My mother owned a ka-1500 since 1977, and when I was 15 I also bought one from ebay (but it was broken..)
I thought I would be able to fix it with a fuse or anything simple but that was'nt a sollution.
The problem is that the left channel sounds verry weird:
Ka-1500 problem - YouTube
Till volume 2 it sounds good ( i think then it's not getting amplified) but when I turn up the volume it sounds really cracky.
Some people I spoke tell me that I need to replace all capacitors, but the right channel is working very well and I can't believe this is a capacitor failure??
But i also am not 100 procent sure it's the output/ driver transistors because you can hear a bit of music.
Actually I really don't know where to start.
I also have a kenwood ka-1500(my mother's) that's working 100procent right, but i don't know wich voltages i need to compair to troubleshoot the faulty one..
I thought I would be able to fix it with a fuse or anything simple but that was'nt a sollution.
The problem is that the left channel sounds verry weird:
Ka-1500 problem - YouTube
Till volume 2 it sounds good ( i think then it's not getting amplified) but when I turn up the volume it sounds really cracky.
Some people I spoke tell me that I need to replace all capacitors, but the right channel is working very well and I can't believe this is a capacitor failure??
But i also am not 100 procent sure it's the output/ driver transistors because you can hear a bit of music.
Actually I really don't know where to start.
I also have a kenwood ka-1500(my mother's) that's working 100procent right, but i don't know wich voltages i need to compair to troubleshoot the faulty one..
Last edited:
This is an oldie. You need the service manual to see what you are doing,
Kenwood KA-1500 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
The amp has no preamp section (apart from one for vinyl) so assuming you are not using that then the fault is in the power amp section. Unusually it has a tone control wrapped around the feedback network for the power amp.
This is what you check first.
1. The DC voltage at the speaker output with respect to ground. It should be essentially zero to within say 100 millivolts.
2. With no speakers connected check the volt drop across each 0.47 ohm resistor on the output transistors. That's RM73/4/5 and 6. There should be around 60 millivolts across each give or take. Anywhere from around 30 to 90 millivolts would be OK.
Those two tests will show if the DC conditions are correct.
Kenwood KA-1500 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
The amp has no preamp section (apart from one for vinyl) so assuming you are not using that then the fault is in the power amp section. Unusually it has a tone control wrapped around the feedback network for the power amp.
This is what you check first.
1. The DC voltage at the speaker output with respect to ground. It should be essentially zero to within say 100 millivolts.
2. With no speakers connected check the volt drop across each 0.47 ohm resistor on the output transistors. That's RM73/4/5 and 6. There should be around 60 millivolts across each give or take. Anywhere from around 30 to 90 millivolts would be OK.
Those two tests will show if the DC conditions are correct.
Mooly, i will do that.
What i've forgotten to say is that the right channel works great!
about point 1:
You mean with volume set to 0 and no music playing
about everything: could you explain a little more about it not having a pre amplifier? It has tone control right, I Always tought having a pre amplifier was having the tone control? Or does a pre amplifier has is own transistor stage e.d
What i've forgotten to say is that the right channel works great!
about point 1:
You mean with volume set to 0 and no music playing
about everything: could you explain a little more about it not having a pre amplifier? It has tone control right, I Always tought having a pre amplifier was having the tone control? Or does a pre amplifier has is own transistor stage e.d
Yes, the DC conditions are measured with no signal and for measuring the volt drop across the resistors its better with no speakers.
Be careful measuring so as not to touch or short anything.
As one channel is OK you will have to identify which is the faulty both in the amp and on the circuit diagram so that the component numbers make sense. Obviously you only need measure on the faulty one but it might be instructive for you to compare with the good one too.
A preamplifier can mean many things, from a stage with gain to just "signal handling".
This amp has no electronic preamp, the line level inputs feeding direct into the power amp via the input selector and volume control. Tone controls normally do mean there is an "electronic" preamp but in this design the tone controls work within the power amp by modifying the feedback network to give the desired response.
Be careful measuring so as not to touch or short anything.
As one channel is OK you will have to identify which is the faulty both in the amp and on the circuit diagram so that the component numbers make sense. Obviously you only need measure on the faulty one but it might be instructive for you to compare with the good one too.
A preamplifier can mean many things, from a stage with gain to just "signal handling".
This amp has no electronic preamp, the line level inputs feeding direct into the power amp via the input selector and volume control. Tone controls normally do mean there is an "electronic" preamp but in this design the tone controls work within the power amp by modifying the feedback network to give the desired response.
Oke mooly, thankyou for your answer.
The thing i don't understand is:
Modifying the feedback network to give the desired response.
What's the difference with an electronic preamp? I Always thougt bass/ treble where just a bunch of capacitors and resistors to filter some frequencies etc and the potentionmeter makes how much this is done.
For example: a sansui 2000x or a denon dra-825r/ a rotel ra-1412 they have electronic pre-amps right? what's the difference.
About the measuring:
I will be doing that Friday🙂. But i was wondering, if there's a DC problem. Will that be in the drive stage or in the output stage? Or is that the next step?
(what am i going to measure? The signal that goes to the output/drive or that comes from the output?/drive stage)
Sorry if i am asking to much, but I really wan't to know what''s behind everything, and untill now I've never found a site/ books that could explain it to me!
The thing i don't understand is:
Modifying the feedback network to give the desired response.
What's the difference with an electronic preamp? I Always thougt bass/ treble where just a bunch of capacitors and resistors to filter some frequencies etc and the potentionmeter makes how much this is done.
For example: a sansui 2000x or a denon dra-825r/ a rotel ra-1412 they have electronic pre-amps right? what's the difference.
About the measuring:
I will be doing that Friday🙂. But i was wondering, if there's a DC problem. Will that be in the drive stage or in the output stage? Or is that the next step?
(what am i going to measure? The signal that goes to the output/drive or that comes from the output?/drive stage)
Sorry if i am asking to much, but I really wan't to know what''s behind everything, and untill now I've never found a site/ books that could explain it to me!
Your asking all the right questions 🙂
Bass and treble controls work pretty much as you say using R's and C's but as always there's more than one way to skin a cat 😀
You can have a purely passive tone network that has no active (semiconductor) devices, but the downside is that the stage is "lossy" and attenuates the signal greatly.
The most popular is the Baxandall (look it up 🙂) tone control using transistors or an integrated circuit opamp with the R and C's "wrapped around" it. This type is called an active tone control has no loss because it uses the semiconductor amplifier stage to make the gain up.... bit more to it than that but you get the idea.
Your Kenwood power amp behaves just the same as an opamp and so there is no reason why the tone stage can not be incorporated around it, and that is what has been done.
Measuring those DC voltages is the first basic step and tbh, I would expect them to be OK but we have to be sure before jumping in further. If there were a significant DC offset (a voltage across the speaker) then the fuse would be blowing and/or the speaker suffering... but we have to make sure. One step at a time.
Bass and treble controls work pretty much as you say using R's and C's but as always there's more than one way to skin a cat 😀
You can have a purely passive tone network that has no active (semiconductor) devices, but the downside is that the stage is "lossy" and attenuates the signal greatly.
The most popular is the Baxandall (look it up 🙂) tone control using transistors or an integrated circuit opamp with the R and C's "wrapped around" it. This type is called an active tone control has no loss because it uses the semiconductor amplifier stage to make the gain up.... bit more to it than that but you get the idea.
Your Kenwood power amp behaves just the same as an opamp and so there is no reason why the tone stage can not be incorporated around it, and that is what has been done.
Measuring those DC voltages is the first basic step and tbh, I would expect them to be OK but we have to be sure before jumping in further. If there were a significant DC offset (a voltage across the speaker) then the fuse would be blowing and/or the speaker suffering... but we have to make sure. One step at a time.
After listening to that clip again, and also you saying its OK until turned up I thought of something else for you to check.
Make sure the amp is OFF and then measure those 0.47 ohms resistors on your meter using the ohms range. Depending on your meter (which may not accurately measure such low values) you can compare readings from the good channel.
Still with the amp OFF and with your meter on ohms locate both output transistors on the faulty channel. Now measure from the centre pin to each of the other pins. Do any read really low ? Compare with the other channel and keep the meter lead polarity the same between comparisons.
(reason for this is that the amp would still "work" if the outputs had failed short circuit and the 0.47 ohms had gone open circuit. The speaker would still be driven from the drivers and the 330 ohm emitter resistors)
Make sure the amp is OFF and then measure those 0.47 ohms resistors on your meter using the ohms range. Depending on your meter (which may not accurately measure such low values) you can compare readings from the good channel.
Still with the amp OFF and with your meter on ohms locate both output transistors on the faulty channel. Now measure from the centre pin to each of the other pins. Do any read really low ? Compare with the other channel and keep the meter lead polarity the same between comparisons.
(reason for this is that the amp would still "work" if the outputs had failed short circuit and the 0.47 ohms had gone open circuit. The speaker would still be driven from the drivers and the 330 ohm emitter resistors)
wow thanks for the explanation, will check it tomorrow (coming back from a late night shift.
Just wondering why: if you've got a significant dc offset is that before the output stage or after? (or more than one option haha)
Just wondering why: if you've got a significant dc offset is that before the output stage or after? (or more than one option haha)
If there is a DC offset then it can be caused by a fault at any location in the circuit. That ones too complex to explain in a few lines but basically the whole power amp can be though of as an "opamp" with two inputs, inverting and non inverting. But a problem anywhere from the first transistor to the last can cause a DC problem.
Most faults usually have a simple cause though 🙂
Most faults usually have a simple cause though 🙂
Just a guess form an old time repair guy. How does it sound in headphones with speakers turned off? I'm guessing it's considerably better. If that is the case, I'd look for damaged output transistors - not shorted but open and/or an open emitter resistor on an output device or a bad solder connection in that area. BTW this kind of fault would show up a DC problems as you describe.
G²
G²
with headphones it is indeed better (like almost acceptable to listen to)
edit:
I tested the 0.47 ohm resistors with my multimeter on the ''200'' (lowest scale on mine)
And all the resistors it says 2.9 so they are good, i am now going to test the transistors
edit:
I tested the 0.47 ohm resistors with my multimeter on the ''200'' (lowest scale on mine)
And all the resistors it says 2.9 so they are good, i am now going to test the transistors
Last edited:
That would fit in with what I mentioned in post #7 and what status46 mentioned. It appears the amp is using only the driver stages.
Do the checks I mentioned and report back 🙂
Do the checks I mentioned and report back 🙂
All four (both channels) read the same ?
You need to do the voltage check across each 0.47 next with the amp on and no speakers connected.
A very good test on the transistors is to measure the base to emitter volt drop with the amp on. That means reading the DC voltage across the end pins of each output transistor, but be very careful not to short to the next pin. You should measure around 0.7 volts on each.
Check power supply, lack of energy.... poor biasing, lower voltage than needed
This is to go checking fuses, power supply rectifiers, transformer and filters..then check into the power amplifier channels if you have voltage (and current enougth)
regards,
Carlos
This is to go checking fuses, power supply rectifiers, transformer and filters..then check into the power amplifier channels if you have voltage (and current enougth)
regards,
Carlos
I would check for dry out electrolytic capacitors! Use an E.S.R meter and make sure to check the caps that are NOT "shorted" with other components on the circuit when you check them.
Do you have access to a simple oscilloscope?
Since the right channel works, look at the schematic (bolded lines represent signal path) so inject a signal in both channel (sine wave is better but others will work too) and using the oscilloscope, check both channels starting with the output transistors, tracing back to the pre-amp stages, at some point you will see a true sine wave on the scope and beyond that a clipped signal... this is where your fault lies.
possible culprits are a blown or open transistor on one side of the rail, an open emitter resistor.
I doubt lack of bias will make it sound like this... this sounds like one of the transistors is switched off completely either with no supply coming to it or it is blown or it has blown it's coupling resistors... simple amp... should be easy fix.
If you don't have a scope, just compare all ohm values of all parts between the good and the bad channel... easy.
Since the right channel works, look at the schematic (bolded lines represent signal path) so inject a signal in both channel (sine wave is better but others will work too) and using the oscilloscope, check both channels starting with the output transistors, tracing back to the pre-amp stages, at some point you will see a true sine wave on the scope and beyond that a clipped signal... this is where your fault lies.
possible culprits are a blown or open transistor on one side of the rail, an open emitter resistor.
I doubt lack of bias will make it sound like this... this sounds like one of the transistors is switched off completely either with no supply coming to it or it is blown or it has blown it's coupling resistors... simple amp... should be easy fix.
If you don't have a scope, just compare all ohm values of all parts between the good and the bad channel... easy.
My test results:
Dc voltage across speaker outputs (amplifier on, volume to zero, no speakers connected):
Left channel: 76 mV
Right channel: 52mV
(note that when the amplifier is off), the left channel has 0mV, but right says 130mV
Volt drop across each 0.47 ohm resistor(from left to right, standing in front of the amplifier, amp on)
1:0mV
2:0mV
3:22mV
4:21mV
Amp Off measuring the 0.47ohm resistors
(multimeter on 200 ohm range, it goes from 1.2 slow to 1.0)
1:01.0
2:01.0
3:01.0
4:01.0
Amp off all transistors
1: (meter on 2k ohm range)
Center to B: 732
Center to e: 887
2:
Center to b: no connection
Center to e:no connection
3:center to b:no connectin
Center to e: no connection
4:center to b: 722
Center to e: 699
Amp on: volt drop across transistors (b to e)
1:0v
2:0v
3:0.6v
4:0.6V
The thing I noticed is The volt drop across the 0.47 ohm resistors and the volt drop across the transistors. But i don't get if that's a cause of the problem or a result!
But if the headphones are after the driver stage and not using the output stage (asuming the problem is in the output stage), why isn't the sound 100 procent right then? It sounds a lot better then using a speaker but it's not 100 procent.
I am also curious: You give me all sort of values like: acros the b and e pins you need to measure anything like 0.7V
0 mv, or at the speaker outputs evertything between 30 mv to 100mv is ok. Are these values that are the same for every amplifier? Or how do you know such stuff? (manual?)
Dc voltage across speaker outputs (amplifier on, volume to zero, no speakers connected):
Left channel: 76 mV
Right channel: 52mV
(note that when the amplifier is off), the left channel has 0mV, but right says 130mV
Volt drop across each 0.47 ohm resistor(from left to right, standing in front of the amplifier, amp on)
1:0mV
2:0mV
3:22mV
4:21mV
Amp Off measuring the 0.47ohm resistors
(multimeter on 200 ohm range, it goes from 1.2 slow to 1.0)
1:01.0
2:01.0
3:01.0
4:01.0
Amp off all transistors
1: (meter on 2k ohm range)
Center to B: 732
Center to e: 887
2:
Center to b: no connection
Center to e:no connection
3:center to b:no connectin
Center to e: no connection
4:center to b: 722
Center to e: 699
Amp on: volt drop across transistors (b to e)
1:0v
2:0v
3:0.6v
4:0.6V
The thing I noticed is The volt drop across the 0.47 ohm resistors and the volt drop across the transistors. But i don't get if that's a cause of the problem or a result!
But if the headphones are after the driver stage and not using the output stage (asuming the problem is in the output stage), why isn't the sound 100 procent right then? It sounds a lot better then using a speaker but it's not 100 procent.
I am also curious: You give me all sort of values like: acros the b and e pins you need to measure anything like 0.7V
0 mv, or at the speaker outputs evertything between 30 mv to 100mv is ok. Are these values that are the same for every amplifier? Or how do you know such stuff? (manual?)
Last edited:
Voltage drop of 0.6v across BE suggest ok biasing ... 0 suggests trannie is switched off. Either not getting bias form before it, or a resister on it's base is open/ fried or the driver is open/ fried.
Last edited:
Easy ones first...
The base to emitter volt drop at 0.7 volts is essentially determined by the physics of the materials used and all (for what we are talking) silicon transistors will be the same.
Given the supply voltages and a circuit diagram its only a couple of minutes work to write in all the voltages and currents throughout the circuit... but that comes with experience and a good grounding in circuit theory.
For a DC coupled amp like this... we call it DC coupled because there is no coupling capacitor to the speaker... always calls for an output voltage of 0.00 volts DC across the speaker.
The headphones... in your amp they use the main output of the amplifier and the feed is just via a resistor to limit power. The reason it's not perfect is because you have fault, the reason its a lot better than via the speakers is because the headphones only lightly load the output stage.
I'm going to look at your readings now but it seems the output stage in the faulty channel is drawing no current. The devil is in the detail with all the readings, and it looks like we'll have to get more involved. The fact the resistors (0.47 ohms) appear intact suggest a less then usual problem.
I'll have a look 🙂
The base to emitter volt drop at 0.7 volts is essentially determined by the physics of the materials used and all (for what we are talking) silicon transistors will be the same.
Given the supply voltages and a circuit diagram its only a couple of minutes work to write in all the voltages and currents throughout the circuit... but that comes with experience and a good grounding in circuit theory.
For a DC coupled amp like this... we call it DC coupled because there is no coupling capacitor to the speaker... always calls for an output voltage of 0.00 volts DC across the speaker.
The headphones... in your amp they use the main output of the amplifier and the feed is just via a resistor to limit power. The reason it's not perfect is because you have fault, the reason its a lot better than via the speakers is because the headphones only lightly load the output stage.
I'm going to look at your readings now but it seems the output stage in the faulty channel is drawing no current. The devil is in the detail with all the readings, and it looks like we'll have to get more involved. The fact the resistors (0.47 ohms) appear intact suggest a less then usual problem.
I'll have a look 🙂
You will see that if you overbias the bad channel, the headphone sound will play even more clearer... reason, high impedance of the headphones and your amp running is more class-A so it compensates for the bad half which is not working. However if you connect via a low impedance speaker, there is not enough bias to drive the speaker with only half the rail working... essentially, one half of the sinewave is not being constructed hence the distorted sound...
- Status
- Not open for further replies.