Burned resistors question. Yamaha console

[JW]

I have a Yamaha PM1000 channel with 3 blown 22R resistors off the 44V power rail. See schematic. Before I replace 22R's, is it reasonable to assume that top output transistor failure? Especially since it's slightly deformed from heat. Or maybe other output transistor? Any other obvious possibilities that would cause this failure?

 

[pinebox]

Have you been hot swapping modules?

 

[Voyager10]

D2 sets the bias current through the output transistors, if there's a fault with it (e.g. open circuit) that would cause the problem you saw.

Can you test this with a bench power supply (with current limiting) first? Should be getting 35-40mA through the output transistors when idle.

 

[JW]

Not hot swapping intentionally. There may have been a couple times I forgot to turn it off in the past.

Thanks. I don't have a current limiting supply. I can of course shotgun parts . . . For the diode, is a 1N4xxx what we're talking about here for a replacement?

Forgot to mention one more clue. Previous to the fault, this channel had been popping when the multideck switch was adjusted. It was fine when the setting was arrived at, but had some loud low endish pops between gain settings.

 

[chrispbass]

Resistors usually “burn” for a reason.

 

[Voyager10]

The diode is marked as an STV-3. Googling this suggests it's a special purpose part used mainly in Japanese audio gear, but can be substituted by 3 normal (1N914 type) diodes in series. Looks like it should have about a 2V drop across it.

 

[chrispbass]

I’d whip the transistor out and test it

 

[NOON]

Popping on switches indicates DC present. Most common cause of this is electrically leaky electrolytics. Could be other issues, but also possible that the big spikes from the switch was enough to kill a semiconductor junction somewhere in the path (transistor or diode)
Recap the board while you're at it and consider doing the rest of the channels.

 

[JW]

Thanks everybody. My first thought was there's DC on the switch as well. It's one of the tants or films if so. This console probably has less than 100 hours on the new electrolytics.

I replaced the burned resistors, the transistor that looked questionable, and the diode. It works now, but still big popping at the switch.

Don't tants short when they go bad?

 

[JohnRoberts]

Thanks everybody. My first thought was there's DC on the switch as well. It's one of the tants or films if so. This console probably has less than 100 hours on the new electrolytics.

I replaced the burned resistors, the transistor that looked questionable, and the diode. It works now, but still big popping at the switch.

Don't tants short when they go bad?

yes they generally do.

Big mixers often use that sized resistors as cheap fuses to protect against short circuits. (Often the resistors are FP or flame proof so they don't catch fire).

If hearing noisy pots, switches from suspected DC use a VOM to measure for DC voltage where it shouldn't be. Often an old leaky electrolytic capacitor is the cause.

JR