I have checked the power amplifier quite extensevly and it started with not going from "wait" to "on". I discovered some 5 volt of DC on the output. After measuring the fused resistor and driver transistors (output ok), I found out that Q435 (2SC2238) was not ok, so I swapped it. Now I have no DC on the output, but the "on" is still not lighting up. Anyone that had problems with the protection circuit when repairing a HK870?
The circuit for power amplifier and protection circuit is attached . For a clearer picture and smaller file I chose the right channel. This makes no difference apart from identifying corresponding parts in the left channel since either of the two is able to activate the protection circuit.
If there is enough dc at TP1,2,5, and 6 in one channel and TP8,4,7, and 3 the protection circuit will be activated.
For the left channel where Q5 lives you can measure between TP5 and TP6 and deduce the current flow between Q437 and Q441 by dividing the voltage drop by 0.44
Similarly you can repeat this process for Q439 and Q443 by measuring the drop between TP1 and TP2.
Since the standing current when correctly set should be within 5 mA of 75mA the voltage measurements between test points should be within a couple of mV of 33 mV in both the above instances.
It is a bit odd that Q435 should fail alone however the schematic has symbols of a triangle with an exclamation mark within. In the parts list these are designated as fusible so it will be worth check that those in output stage are intact - R305, R491, and R493 connecting to Q435 might have gone open circuit without showing any visible signs.
Attachments
Thanks for your comments. I will check the suggested TPs. I believe that there is a timer function of some second to stabilize the amplifier before delivering sound and light up "on". what components are involved in this?
I suggest you look at Q4 and C16 in the power supply section feeding B1. This combination is feed a rectified wave form which brings Q4 to the threshold of conduction and the output is smoothed by C16 which could be down in capacitance.
B2 is a separate rectified feed to the protection circuit where you have C21 and C22. You should look at all of these parts - I suspect the immediate problem is most likely one of these capacitors. Working backwards first check the voltage on C22 and if that is OK trace the route back through B2 to the diode bridge in the power supply section.
You can download the service manual for this from Hi-Fi Engine you will need to create an account but the service is free.
B2 is a separate rectified feed to the protection circuit where you have C21 and C22. You should look at all of these parts - I suspect the immediate problem is most likely one of these capacitors. Working backwards first check the voltage on C22 and if that is OK trace the route back through B2 to the diode bridge in the power supply section.
You can download the service manual for this from Hi-Fi Engine you will need to create an account but the service is free.
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