Expert: Cleggsan - 5/9/2010

Question
Unit does not turn on. Startup power supply okay. Power switch okay. The signal to turn on the relay to apply 120 to the main power supply not received. Where can I get a detailed circuit diagram so I can traced the cause. Does this unit have a safety shut down circuit? Placed a load on the front speaker posts of the proper impedance as required by the owners manual.

Answer
Yes, it probably has a protection system that monitors current and when it is exceeded it turns the power off to the supply.  

Remove the speaker connections to see if that relieves the shut-off. If so the problem is likely a shorted speaker wire or voice coil.

Also, check the output transistors which are famous for breaking down placing a short load on the power supply.

You can purchase the service manual online but I could not find a "freeby" for you. If you know how to trace the circuit start with the relay wires and see if you can find out where the feed line is that operates the relay (turning it off).....

Yes, if you operate the amp with loads removed you should make sure all volume controls are down to zero and there is no input signal at any input.  Under this condition it should not harm the amp section to operate with speaker loads removed.  An alternate is to check the speaker lines and speaker voice coils. If you have an ohmmeter you should find the dc resistance of each speaker to be just a little less than the impedance rating of the speaker.  Hence, an 8 ohm speaker will have a dc resistance of around 6 ohms.  

Another alternative is to just lift the power rails to the output stages of the amplifiers (this may be easy or difficult depending on how the B+ and B- lines are configured).  If they are lifted and the power amp comes on properly it infers that one or more of the outputs has shorted down.  But, unless you know clearly what you are doing this could be a difficult test.

Another idea, also a little dangerous, is to manually or electrically close the power supply relay forcing the supply to fire up and then watch quickly for some indication of heating and if you have the voltmeter leads on the main dc voltage lines see if they come up to full voltage.

You can google for the service manual but they will cost $15 to $30.

Let me know if you need more.