Hello gents, ive a ka-1500 amp thats outputting raw power to the speakers, similar to that of a damaged input lead. Theres no trace of a signal, just massive buzzing noise..
I read on this forum this fault or similar faults are usually in the 'differential stage' transistors. I have basic understanding of electronics&components but am new to amplifier architecture.
Is it a good start to replace all transistors & capacitors if unable to source the fault? What components should i be testing with multimeter, bearing in mind iv only a volt/resist/current meter? Can you test internal components in a solid state amp without an output load?
Any help much appreciated!
Colin
I read on this forum this fault or similar faults are usually in the 'differential stage' transistors. I have basic understanding of electronics&components but am new to amplifier architecture.
Is it a good start to replace all transistors & capacitors if unable to source the fault? What components should i be testing with multimeter, bearing in mind iv only a volt/resist/current meter? Can you test internal components in a solid state amp without an output load?
Any help much appreciated!
Colin
Mooly, you say a 100w bulb in series with incoming supply, i assume you mean after the transformer but before the speaker outputs? Or in place of the speaker load?
Jim,the power transistors i see are marked 'T D525' and 'T B595', are these the same as 2SD525 and 2SB595?
Does anyone have a suitable protection circuit i could incorporate?
Thanks for the help
Colin
Jim,the power transistors i see are marked 'T D525' and 'T B595', are these the same as 2SD525 and 2SB595?
Does anyone have a suitable protection circuit i could incorporate?
Thanks for the help
Colin
No... before the transformer. It's wired in series with the amplifier mains supply to limit current. The filament resistance is low enough to allow the amp to run (with no load... speakers etc) so you can test it.
Havent done any tests as im awaiting my multimeter back from repairs, which incidentally was jumping up and down from the voltage figure being tested and wouldnt settle on a value, any ideas why?
Yes please, quote some tests, ive gotten some useful information from various sites but any information is good. Do you possibly have a schematic of a protection circuit i could incorporate? Thank you
Yes please, quote some tests, ive gotten some useful information from various sites but any information is good. Do you possibly have a schematic of a protection circuit i could incorporate? Thank you
no, it is for thermal compensation of the bias current. Open causes outputs to fail.Theres two of these screwed to the heat sinc(thermistor?), is this some kind of overload protection?
Would have to see the meter in action to know what was going on... and unfortunately you can't do much without it.
As for testing... well you need first of all to rig the bulb up before any powered tests.
You need to measure the output transistors on low ohms or "diode test" to see if any show as obviously short circuit. Your reading from the case of the transistor to the emitter as a first step. The emitter of each goes to those 0.47 ohms resistors. Make sure the amp is off before doing any testing on ohms or diode check ranges on your meter.
That's your first step... see if any are short circuit.
If they are then check also those 0.47 ohm resistors to maje sure they haven't gone open circuit or high in value due to overload. Also you can check the driver transistors for any obvious shorts or suspicious readings.
I think this amp will "run" OK with the output devices totally removed from the amplifier. By that I mean it will enable you to power it up (No load attached) and check some key voltages to make sure all is well. When you do this the drivers take over as "outputs" via the 330 ohm resistors... it sounds an obscure way of testing, but it works. Keep an eye out for anything getting hot... like the two 330 ohms.
So powering up with the outputs removed (you only need remove the faulty channel ones if the other channel is OK) should give zero volts on the "output" line which is the junction of the 0.47 ohm emmiter resistors.
That's one of the most important readings... and if that is OK then check the voltage across resistors rm69 and rm71 (and the corresponding other channel of course if that's the one that is faulty). I would expect around 1 volt or less. Altering the bias pot should swing this voltage higher or lower. If all that is OK turn the bias down to minimum... which is minimum volt drop across the 330 ohm resistors and then refit new power transistors.
That should be enough to be going on with...
As for testing... well you need first of all to rig the bulb up before any powered tests.
You need to measure the output transistors on low ohms or "diode test" to see if any show as obviously short circuit. Your reading from the case of the transistor to the emitter as a first step. The emitter of each goes to those 0.47 ohms resistors. Make sure the amp is off before doing any testing on ohms or diode check ranges on your meter.
That's your first step... see if any are short circuit.
If they are then check also those 0.47 ohm resistors to maje sure they haven't gone open circuit or high in value due to overload. Also you can check the driver transistors for any obvious shorts or suspicious readings.
I think this amp will "run" OK with the output devices totally removed from the amplifier. By that I mean it will enable you to power it up (No load attached) and check some key voltages to make sure all is well. When you do this the drivers take over as "outputs" via the 330 ohm resistors... it sounds an obscure way of testing, but it works. Keep an eye out for anything getting hot... like the two 330 ohms.
So powering up with the outputs removed (you only need remove the faulty channel ones if the other channel is OK) should give zero volts on the "output" line which is the junction of the 0.47 ohm emmiter resistors.
That's one of the most important readings... and if that is OK then check the voltage across resistors rm69 and rm71 (and the corresponding other channel of course if that's the one that is faulty). I would expect around 1 volt or less. Altering the bias pot should swing this voltage higher or lower. If all that is OK turn the bias down to minimum... which is minimum volt drop across the 330 ohm resistors and then refit new power transistors.
That should be enough to be going on with...
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Adding a protection circuit... tbh I wouldn't.
Having said that it is possible, but is it worth it ? The amp should be fine when fixed.
Protection comes in different forms. There is DC offset protection which would disconnect the speakers via a relay if any fault developed causing a DC offset. An effective circuit isn't particularly simple... not one that actually works properly that is.
There is overcurrent protection... which could be added by sensing the volt drop across the 0.47 ohm emitter resistors with an NPN/PNP pair of transistors... again it would be messy to implement unless you added a little PCB etc.
So I would just keep it all original I think
Having said that it is possible, but is it worth it ? The amp should be fine when fixed.
Protection comes in different forms. There is DC offset protection which would disconnect the speakers via a relay if any fault developed causing a DC offset. An effective circuit isn't particularly simple... not one that actually works properly that is.
There is overcurrent protection... which could be added by sensing the volt drop across the 0.47 ohm emitter resistors with an NPN/PNP pair of transistors... again it would be messy to implement unless you added a little PCB etc.
So I would just keep it all original I think
Mooly thanks for the specific advice, should be able to test all important values. One question though, the 'output' transistors are the power transistors on the heat sync right? Which are the driver transistors though? And is it necessary to set the bias pot to minimum if replacing the output transistor on that channel?
Thanks again
Thanks again
Output are the ones on the big heatsink... the drivers are the ones that feed directly into these (Qm15 and Qm17)
It's good practice to start with the bias on minimum and then set the value correctly when it's repaired.
A fault like this is most likely one or more outputs short circuit. Quite likely that may damage the drivers. The 0.47 ohms may be damaged by all this.
It's good practice to start with the bias on minimum and then set the value correctly when it's repaired.
A fault like this is most likely one or more outputs short circuit. Quite likely that may damage the drivers. The 0.47 ohms may be damaged by all this.
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