Yamaha RX1130 R&L input signals mix in the output section

[rfitts46]

Also measured resistance between rails and with the meter on the 200ohm scale it wont measure but in the audible continuity setting it rings.
Hard short on the rails? Just surprised because this amp will power up?

 

[anatech]

Hi Bob,

removed muting transistors Q105 and Q106. Powered on and exhibiting same symptom. When balance knob is turned right or left both channels on.

Measure the signal common (ground) to the balance control please.

the US models were supposed to have a 10/25.

Go ahead and put the 10/25 capacitor in. Canadians like low bass -
Really, Yamaha went through a period where they lost a lot of power transformers in Canada. The CSA people (no comment) decreed that the thermal fuse in transformer primaries shall be 10 °C lower than the US one. It seems that the transformer size was marginal, because transformers then dropped like flies! Open primaries. Cost another fortune (they had to replace the transformers with the wrong fuses earlier). Excellent demonstration as to why accountants should not make engineering decisions (or run companies).

Also removed and replaced the 0.22ohm emitter resistor. If only this could have been seen from the top side?

We got used to this. The burnout occurs most often between he PCB and lower surface of the resistor. <------- Clue! Can you tell me why this is?

If you lose continuity, no current flows. However, the large filter capacitors will store a charge for a long time. Does your meter have a fuse for the resistance function? Will it properly measure a resistor out of your parts box? The other possibility is that the caps are big, taking a lot of time to charge so your meter reads either 0 (zero) or something silly.

-Chris

 

[rfitts46]

Hi Bob,

Measure the signal common (ground) to the balance control please.

Chris why are we going upstream? May have to eventually. Remember after removing jumpers between amp and preamp the amp section was tested by inputting right and left signal. If one signal was removed still had sound from both channels.

We got used to this. The burnout occurs most often between he PCB and lower surface of the resistor. <------- Clue! Can you tell me why this is?

Yes I can see why. In this situation these resistors are up off the board, a good 3/8" or so.

If you lose continuity, no current flows. However, the large filter capacitors will store a charge for a long time. Does your meter have a fuse for the resistance function? Will it properly measure a resistor out of your parts box? The other possibility is that the caps are big, taking a lot of time to charge so your meter reads either 0 (zero) or something silly.

Chris did check a 15 ohm resistor with the meter and it measured correctly. Reading in the manual the audible continuity test is combined as a diode test. Something in the manual that it is 50 ohm or less?

Bob

 

[rfitts46]

Also measured resistance between rails and with the meter on the 200ohm scale it wont measure but in the audible continuity setting it rings.
Hard short on the rails? Just surprised because this amp will power up?

Chris you were right in the diode/audible tone resistance check the meter is trying to charge the caps. Learned not to use this function for incircuit resistance checks.

R200 was replaced and the amp was powered up. Everything okay except for the symptom we are trying to resolve.

 

[rfitts46]

QUOTE=anatech;4267177]Hi Bob,

Measure the signal common (ground) to the balance control please.


Chris, measure the signal common (ground) to the balance control. It looks like E (signal common) runs through a line amp immediately upstream from the power amp.

Upstream from the line amp looks like a digital tone board (this has the analog switch for CD direct and other functions). There is no analog control for audio muting this is done using the remote. There is no analog control of mono/stereo this might be a selection using the remote. The digital control board allows these controls while the analog tone control board has the old school knobs on it.

Upstream from this board is the board that has the analog balance adjustment knob.

When you say measure are you looking for me to confirm connection or measure resistance?
I would like to disconnect the jumpers and hook the function generator up to one channel and use scope to probe the power amp. Probe the channel that the signal should not be showing up on. Maybe it can be determined it the signal is mixing upstream from the predrivers or downstream from the outputs?


regards Bob

 

[rfitts46]

I am aware that the common scope probe is earth ground so where should I make the connection of the common scope probe to the Yamaha?

 

[rfitts46]

Reviewing the schematic it shows the center tap of the transformer secondary going to ground as well to the speaker negative outputs. I believe that would be a safe bet?

 

[anatech]

Hi Bob,

Remember after removing jumpers between amp and preamp the amp section was tested by inputting right and left signal. If one signal was removed still had sound from both channels.

Sorry Bob, I forgot about that. Your fault appears to be with the power amp only. Very, very weird! I can't think of any problem that would make the signal common to both amps without blowing them.

Learned not to use this function for incircuit resistance checks.

In circuit resistance checks are suspect at the best of times. You could get away with this at times with tube circuits, but not with solid state.

When you say measure are you looking for me to confirm connection or measure resistance?

Yes. If you lose ground, or the center tap on some balance controls you will have L to R blending no matter where the knob is. That sounded exactly like your problem, and I have seen it before. Not often, but enough to remember the problem.

When you say measure are you looking for me to confirm connection

Confirm connection.

I would like to disconnect the jumpers and hook the function generator up to one channel and use scope to probe the power amp.

Yes, please do. This is a test I would do earlier on. Everyone has their own way of working on things that's fine.

Reviewing the schematic it shows the center tap of the transformer secondary going to ground as well to the speaker negative outputs. I believe that would be a safe bet?

Yes, and now that you have mentioned this, measure between the speaker ground and the power supply common ground. I would use the common point between the filter capacitors for that. Make sure the unit is off before making your measurement. An open speaker common would also cause this exact thing. Maybe you just found your problem. If the ground is open, that's it!

-Chris

 

[rfitts46]

Hi Bob,

Sorry Bob, I forgot about that. Your fault appears to be with the power amp only. Very, very weird! I can't think of any problem that would make the signal common to both amps without blowing them.


Yes. If you lose ground, or the center tap on some balance controls you will have L to R blending no matter where the knob is. That sounded exactly like your problem, and I have seen it before. Not often, but enough to remember the problem.

Confirm connection.

Yes, please do. This is a test I would do earlier on. Everyone has their own way of working on things that's fine.

Chris, I set the signal generator and scope up on it and will start probing the predriver and output section to see if that tells me anything. Any suggestions on how to approach this? I assume just follow the signal through checking each transistor base and emitter for signal?

I will bring an amp done on the bench and connect the Yamaha preouts into it to see what balance controls are being offered upstream from the power amp.

Yes, and now that you have mentioned this, measure between the speaker ground and the power supply common ground. I would use the common point between the filter capacitors for that. Make sure the unit is off before making your measurement. An open speaker common would also cause this exact thing. Maybe you just found your problem. If the ground is open, that's it!

There is continuity between secondary center tap and speaker negative (E).
Soldered the muting transistors back on to the board.
Currently my bias current is low 1mV and 3mV s/b 6 to 8mV.
Still have 50-60mV DC Offset on repaired channel and 90mV on the other channel. There is no DC offset adjustment on this amp?


-Chris

 

[anatech]

Hi Bob,

There is continuity between secondary center tap and speaker negative (E).

Fantastic, it's not that then. This would have been easy to fix though.

Soldered the muting transistors back on to the board.
Currently my bias current is low 1mV and 3mV s/b 6 to 8mV.

Don't worry about bias yet. Low bias will at most give you higher distortion, but will not affect your troubleshooting at all. You can fix that later.

Still have 50-60mV DC Offset on repaired channel and 90mV on the other channel. There is no DC offset adjustment on this amp?

No, DC gain = 1 as long as C193/194 (220uF 35V) are okay. If these capacitors are okay, your DC offset is as a result of an unmatched differential pair. You need to exactly match Q107 to Q109 and Q108 to Q108, then thermally couple them. If you do this properly, two things will happen. You will have very good DC offset values ( < 10 mV), and lower distortion. The proper way to match these transistors is to create a diff pair on an experimental "proto board" (you can plug parts in and there are contacts inside). If you can balance the transistors ... The problem with measuring transistors is that they are highly sensitive to temperature. Do not expect to measure them and choose units with the same Beta. That is only good for pre-selection because some matches will not be even close to what you need.

Chris, I set the signal generator and scope up on it and will start probing the predriver and output section to see if that tells me anything. Any suggestions on how to approach this?

Yes. Send the signal in, do not probe anything. Just use your speakers to see where you get sound and list the results. Just do that for now, we can tackle one thing at a time.

-Chris

 

[rfitts46]

Hi Bob,

Fantastic, it's not that then. This would have been easy to fix though.

Don't worry about bias yet. Low bias will at most give you higher distortion, but will not affect your troubleshooting at all. You can fix that later.

No, DC gain = 1 as long as C193/194 (220uF 35V) are okay. If these capacitors are okay, your DC offset is as a result of an unmatched differential pair. You need to exactly match Q107 to Q109 and Q108 to Q108, then thermally couple them. If you do this properly, two things will happen. You will have very good DC offset values ( < 10 mV), and lower distortion. The proper way to match these transistors is to create a diff pair on an experimental "proto board" (you can plug parts in and there are contacts inside). If you can balance the transistors ... The problem with measuring transistors is that they are highly sensitive to temperature. Do not expect to measure them and choose units with the same Beta. That is only good for pre-selection because some matches will not be even close to what you need.

Yes. Send the signal in, do not probe anything. Just use your speakers to see where you get sound and list the results. Just do that for now, we can tackle one thing at a time.

Chris do not use scope to evaluate. Connect speaker connection wires to some probes and probe to determine where the signals are ? Use ears not eyes for this.

-Chris

 

[anatech]

Hi Bob,
NO!
Connect the speakers as they normally would be. Just listen while trying the controls with an input signal. A sine wave (audio generator) is the easiest, but any signal source will do. Vary things like tone controls, balance, loudness button and tape loops. Just change each thing to see if there is any difference. List what changes you hear (apart from the obvious like more bass when you increase the bass control).

Probing with a low impedance thing like a speaker lead can cause a lot of damage. About the only thing you should probe with is an oscilloscope lead (insulated except for the very point tip.).

Happy hunting!!