Hello guys, recently i got this amp from a firend. The amp is working(sort of), the problem is i can only turn it on if I apply power to B+ and Remote at the same time. This way it works ok, drawing around 1.3-1.1A in idle, and producing music. But if i disconnect remote and now reconnect, the amp will draw 35mA and the powersupply wont turn on although the amps powerLED comes on. On further investigation i realised somebody has changed q802 which is a biased transistor in the B+ Switch circuit to some sort of random SMD transistor, without chechking the voltage divider before it..(the voltage divider had one open resistor the 3.6k one, which i changed now)
Could you guys give me some directions to start from? Im testing off a lab supply, and iv got pretty professional gear(student at an electrotechnics university). I actually set the power rails as described in the manual, but i could not set the idle since it says i should read 3+-1mV but i read 90V between the test points given so i dont really understand this but hey.
Here is the ful lservice manual: https://we.tl/LdGveqXcVo
Thanks for your time and help!
Could you guys give me some directions to start from? Im testing off a lab supply, and iv got pretty professional gear(student at an electrotechnics university). I actually set the power rails as described in the manual, but i could not set the idle since it says i should read 3+-1mV but i read 90V between the test points given so i dont really understand this but hey.
Here is the ful lservice manual: https://we.tl/LdGveqXcVo
Thanks for your time and help!
okay so i have realised that no matter what is up with q802 if the green LED comes on its meaning that the B+circuit is functioning correctly. So i measured some more and iv got to IC602 which had verry strange voltages. So i started chechking and i got to pin4 which had 5 volts to it(this was extremely high since it should be something like 330mV) so i checked the datasheet for that IC ( PC494C pdf, PC494C description, PC494C datasheets, PC494C view ::: ALLDATASHEET ::: ) and i saw that the number 4 leg is dead time control, which should be max 3.3 where the duty cycle is 0) So i started searching where does that 5V come from, i checked every diode and resistor where it could possibly come from and they were all fine so the last thing was a transisto(q706, overload spkr etc protection circuit) which had 5v on its collector instead of -525mV. So i desoldered that transistor, at that point i could not think about anything else but that transistor being bad. So tranny out and the amp is now working suddenly, by its remote totally fine. I just have to get a replacement for that KTA1266 and an explanation exactly what is its role. Thanks Agai nfor your response!
Virtually all protection circuits pass through that transistor. What happens if you replace it and remove Q705?
iv just checked q705 and q705 does not have its base voltage. the emitter and collector voltages are looking fine but if im correct there is a voltage divider which grounds through q705, and if q 705 does not have its base voltage which should be like 0.5v and its like 0.02.. Ill try that, the problem is i dont really have parts on me right now :S
okay so i have replaced q706 and removed q705. Now the amplifier starts up just as normal, but still has 1.5V instead of -0.5V on the collector. Also its getting 5.05V instead of 4.86 on the base. The emitter is getting its 5V ok. As far as the q705 goes Emitter is ok collector has its 5v and the base voltage is missing.
okay so i just realised that if i power it on with the b+ and remote at the same time il lget the -0.5V but when i remove the remote and restart the amp, ill get around 2 volts decaying to 1.5V. Then i started lowering the voltage on the supply, to around 13V, the 1.5V started decaying to around 1.3V where it fell to like -5V and it went back to -0.5V
The 1.5v may be from the PWM rail regulator. If so, that voltage will vary greatly.
Measuring the rail voltage, what is the range of input from the 12v supply where the rail voltage doesn't vary?
Measuring the rail voltage, what is the range of input from the 12v supply where the rail voltage doesn't vary?
sorry, but its not too clear what you meant by rail voltage and range of input if you could be more specific id appreciate it(sadly english is not my native language) I will try to figure it out tho
If i understood corretly you are asking what is the range of voltage(varied by my supply) where the rail voltage wont change? By rail voltage im not exactly what do you mean.. but i guess the supply voltage of the amp(+-45V)? If so its about 13.5V where the power rail starts dropping quickly
Okay so ihave been able to narrow the things down. Q701 is triggering the protection circuit. I have followed it back and it looks like q701 is being driven by q523/q525 and q524/q526 but i cant seem to figure what is driving them. Also to note r557 and r567 have been changed by someone to 620ohm resistor. can you guys help me i nthis?
It looks like.. I am not a professional tho, just a student... So im not 100% sure but if i understood the scematics correctly then yes it is driven by them please correct me if im wrong!
If that's right, reinstall Q701 and remove the transistors driving Q701, one at a time to see which one is triggering the protection circuit.
Those transistors are driven by the voltage drop across the source resistors connected to the output transistors.
Those transistors are driven by the voltage drop across the source resistors connected to the output transistors.
I did not do that but i removed the 10kohm resistors that were the connection between these transistors and q701... Yeah guess what, q701 still triggered... I might have missunderstood the concept of the low impedance switch(s701-1)...
q701 is fine because i removed the 1k resistor between its base and the line marked 15(which is controlled by the low impedance swithc if im right, and those transistors i mentioned earlier) And now the thing works again... its getting about 6 volts to its base(q701)...
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