Yamaha Stagepas 300 no left channel

[nosaj]

I have a yamaha stage pas 300 with no left channel. The output modules are interchangeable and I have verified both work in the right channel slot. So i ran through the Troubleshooting diagram. And all my voltages are low.
http://www.go-gddq.com/upload/2014-0...2721337905.pdf
I'm pretty sure for a voltage rectifier ex a 12vdc regulator will need a higher than 12v input.


Above image has my voltages. I'm thinking the switch transformers have a voltage issue??

Thanks,
nosaj

[nickb]

The voltages on the primary filter caps seem wrong. With 120V line voltage and the 120/240 strap JP801 fitted you should have 120 x 1.4 = 168V across each capacitor. Yet you see a very odd +121 and -121. It begs the question what are you using as the reference when you make that measurement?

I would use the -ve end of the lower cap (ref obscured by arrow). So you should see 168V and 336V. If you don't the relay contact RY801 may not be closing.

[nosaj]

The voltages on the primary filter caps seem wrong. With 120V line voltage and the 120/240 strap JP801 fitted you should have 120 x 1.4 = 168V across each capacitor. Yet you see a very odd +121 and -121. It begs the question what are you using as the reference when you make that measurement?

I would use the -ve end of the lower cap (ref obscured by arrow). So you should see 168V and 336V. If you don't the relay contact RY801 may not be closing.

Ground tab on PCB. -ve are you talking about c808 negative side?
included non reference screen shot

163vdc and 326vdc. I will retake the voltages on the voltage regulators using + rail of one of the fan connections(reasoning being the ground symbol is the same as the one on the Voltage regulator but pins are too close for I might slip.)
Thanks,
nosaj

[nickb]

Ground tab on PCB. -ve are you talking about c808 negative side?
included non reference screen shot
Thanks,
nosaj[ATTACH=CONFIG]44596[/ATTACH]

Yes, the lower end of C808. The GND tab is isolated from the primary by the transformers, of necessity. Take care - lots of volts and lots of energy.

[nosaj]

Yes, the lower end of C808. The GND tab is isolated from the primary by the transformers, of necessity. Take care - lots of volts and lots of energy.

163vdc and 326vdc. I will retake the voltages on the voltage regulators using + rail of one of the fan connections or d819(reasoning being the ground symbol is the same as the one on the Voltage regulator but pins are too close for I might slip.)
Thanks,
nosaj

[nosaj]

163vdc and 326vdc. I will retake the voltages on the voltage regulators using + rail of one of the fan connections or d819(reasoning being the ground symbol is the same as the one on the Voltage regulator but pins are too close for I might slip.)
Thanks,
nosaj

Ok the power rails are all spot on. Guess ground reference is super important. Now the issue is when a module is plugged into the left channel the machine turns on then off. Removing module from left slot allows machine to power up and run. Both modules have been tested and work in the right slot..

I can take voltages on the pins for that slot and compare them to the other working slot. And then power it off and check for continuity between any pins that does or doesn't exist on the working slot?

Thanks,
nosaj

[nickb]

The first thing I'd check is the output is not shorted. Unplug any speaker/dummy load. With power off and module unplugged check for an open between the OUT pin and GND on the left module connector. If that is OK, power up and check all the voltages on the module connector.

[nosaj]

The first thing I'd check is the output is not shorted. Unplug any speaker/dummy load. With power off and module unplugged check for an open between the OUT pin and GND on the left module connector. If that is OK, power up and check all the voltages on the module connector.

No shorts. .3 ohm on pins 1 and 3 Shared ground I believe. This was on Cn9 (4 pins) Now looking at schematic I should also do this on cn4 and cn5 between gnd and output. CN6 and cn7 showed same voltages as CN4 and cn5. So this leads me to belive the issue will be in cn5 somewhere.
Power up
CN4
Pin1 12.5
pin2 No Voltage
Pin3 -12.5
Pin4 No Voltage
Pin5 -38
Pin6 -38
Pin7 +38
Pin8 +38

CN5 No Voltages on Pins 1-9

Thanks,

nosaj

[nickb]

If the power rail rails are OK, then I'd next suspect excessive output voltage offset.

The detection circuit uses Q182/3 and shuts down the PSU via PC802 if the output voltage has an average level of about >3.6V or <-3.6V. Since the module is known to be OK check the output voltage and signal input voltages on CN5 pin 2 & 3 with the module in place and compare with the right channel. Don't connect a load. I'd expect to get a second or so before it shuts down. If it's too fast to make a measurement you'll have to defeat the shutdown by shorting out R877.

Or, if you want to try something quickly C137 and/or C142 might be leaky so pull one or both and see.

[nosaj]

If the power rail rails are OK, then I'd next suspect excessive output voltage offset.

The detection circuit uses Q182/3 and shuts down the PSU via PC802 if the output voltage has an average level of about >3.6V or <-3.6V. Since the module is known to be OK check the output voltage and signal input voltages on CN5 pin 2 & 3 with the module in place and compare with the right channel. Don't connect a load. I'd expect to get a second or so before it shuts down. If it's too fast to make a measurement you'll have to defeat the shutdown by shorting out R877.

Or, if you want to try something quickly C137 and/or C142 might be leaky so pull one or both and see.

Ok I just went ahead and replaced c137 and c142 as I have plenty of spares and they're cheap. So now left and right channel work but only if one module is removed it does not matter which one though.

What I'm about to try though I have no basis for it.

Is to check DC offset on both modules to see what they are. And then I'm lost. I took it apart to make sure it wasn't a connection issue but still repeats earlier issue. Both modules installed it powers off after turning on.

Thanks for any insight,
nosaj

[nickb]

Measuring the offsets is the next logical step. If you want to try both you may need to short R877 to defeat the protection to give you enough time to measure, or use a scope.

[nosaj]

Measuring the offsets is the next logical step. If you want to try both you may need to short R877 to defeat the protection to give you enough time to measure, or use a scope.

Is there a reason to try and measure both at the same time or individually. Would it make a difference?
thanks,

nosaj

[nickb]

Try both at the same time first as that should give you enough info to move forward. Seems easier to debug the fault configuration.

But, has the behaviour actually changed since the outset after swapping the caps? You said both modules worked in the right position but did they also both work in the left? I was going on the assumption that the left slot was bad - that may not have been true.

[nosaj]

Try both at the same time first as that should give you enough info to move forward. Seems easier to debug the fault configuration.

But, has the behaviour actually changed since the outset after swapping the caps? You said both modules worked in the right position but did they also both work in the left? I was going on the assumption that he left slot was bad - that may not have been true.

Since changing the caps I can use left or right module in left or right slot but just not at the same time.

previously before cap change left slot would not work at all.

thanks,
nosaj

[nosaj]

Since changing the caps I can use left or right module in left or right slot but just not at the same time.

previously before cap change left slot would not work at all.

thanks,
nosaj

Both modules read about 195mv DC in either slot. I'll hunt down that resistor to disable the protection circuit then read the DC offsets again.

Thanks,
nosaj