| Electronics > Repair |
| Marantz SR5200 amp damaged center channel + DAC issues |
| (1/8) > >> |
| poot36:
I got the amp from a friend and they reported that the digital inputs as well as any analog input that was not set to sound direct would not produce any sound out of any of the speakers. When I was trouble shooting this I found that the surround sound DAC was getting very warm and was not outputting any sound out but there was signal into it. During this process I had no speakers connected to the amp and was measuring the output of the transformer when I had a fire ball and smoke come out of the amp for no reason. I do not believe there was any audio source but I could have been on the internal tuner. I traced this down to the SAP16NY Darlington transistor on the center channel going short between source and collector. I disconnected the emitter source and collector of both the SAP16NY and SAP16PY Darlington transistors and started checking the driver circuitry and found that I was missing the negative section of the sine wave at the base of both Darlington transistors all together. I have tested all of the resistors and they all check out as well as all the pre-driver and driver transistors. The caps are not shorted and the few I was able to test in circuit passed. Is the reason that I am getting half of the sine wave because there is no feedback from the collector pins of the Darlington pairs? I don't want to buy parts and have them blow as well due to another fault in the circuit. I have attached a screen shot of the relevant section of the schematic to this post. |
| coromonadalix:
I think you have to remove q 205, q206, q207, q 208 q 209 check them all, check all your supply rails Q204 Q203 act as an current regulator for the differential input transistors, they may be damaged ??? This amp is direct coupled, if something at the output transistors goes bad, they can damage almost entirely the amp section up to the input. Check all capacitors, all the low values ohms resistors (all of them loll) , and your r212 resistor in the squared markings. And finally your dc output protection circuit q 215. |
| poot36:
I have checked all the transistors, the only one that was shorted was the SAP16NY and that has been removed from circuit except for its base and drain connections. All of the resistors check out. I will test all of the capacitors that I cannot test in circuit. I used one of the cheap component testers from ebay to test the capacitors and transistors. All the supply voltages are ok and the rest of the output channels work with no issues. |
| Zucca:
No scope, uh? |
| poot36:
I have a scope, that is how I figured out that I was missing half of the sine wave. I have removed the amp section and have hooked it up to a +- 30V supply (it is normally run around +- 40V in the amp) and with no signal attached to it Q205 and Q209 get warm and on both of the bases of the output transistors I have around -19V. On the good channels I get +- 1V on the bases of the output transistors. Is it because there is now no bias/feedback loop from the emitters of the output transistors? Since only one of the outputs transistors failed can I just replace the one or will I have to replace both of them? I know I cant get the SAP16 transistors anymore and will have to replace them with STD03 along with a emitter resistor. |
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