Harman Kardon AVR20ii

[carlosraj]

Hi

This amp was on standby and not powering up. Upon checking Q201(relay drive) was open. After replacing the transistor, the amp turned on as normal. Some of the input source (tuner,tape1) were not functioning. Upon inspecting the front board, I realise that the solder joints were bad, so I took out the front board and resoldered the whole board( I have resoldered the other boards earlier and the amp was working fine). After reassembling, the amp does not turn on. It is on standby again. There is 5V present on pin34(Vdd) of the cmos microproccesor, but no voltage on pin80(standy relay). No voltage on pin80 means no base voltage for Q201, the relay drive. I am not familiar with CPUs, and wonder what could have gone wrong. I have checked Q201 and all other components on the power supply board, they are OK. Could the CPU itself be the problem. I would like to know these 4 things (1) Once the CPU is receiving its Vdd(5v), then the CPU will output 5V to the standy relay thru pin 80. Is this correct. (2) Could the resoldering or touching the CPU pins damage the CPU(CMOS). (3) Pin 32 is the reset pin, but it is not connected to anything on the board, so how do I reset the CPU. (4) Apart from the Vdd, is there any other voltages or signals the CPU must receive to do a start-up. I am unable to attach the schematic as it is too large, but can be download from the web. Please assist.

Thanks

 

[Eppie]

This might be better asked in an electronics repair forum but I downloaded the schematic from elektrotanya and had a quick look. Review page 13 on the protection circuits and see if Q221 is turned on which pulls the CPU "protection port" low and turns off power. There may be other things besides Vdd on the CPU which causes it to put 4V to Q201. Hard to follow the schematic when it is spread over so many pages.

Could resoldering damage the CPU? Perhaps with high temperatures. Did you apply flux and use fresh solder which has a lower melting temperature than factory solder?

You could look up the data sheet on the CPU to see if it describes how to reset the CPU. If pin 32 is high, then forcing it low may reset the CPU but do not ground it directly, use a resistor. This would be a last resort attempt as you risk damaging a good CPU.

A computer controlled AVR is a difficult repair. If the power supply and amplifier stages are fine then you may have controller issues which require a logic probe and oscilloscope to trace, and even then it is a challenge to figure out what the proper sequence of events needs to be to trigger the power relay.

 

[carlosraj]

Hi

After a few days , today, I tried the amp again. Surprisingly it turned on, stayed on for about 5 secs and then turned off. Now everytime I press the standby switch, it turns on for approx 5secs and turns off. Does this mean that the CPU is OK, not shorted. Something else is causing it to shut down. Any suggestions.

Thanks

 

[Eppie]

Hi

After a few days , today, I tried the amp again. Surprisingly it turned on, stayed on for about 5 secs and then turned off. Now everytime I press the standby switch, it turns on for approx 5secs and turns off. Does this mean that the CPU is OK, not shorted. Something else is causing it to shut down. Any suggestions.

Thanks

If the front display turns on, then the CPU is likely ok. As I mentioned above, check page 13 of the service manual which describes the speaker protection and thermal protection circuits. A faulty thermal protection circuit is possible but less likely. I would look more closely at the speaker protection circuitry. I would probably unsolder the output transistors as a precaution as they tend to be the most likely transistors to fail.

 

[carlosraj]

Yes, not only does the display turns on, the selector switchs work, the manual tuning works as well I will try with the 2 protection transistors removed.

Thanks

 

[TLS Guy]

Yes, not only does the display turns on, the selector switchs work, the manual tuning works as well I will try with the 2 protection transistors removed.

Thanks

That is a very dangerous idea. The protection is likely doing what it should. The main reason for protection circuits is to prevent DC offset. There is every prospect that the protection is doing just that. So, of you remove protection there is a high probability, in fact darn near a certainty, that the DC rail voltage will be sent to your speakers and there will be flash and fire. Just don't.

 

[Eppie]

I would hope that the op is not testing the unit with speakers attached, but yeah, disabling the protection circuitry is a bad idea. Check the amplifier stages first for any shorted components and work backwards. If the AVR stays on long enough to use the selector switch then it should be possible to check for DC offset at the outputs during the few seconds that the AVR is running.

When the unit shuts off, does it switch off the standby relay? Looks like the 49th page of the service manual shows the standby relay (part of page 35 Main PCB1). There should be 4V feeding R209 3.3k which connects to the base of Q201 which controls the relay. IC201 looks to be a 6V voltage regulator. You could follow the 4V line from R209 to see what controls the relay.

 

[carlosraj]

The problem is there is -50V DC present at the R channel speaker output. I totally isolated the R channel, then the amp works without shutting down, but the left channel sound distorted. I cannot trace where the -50V is coming from, all transistors,diodes ,caps and resistors are OK on the right channel. What could the problem be.

Thanks

 

[Eppie]

You need to look at the schematic for Main(PCB1). There is a +45V and -45V rail feeding the collectors of the output transistors. There are only so many places where that voltage can pass through to the speaker terminals. The schematic also shows the signal path in a thick black line with arrows for one of the channels. The signal is on the emitter of the output transistors (Q216 & Q217) and then passes through a coil L201 and goes to the speaker A/B selector switch. If you remove the output transistors there should be no voltage on the speaker terminals unless current is leaking through Q218.

You can see the thermal control circuit to the right of the amplifier circuit on the schematic, but that gets fed only 15V, so that is likely not the issue. The majority of times if there is 45V at the speakers then the output transistors or bias transistors are shorted. Q218 and Q219 need to be checked too as they tie into the output stage. The schematic shows voltages at different points in the amplifier circuit. This repair will take careful examination of the schematic and an understanding of how the amplifier circuit operates.

 

[carlosraj]

When I removed the transistors of both the L and R channels to check them out of circuit, the circuit trace peeled off and broke at a few places. I used wire to reconnect them back. I do not know, maybe certain reconnection were not correct. I have to closely trace back the connections following the schematic.

Thanks