Hi,
Been lurking this forum for many years, its a great resource. This is my first post though. I was hoping you could help me figure out what I am missing trying to fix my Yamaha RX-V3800:
I picked the amp up for a song from a film studio, who were selling off about 10 of them at once (I would have bought more if I had the room to store them). The problem listed was that the amp was not powering on – I was hoping that a power supply capacitor was blown, which is a very common problem for these RX series Yamaha amps. I’ve got another one that had exactly this problem.
Unfortunately, this is not the case, instead the amp powers on, and then shuts off after three seconds. I managed to get my hands on the service manual, which tell me that this is the protection circuitry cutting in, shutting off the amp.
Fortunately you can override the protection circuity by pressing a series of keys on the front panel together:
This brings up a diagnostic test, and also allows you to power on the amp (protection circuitry disabled obviously). The fault displayed is : Diagnostic DC PRT : 057. Consulting the service manual tells me that this indicates a DC voltage across the output.....
Bingo! 39V across the Right Rear Surround channel. All the other channels seem fine and have around 10mv of DC offset. My first thought was blown output transistors slamming the output into one of the power rails. I spent the next few hours pulling the amp apart to get to the main amp board (of course you have to pull literally every other board out to get to it):
Here my problem; all the output transistors seem fine. No short circuits, and responded appropriately in diode test mode. I even found the schematics for the channels and started checking all the ICs in the circuit:
All the ICs Ive ticked above are good; I was going to start testing all the passives, and even start desoldering and testing out of circuit, but thought I would check here first, in case I am missing something obvious.
The other thing that is causing confusion is that i checked the primary supply rails, and they are 60v +- rails, not 39V. My understanding is that a shorted output transistor would leave a 60v DC offset on the speaker terminals, not 39V.
So my question is, what am I missing? Is there something obvious that would put a 2/3 of the rail voltage on the speaker outputs, without damaging the output transistors (as far as I can tell)?
Thanks for any suggestions!!!
Been lurking this forum for many years, its a great resource. This is my first post though. I was hoping you could help me figure out what I am missing trying to fix my Yamaha RX-V3800:
I picked the amp up for a song from a film studio, who were selling off about 10 of them at once (I would have bought more if I had the room to store them). The problem listed was that the amp was not powering on – I was hoping that a power supply capacitor was blown, which is a very common problem for these RX series Yamaha amps. I’ve got another one that had exactly this problem.
Unfortunately, this is not the case, instead the amp powers on, and then shuts off after three seconds. I managed to get my hands on the service manual, which tell me that this is the protection circuitry cutting in, shutting off the amp.
Fortunately you can override the protection circuity by pressing a series of keys on the front panel together:
This brings up a diagnostic test, and also allows you to power on the amp (protection circuitry disabled obviously). The fault displayed is : Diagnostic DC PRT : 057. Consulting the service manual tells me that this indicates a DC voltage across the output.....
Bingo! 39V across the Right Rear Surround channel. All the other channels seem fine and have around 10mv of DC offset. My first thought was blown output transistors slamming the output into one of the power rails. I spent the next few hours pulling the amp apart to get to the main amp board (of course you have to pull literally every other board out to get to it):
Here my problem; all the output transistors seem fine. No short circuits, and responded appropriately in diode test mode. I even found the schematics for the channels and started checking all the ICs in the circuit:
All the ICs Ive ticked above are good; I was going to start testing all the passives, and even start desoldering and testing out of circuit, but thought I would check here first, in case I am missing something obvious.
The other thing that is causing confusion is that i checked the primary supply rails, and they are 60v +- rails, not 39V. My understanding is that a shorted output transistor would leave a 60v DC offset on the speaker terminals, not 39V.
So my question is, what am I missing? Is there something obvious that would put a 2/3 of the rail voltage on the speaker outputs, without damaging the output transistors (as far as I can tell)?
Thanks for any suggestions!!!
