood off and see nothing obviously burned up. surely the reciever going out must have some significance, could a power surge have done this?
anyways, I have a service manual, if anyone can point me in the right direction for diagnostic procedures and parts it would be greatly appreciated.
Dont expect anyone in the forum to say ""do this"" or ""do that"" to a person that poped the hatch and did not look at the obvious ( and especially when schamatics is available )
check all fuses
check all voltage according to the schematic
obviously and as long as it looks that you have no expirience you need to be very carefull when working with mains voltage .....
check all fuses
check all voltage according to the schematic
obviously and as long as it looks that you have no expirience you need to be very carefull when working with mains voltage .....
I'm asking for help because I have searched to the hilt and found no basic diagnostic procedures or where to start checking. If the fuses were blown don't you think I would have replaced them?
Here are the basics:
Remove all jewelry so you don't burn your fingers off. Caps can hold charge after the power is off, and over 25V can kill you if it crosses your heart, so read aikenamps.com tech info technician safety button about proper rules for people repairing amps. He also has some good amp operation basic screens. R.G. Keen also has a great amp repair tutorial, although tuned for guitar amps: tube amp repair www.geofex.com/ampdbug/ampdebug.htm
Somewhere around ten to twenty years the electrolytic capacitors in electronic gear leak out the water and lose capacity. The cheap ones used on consumer gear are sealed with rubber that is attacked by oxygen, operating or sitting still. The good news is, amplifiers don't have very many and it can be economic to replace the caps if you do it yourself. I just change them all, without measuring; I understand others believe in taking them all out and measuring them once a month. Like a twenty year old tire, just because it holds air today doesn't mean it will hold air tomorrow.
These are aluminum cans with plastic wrap with a bunch of minus's in balls pointing at one lead, or small peanut M&M's with a plus on one lead. Usually they are marked 1 mf or higher. New marking at the distributors turns 1 mf into 1 uf, as if a Uniform was a deformed Micro. You look them up at distributors by clicking passive components, then capacitors, then aluminum electrolytic. I like to buy the 2000 hour up expected life ones, as I have recapped my dynakit equipment 3 times and that is too often if they sell longer life caps now. The peanut ones are tantalum:80% of the tantalums I have owned or bought have been defective, so I replace tantalums with aluminum plus a 0.1 mf ceramic cap wired parallel. Time marches on, and frequently in 1-10 mf caps you can replace them with film dielectric caps, if you can make it fit. Some electrolytics are marked "NP" for non-polar. The small ones I make it a point to replace with film.
WIth your unit going dead, probably an electrolytic cap has activated the protection circuit (good) or shorted out a rectifier or resistor in the power circuit (requires further debug). Check your components before the first big cap after the transformer to save yourself an additional shipping charge.
I started on car radios using the Ford car service manual, things are a lot easier now with the internet. Look for your schematic with a search engine.
While debuggging I would replace your speakers by 8 ohm 225 watt (or whatever you amp puts out) resistors so you don't blow up your speakers with DC. You can cobble some up out of surplus some times instead of paying the list price.
Remove all jewelry so you don't burn your fingers off. Caps can hold charge after the power is off, and over 25V can kill you if it crosses your heart, so read aikenamps.com tech info technician safety button about proper rules for people repairing amps. He also has some good amp operation basic screens. R.G. Keen also has a great amp repair tutorial, although tuned for guitar amps: tube amp repair www.geofex.com/ampdbug/ampdebug.htm
Somewhere around ten to twenty years the electrolytic capacitors in electronic gear leak out the water and lose capacity. The cheap ones used on consumer gear are sealed with rubber that is attacked by oxygen, operating or sitting still. The good news is, amplifiers don't have very many and it can be economic to replace the caps if you do it yourself. I just change them all, without measuring; I understand others believe in taking them all out and measuring them once a month. Like a twenty year old tire, just because it holds air today doesn't mean it will hold air tomorrow.
These are aluminum cans with plastic wrap with a bunch of minus's in balls pointing at one lead, or small peanut M&M's with a plus on one lead. Usually they are marked 1 mf or higher. New marking at the distributors turns 1 mf into 1 uf, as if a Uniform was a deformed Micro. You look them up at distributors by clicking passive components, then capacitors, then aluminum electrolytic. I like to buy the 2000 hour up expected life ones, as I have recapped my dynakit equipment 3 times and that is too often if they sell longer life caps now. The peanut ones are tantalum:80% of the tantalums I have owned or bought have been defective, so I replace tantalums with aluminum plus a 0.1 mf ceramic cap wired parallel. Time marches on, and frequently in 1-10 mf caps you can replace them with film dielectric caps, if you can make it fit. Some electrolytics are marked "NP" for non-polar. The small ones I make it a point to replace with film.
WIth your unit going dead, probably an electrolytic cap has activated the protection circuit (good) or shorted out a rectifier or resistor in the power circuit (requires further debug). Check your components before the first big cap after the transformer to save yourself an additional shipping charge.
I started on car radios using the Ford car service manual, things are a lot easier now with the internet. Look for your schematic with a search engine.
While debuggging I would replace your speakers by 8 ohm 225 watt (or whatever you amp puts out) resistors so you don't blow up your speakers with DC. You can cobble some up out of surplus some times instead of paying the list price.
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Ok, thanks allot for the info. I know about caps but I had previously thought that they realy only go out over time, not blow out at once. I have the schematic.
So the game plan should be to first check continuity of the caps, if none bad replace anyways and then check the circuit up for voltage with a dummy load on the amp?
also not sure what you mean by "Check your components before the first big cap after the transformer to save yourself an additional shipping charge"
So the game plan should be to first check continuity of the caps, if none bad replace anyways and then check the circuit up for voltage with a dummy load on the amp?
also not sure what you mean by "Check your components before the first big cap after the transformer to save yourself an additional shipping charge"
Continuity check on electrolytic caps tells almost nothing. The serious people around here measure ESR in addition to capacitance, or tan theta. The cheapest calibrated meter I have found that does that is $150. (there is a build it yourself one somewhere for $90). The cap function on your $30 DVM does not do that and gives stupid numbers on electrolytic caps usually. The electrolytics are also very temperature sensitive when dry and nobody measures them in an oven I have heard of.
I can recap a whole amp with 2000hour up caps for <$40 usually, I don't see measuring the **** things when I can look at a calender & read. I've had to recap my ST70 amp 3 times in 40 years, you can check the output power with a meter and see they are gone again. (The MacIntosh salesman told me the first time why my ST70 put out 12 watts). Solid state amps are not as forgiving as tube amps, when the caps gets leaky it blows a $.03 diode usually or a $2 bridge rectifier. You can check SS diodes with a dVM, about 600 ohm forwards and 1999 backwards on the 2000 ohm scale on my Sears DVM. (200 ohm scale is too low in voltage). Low power resistors in the input circuits, or negative temperator coefficient resistors (NTC thermistor) can burn out in the same place.On SS equipment Usually the cap deteriorates gradually, then one day the power circuit just gives up & burns out. Some cheapskates on here like to measure them and put them back, as do some organ repairmen. Like 20 year old tires, they could blow out tomorrow. There were some high quality caps sealed with something better than rubber after 1966, but not in a Kenwood.
Other problems with old amps are bad controls and corroded tin or brass connections, but since yours blew up instead of dropping one channel, you can worry about fixing that stuff later. Read the thread about the Marantz about how to make an AC signal chaser, when you get that far. I've got a Simpson 260 VOM with 2 VAC scale (good) and now a B&K 2120 oscilloscope, which is even better.
I can recap a whole amp with 2000hour up caps for <$40 usually, I don't see measuring the **** things when I can look at a calender & read. I've had to recap my ST70 amp 3 times in 40 years, you can check the output power with a meter and see they are gone again. (The MacIntosh salesman told me the first time why my ST70 put out 12 watts). Solid state amps are not as forgiving as tube amps, when the caps gets leaky it blows a $.03 diode usually or a $2 bridge rectifier. You can check SS diodes with a dVM, about 600 ohm forwards and 1999 backwards on the 2000 ohm scale on my Sears DVM. (200 ohm scale is too low in voltage). Low power resistors in the input circuits, or negative temperator coefficient resistors (NTC thermistor) can burn out in the same place.On SS equipment Usually the cap deteriorates gradually, then one day the power circuit just gives up & burns out. Some cheapskates on here like to measure them and put them back, as do some organ repairmen. Like 20 year old tires, they could blow out tomorrow. There were some high quality caps sealed with something better than rubber after 1966, but not in a Kenwood.
Other problems with old amps are bad controls and corroded tin or brass connections, but since yours blew up instead of dropping one channel, you can worry about fixing that stuff later. Read the thread about the Marantz about how to make an AC signal chaser, when you get that far. I've got a Simpson 260 VOM with 2 VAC scale (good) and now a B&K 2120 oscilloscope, which is even better.
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Ok so a good start would be to just replace all the caps regardless. hopefully there is'nt anything else wrong with it too major. I just took her apart so no turning back now. also when i turn the amp on it idles at around -20mv of dc output on each channel, does this mean anything?
even though things said from indinajo are correct id say its wrong practice .... changing the capacitors is a move towards upgrade not repair .
better practice will be to find out what is wrong first and then decide if, how, and what parts to replace.
any amplifier that idles at about 20mv offset ( better term will be settle at 20 mv ) means that 90% you have a perfectly working amplifier ( ta least the main stage )
if no sound comes out of it as you said check again all voltage according to the schematic there is 52.7/-31v/+30v safety resistors here and there
then offset since your amplifier features protection relay is to be measured before the relay not after in the output...
Does the relay click ? if not you either have a failure on the output stage ( protect circuit senses Excessive DC offset in the output and a few other things )
If the relay clicks and still there is no sound is the voltage present in the preamp stages?
check and verify 30+30 volt ( secondary psu ) is present were needed ...notice that locally this is regulated to 24+24 volts verify that this is present also ....
better practice will be to find out what is wrong first and then decide if, how, and what parts to replace.
any amplifier that idles at about 20mv offset ( better term will be settle at 20 mv ) means that 90% you have a perfectly working amplifier ( ta least the main stage )
if no sound comes out of it as you said check again all voltage according to the schematic there is 52.7/-31v/+30v safety resistors here and there
then offset since your amplifier features protection relay is to be measured before the relay not after in the output...
Does the relay click ? if not you either have a failure on the output stage ( protect circuit senses Excessive DC offset in the output and a few other things )
If the relay clicks and still there is no sound is the voltage present in the preamp stages?
check and verify 30+30 volt ( secondary psu ) is present were needed ...notice that locally this is regulated to 24+24 volts verify that this is present also ....
Plastic film caps, and ceramic disk caps, are long life parts and not suspect.
You don't have to change all the electrolytic caps, you can change them 1 at a time paying $7.07 apiece as the smaller caps are about $.07 and the shipping is minimum $7. Sakis is a pro with a shelf full of parts. I keep various transistors around but having electrolytic caps in the closet is just asking for stuff to go bad in a box. Buying all at once for a project is smart, changing 1 or 2 at time to see if you made it better or worse helps you find your bad solder joints without a scope.
Taking measurements to figure out what else is wrong first will also save money on shipping. I don't have an in circuit transistor diode tester either, and DVM can often fool you in circuit. Also removing transistors to measure often damages them, or the PCB. Playing music from a radio into a resistor exercises the circuit best for measurements, I believe.
I totally agree with sakis here.
Sticking everything on capacitors as indianajo seems to be doing is not the same as methodically tracing the problem. Have a look at http://www.diyaudio.com/forums/solid-state/198955-marantz-pm-7200-left-channel-distortion.html for some conspicuously similar comments on an amp-problem with a completely different failure mode, and you'll probably see what I mean.
Trace the problem, sakis has already given some good tips how to start.
Good luck.
And yes, in the end the culprit might turn out to be a bad cap...
It isn't, that's true, but if an upgrade is desirable, replacing everything could kill two birds with one stone.
I found one capacitor on the input board that looks like one leg came loose from the board. problem is i cant find any place for it on in the wiring diagram, I'm wondering if it is supposed to be like this, does a cap serve any function with only one leg used?
pic
pic
Unfortunately the picture is too blurred to get a good look, but from it, I would assume the black wire is connected to it.
A cap with only one leg connected indeed doesn't serve any function. But I find it hard to believe that this could have happened with the amp just sitting somewhere playing music.
If you were to bend the cap upright, is there a hole in the pcb the loose leg could fit in?
I just checked the collector of the output transistors with the volt meter and at least 6 resisters burned up...
At this point you may want to think about spending the time on a kenwood, or look at for something more competent to spend your time on. Learning to fix an amp is fun, but most of the kenwood's I have seen are very low end. You mind find something just as worn out but with better parts & heat sinks at a flea market or something.
I find replacing things knocks the bias circuits out of balance, so make sure you make a full test with dummy resistors before attaching speakers. After replacing the burnt O.T.'s , PC board lands, and all the 1994 capacitors, I'm chasing something now on a Peavey that makes it put out 76 VDC - safe on the dummy load resistors but deadly for speakers. Probably a $.01 diode that shorted silently instead of exploding messily the way the driver transistors and other diodes did.
I find replacing things knocks the bias circuits out of balance, so make sure you make a full test with dummy resistors before attaching speakers. After replacing the burnt O.T.'s , PC board lands, and all the 1994 capacitors, I'm chasing something now on a Peavey that makes it put out 76 VDC - safe on the dummy load resistors but deadly for speakers. Probably a $.01 diode that shorted silently instead of exploding messily the way the driver transistors and other diodes did.
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Some are, I suppose, but the "low-end" Kenwood I redid (KA-76) sounded spectacular after a recap and transistor replacement (and bias re-adjust) - I like it better than the combo I'm using now (NAD 116 and Hafler DH-220) and will probably go back to it. I was very surprised because it was nothing special before the refit.
I stand corrected. Actually, if you've got heat sinks and a transformer you can make about anything of discrete transistors,for almost no cost. This reviled ST120 that has melted everything semiconductor twice, and melted the solder last time, has settled down to a pleasant 16 hour a day music appliance if you play it loudly enough to drown out the fans I installed.
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