Soundcraft Spirit Power Station -- dead Right channel power output

[xtian]

Everything works except the Right output. All fuses OK. +/-60v rails OK on both channels. All voltages at connector CN5 are OK (see schematic snip, pasted below). Thanks to my new thermal camera, I spotted resistor R110 (highlighted in red) is cooking at over 400°F. This appears to be part of the 5v regulator.

So, considering the solder side of the PCB is not accessible without further disassembly, what's my logical next step? Thanks!

[xtian]

I mean, can a LM317 still output 5v while drawing way too much current? Or maybe that C23 cap is shorted? DC resistance across C23 shows ~7K.

[clintronics]

What is the resistance across C40? now measure the same C40 res with con5 unplugged. Does the highlighted res still get hot with con 5 unplugged?

The input current on LM317T is determined by the load current and the current flowing from the adjustment pin (Iadj). The input current is equal to the sum of these two currents¹. The load current depends on the output voltage and the resistance of the load. The adjustment pin current is typically 50µA and can be neglected in most cases. (BingChat)

Im thinking since the 317 puts out the 5V its possible the load is drawing too much current.

[g1]

That is a 2W resistor, it's supposed to get hot. Are you getting 5V out of that regulator? What is the voltage across the 33R resistor in question? 7V across it would still only be 1.5Watts.
Any power supply fault should cause an issue for both sides.

[The Dude]

......Any power supply fault should cause an issue for both sides.

^^^^^^ That.
If you insert signal directly into the power amp inputs, do you get anything? It could just be a bad switching jack.

[xtian]

What is the resistance across C40? now measure the same C40 res with con5 unplugged.

760R with con5 plugged in, 894R with con5 unplugged.

Does the highlighted res still get hot with con 5 unplugged?

No!

That is a 2W resistor, it's supposed to get hot. Are you getting 5V out of that regulator?

Yes, +5v supply is OK.

What is the voltage across the 33R resistor in question? 7V across it would still only be 1.5Watts.

With con5 unplugged, voltage drop across R110 is only 0.2v. But with con5 plugged in, voltage drop is 7.6v. So, no worries here?


If no worries here, I'm back to my original issue, which is that the Right channel output is dead. As I mentioned at top, I measured the 60v rails, which look good. Where do I go from here?

[Jon Snell]

You must have 0volts DC on L201 to work.
Have you got 0volts? If you have does relay 2 pull in?

[xtian]

I have very near 0v (4mv, 8mv) on L201 and L1 (the big inductors just prior to relays and speaker outputs). On working Left channel, relay works correctly and makes good connection to speaker output when energized, but Right channel does not. Here is a snip of the relay turn-on circuit:



Both D23 and D19 measure as diodes with 0.17v drop when tested in diode mode with my DMM. Measuring voltage drop across D23 when circuit is energized shows 24vDC, and across D19 shows 25vDC.

So, does this data eliminate everything but a faulty relay for Right channel? And if yes, how do I spec the relay? It's marked "Potter & Brumfield T90 series". 24v, I think, based on schematic and voltage across diodes. What amperage?

Maybe this one? https://www.amazon.com/TE-CONNECTIVI...dp/B0195UZNEI/

[Helmholtz]

Both D23 and D19 measure as diodes with 0.17v drop when tested in diode mode with my DMM.

Measured out of circuit?

[xtian]

Measured out of circuit?

No, in circuit and unpowered.

[Helmholtz]

No, in circuit and unpowered.

In circuit you're essentially measuring the voltage drop across the DCR of the coils.
Forward voltage of a 1N4001 should be around 0.6V.

[xtian]

What do you recommend?

[xtian]

Oops--owner just pulled the plug on this repair.

[Helmholtz]

What do you recommend?

Of course measuring the diodes out of circuit.
In circuit diode measurement only makes sense if the surrounding circuit resistance is larger than maybe 10k.

[Grrrg]

What do you recommend?

Using a scope, if you got one.