Yamaha P5000S with Fault in Channel B Output Module

[Delta362]

I'm working on a Yamaha P5000S that wouldn't power on. I've attached the relevant schematic. I narrowed it down to Channel B output module. If the +/- high voltage wires to this module are connected when powering on, it goes into protect mode and shuts off within a second. However, if I disconnect the high voltage lines and leave the low voltage connector connected, I can power it on.

Part of the problem I'm running into is that there are a handful of surface mount components on the back side and I'm avoiding complete disassembly of the good unit unless I have to. So, to check the surface mount component circuitry, with just the low voltage connectors in place and applying an input waveform, I do have the same audio signal on both channels reach the base of Q210.

Knowing that Channel A is working properly is great because I have a known working module to compare it to. With no power applied to the unit and all connectors disconnected from the two output modules, I then began taking passive resistance and diode setting checks on the components. I found no differences on the readings of all heatsink components between the two channels so I don't think I have any blown output transistors at this time.

What I did find odd were readings around R236 and R237, and Q211, Q221 on the first page and Q130 on the second page. When reading the resistance of R236 or R237 of the good channel, I can see that it takes several seconds to charge up to its 75k value. However, on the bad channel, this resistance snaps right to it.

Measurements of components in the same general circuit area are below. If the below measurements were the same, I didn't report them.

For Q211 (2SA1371), all voltages are the same except C to B (red to black probes). On good, it's 1.8V. On the bad, it reads 2.1V.

For Q221 (2SA1924), E to C (red to black) is 2.2V on good, while 2.9V on bad. Also, B to C (red to black) is 2.9V on good, while O/L on bad.

For Q130 (2SC3468), C to B (red to black) on good is 2.9V, O/L on bad. C to E (red to black) is 2.7V on good, O/L on bad.

Any help trying to make sense of the above would be greatly appreciated. Thanks!

[Delta362]

I did pull both Q211 and Q221 and tested them on my transistor tester where both showed ok. I have not pulled Q130 as I'm trying to avoid pulling components unless necessary.

[The Dude]

Have you checked output transistors for shorts?

[Delta362]

I did resistance and diode setting tests on all transistors that are connected to the heatsink and compared them with the working unit. I didn't find any shorts or any readings that were different between the two.That's why I am not suspecting a shorted output transistor at this time.

With the R236 and R237 resistor readings going immediately to their value on the bad unit vs. taking several seconds to increase the reading to meet the right value for the good unit, I'm leaning towards something on sheet 2 is feeding back into the circuit properly for the good unit, and something appears to be open preventing a capacitor from being in the line to cause the charging reading.

[g1]

Check that whole long string of parts running from D205 downward all the way to R294 (also the IC202 across R293 & 294).

[Delta362]

Finally some progress! I found R241 (1 ohm 1/2W) resistor open. It's near the bottom right of the first sheet. I realize the resistor most likely failed from too much current. I replaced the resistor and rechecked those transistors and resistance readings above. Now both channels are giving the same components readings.

Before I power on, I'd like to determine what may have caused R241 to fail to ensure I don't have another failed component lurking in the circuit. Any suggestions on other components I should be checking closer?

[Enzo]

MAke yourself a light bulb limiter and power it up with that first.

R240 and R241 are in the HV rails. To burn one of those out, my first suspects are on the other page in the output devices.

[Delta362]

Finally completed the repair of this. (Have I ever mentioned how much I hate working with thermal grease and mica insulators, especially units with this many transistors mounted like that?)

After replacing R241, I also replaced R188 (470 ohm 1/2w) as it was reading about 455 vs channel A reading 470. Powered on with the bulb limiter and it had the typical initial surge, followed by the charging of the power supply caps and the bulb then began dimming. With the LBL still in the circuit, I compared voltages of the rails, output transistors, and a few other spots. They were all the same.

I then turned off the amp, removed the LBL and powered on hoping I didn't miss anything. The voltage drop across R240 and R241 matched for both channels. Spot checking rail and other voltages also matched. Both channels had nice clean waveforms and I was able to get full power at the output for both. So, I'm baffled as to what caused R241 to open. Whatever it may have been, I'm glad to have this one done.

As always, thank you for the help along the way!