Yamaha RX-V577 - DC: PRT 000L

[stinkincaps]

Manual says DC output of power amplifier is abnormal.

Power supply board looks to be ok, 5.5V is good.

Looks like I need to go deeper, where do I start?

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

Check for shorted output transistors

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Thanks for the tip. So I'm looking on the MAIN amp board, powered off, measuring impedance on all transistors. Will have to do some disassembly first to get to them all.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Ok, the big transistors on the heat sink all seem to be not-shorted. Any advice on next step?

Thanks in advance.

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

Check the voltage on all the output channels, this is usually at the common point of the ceramic emitter resistors of the output transistors or the output to the speakers BEFORE the output protection relay. This offset voltage needs to be close to 0 volts. Upload the service manual

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Service manual attached. What boards do I need to reconnect and power up to check these voltages?

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

Which boards do you have disconnected? I would think you need all of them connected. Check the voltages on (page 108) the center of R2249 for the FL channel. or any point on that line that you can access like SCN-SCN 5P and 4P
You need to check this voltage as soon as you turn on the amp. if you wait the protection circuit will cut in and you will get a false reading. You need to check this voltage on all channels.
You may need to cancel protection mode (page 28) to work on the amp.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

I took the digital board and Operation(4) board out to get to the output transistors. Once I put the boards back, getting to measure more than 2 of these resistors (C + FL) will be a bit tricky to impossible.

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

what about the position on page 21, that should allow access to the main board. Don't forget the ground jumper leads.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

what about the position on page 21, that should allow access to the main board. Don't forget the ground jumper leads.

That looks doable. Let me reassemble and pop the back off.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Which boards do you have disconnected? I would think you need all of them connected. Check the voltages on (page 108) the center of R2249 for the FL channel. or any point on that line that you can access like SCN-SCN 5P and 4P
You need to check this voltage as soon as you turn on the amp. if you wait the protection circuit will cut in and you will get a false reading. You need to check this voltage on all channels.
You may need to cancel protection mode (page 28) to work on the amp.

All the ceramic resistors are 0V, except R2249, showing -9.8VDC on the 3 legs.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Some voltage readings:

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

That will be the problem channel, You should be able to compare resistance etc. to a good channel to find the problem. It is only -10v so it might be hard to locate the cause of the offset.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Thanks R_J,

To confirm, I'll power down and check device resistance & compare to good channel? Looking at everything from resistors, diodes, transistors and caps?

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Found some components that don't meter like the others. Time to order some parts.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Also found C2165 to have lower resistance. This one is a mlyar cap, hard to find. Seems like only NTE has it, so I'll order everything from there, as best they have it.

[R_J]

Re: Yamaha RX-V577 - DC: PRT 000L

I doubt the cap C2165 has a problem, they are very reliable. there are many things that can throw off the readings, The meter uses a small voltage to check resistance so ANY stray voltage in the circuit will throw off the reading one way or the other. I would suspect either a resistor that change value (went high in value) or a leaky transistor, which might not show up in a resistance check.
Remember that the OFFSET voltage you measured is -9.8v, which indicates that Q2137 or Q2151 is very slightly biased on a bit more than Q2136 or Q2150. You might need to chack as farback in the circuit as Q2114, Q2121. There are dc voltages given on the schematic, did you check how close they were?

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

The parts arrived and sat for a few weeks. Finally pulled the unit off the shelf and started pulling parts. C2135 was dead for sure, no capacitance measured. Replaced. I also replaced Q2107 & Q2114.

The rest of the suspect parts I pulled from the board and measured out of circuit, all came in fine.

The unit now powers up fine. I'm getting a 39-40 mV reading on R2249 (all 3 legs are the same). Other resistors are 0.000V. It was -9.8V before, so much better.

Should I be worried about 40mV there? I guess I can start looking at circuit voltages and compare to the schematic.

[stinkincaps]

Re: Yamaha RX-V577 - DC: PRT 000L

Voltages measured in protect mode is showing 50mV. (prior measurement showing 40mV was not in protect mode).

I'm not smart enough with analog circuits to figure out where the issue lies.

[petehall347]

Re: Yamaha RX-V577 - DC: PRT 000L

q21174 q2107 need a matched pair .