Yamaha RX-V375 AV Receiver with a No Power Problem

[Johnsr699]

I have a Yamaha RX-V375 AV Receive that does not power on. I used the tone control/info/pwr process to get the display to come on and it gave me a DC PRT 255 H error before the protection circuit powered it off in 5 seconds. This is an indication to DC output of the power amplifier is abnormal. I do not have a schematic or a service manual. Any suggestions how I can get it to power up and where the best place to check voltages on each output transistor?

 

[shrtrnd]

Did you Google your make and model number and see if you can find a schematic on it?

 

[Johnsr699]

I have found the service manual and downloaded it. There is supposed to be a mode to bypass the protection circuit so you can briefly take voltage measurements, however it doesn't seem to work for me. I am going to try some other modes to see if I can get the unit to stay on long enough to test the output transistors.

 

[shrtrnd]

The REASON your unit is shutting-down is that it has internal protection circuitry and/or devices that are intended
to protect the sensitive circuitry within.
The more often you try to bypass this protective circuitry to do troubleshooting, the greater the chance that you do more
damage to the unit than is what is actually there already. (ie: don't keep trying to cycle power just to see what happens. Try to determine the existing failure first).
The fact that the unit does not power-on, indicates an 'open' circuit within the unit. Something burned open-circuit.
Most receivers have speaker output fuses to protect speakers, but you said your unit will not power-on, so that's
not this particular problem.
You have the service manual. It should have a troubleshooting guide that you should follow first.
You said the fault (what the unit is programmed to display anyway), is 'DC output of the power amp abnormal'.
I'd sure try to find causes for that in your troubleshooting section.
My initial GUESS would be that you're already on the right track, assuming that one or more power output transistors has failed. Your service manual should list the expected voltages at those transistor outputs, helping you to isolate
the problem.

 

[Johnsr699]

Thank You for your reply. I did find Q56 2STC4468 shorted out. This is one of the Power Amplifiers at the output of Surround Sound Right. Once I take the unit apart and remove the transistor I will check other components in the circu8t

 

[shrtrnd]

Good work so far, good luck with the repair.
Because you have an output transistor failure, look for those (internal) speaker output fuses while you're inside your
unit to make sure they didn't also take a hit.

 

[Johnsr699]

This unit has relays on outputs that shut off like a fuse but I will check other components. Does this site have a way to upload Service manuals? If so I could upload the service manual on this receiver for others to use.