Soundcraft Spirit Mixer stuck in PFL solo mode

[substitute]

Hello, hello, hello, looooooooooooooong time no see.

I have a Soundcraft spirit folio mixer that is stuck in PFL SOLO mode that I'm working on.  Here are the details

- The SOLO indicator LEDS on the two stereo channels are constantly on regardless of switch position
- The master PFL indicator LED is constantly on and the master meters behave as though it is in PFL mode
- Even though the SOLO indicator is on either way, audio from the stereo input channels doesn't route to the PFL buss unless the switch is depressed
- The SOLO switches on the channels test appropriately for continuity and behave as expected

I'll attach screen shots of the SOLO area of the schematics for both the stereo channels and the master section following this post.

Here is a link to the complete service manual, pages 18 and 21 are what I'm looking at.
https://www.dropbox.com/s/siya6u7guwva0zd/soundcraft_spirit_folio_racpac_sm.pdf?dl=0

Here's a link to the user manual
https://www.soundcraft.com/en/products/spirit-racpac

My current suspicion is that some component (perhaps Q3) in the master section has failed and is letting the +17 supply through to the EN buss and also activating the main PFL LED.

 

[substitute]

Stereo Input Solo detail attached

 

[substitute]

Master PFL detail attached

 

[JohnRoberts]

it's after beer o'clock so I will just answer in general.

Look for something wrong with the solo/pfl logic bus...  it may be shorted or floating at some incorrect voltage.

JR

 

[substitute]

It is (or was) and on a Friday no less. Now it's coffee o'clock. Unfortunately the schematics don't have any voltages notated and I don't know enough about these types of transistor switching circuits to know what to expect. I'll post back with voltage measurements a little later.

 

[substitute]

Here's some intel for the transistors in the master PFL section...

Q3
B 16.35v
E 17.02v
C 17.91v

Q4
B 6.79v
E 0v
C 16.33v

Q5
B -16.26v
C -16.85v
E -16.84v

Q6
B -0.64v
E 0v
C -0.14v

Does anything jump out at anyone?

 

[JohnRoberts]

Here's some intel for the transistors in the master PFL section...

OK there is trouble in PFL 'ville

Q3
B 16.35v
E 17.02v
C 17.91v

Q3 is almost valid... it appears turned hard on, while a little unusual for collector voltage to be higher than emitter that is pulling it up.  I will ignore that as a measurement artifact.

Q4
B 6.79v
E 0v
C 16.33v

now your talking,,, Q4 is busted. Unless base is actually 0.67V instead of 6.7V 

Q5
B -16.26v
C -16.85v
E -16.84v

q5 looks OK

Q6
B -0.64v
E 0v
C -0.14v

q6 looks OK..

Does anything jump out at anyone?

Recheck base voltage at Q4, if actually 6.7V that is your problem.

JR

 

[substitute]

Just double checked, the base of Q4 is indeed 6.7v.  I'll report back once I've replaced it.

Thanks for the insights!

 

[moamps]

Q3
B 16.35v
E 17.02v
C 17.91v

Collector of Q3 can't be on higher potential than emitter. Check this transistor also.
Q3 should have base or emitter resistor, or it is just schematic error.

 

[substitute]

Will do.

 

[JohnRoberts]

Collector of Q3 can't be on higher potential than emitter. Check this transistor also.
Q3 should have base or emitter resistor, or it is just schematic error.

I already ignored that as a measurement artifact (error) since the PS rail voltage is 17V making 17.9V extremely unlikely.

JR

 

[JohnRoberts]

I already ignored that as a measurement artifact (error) since the PS rail voltage is 17V making 17.9V extremely unlikely, and a bad transistor will not generate extra voltage.

JR

 

[moamps]

I already ignored that as a measurement artifact (error) since the PS rail voltage is 17V making 17.9V extremely unlikely.

What's more confusing to me is connection of Q4 and Q3. It can't work that way.

 

[JohnRoberts]

What's more confusing to me is connection of Q4 and Q3. It can't work that way.

I suspect there are parts missing off the page,,, like a resistor in series with base of the npn to keep collector current modest.

it is not unheard of for mistakes in old hand drawn schematics.

JR

 

[abbey road d enfer]

What's more confusing to me is connection of Q4 and Q3. It can't work that way.

Why? Seems perfectly legit to me. Collector of NPN Q3 sinks current from base of PNP Q4.

 

[moamps]

Why? Seems perfectly legit to me. Collector of NPN Q3 sinks current from base of PNP Q4.

Q4 is NPN, Q3 is PNP. Both work as switches. What's the base current of Q3 when Q4 is in ON state?

 

[abbey road d enfer]

Q4 is NPN, Q3 is PNP.

That's correct, so Q4's collector sinks current from Q3's base.

Both work as switches. What's the base current of Q3 when Q4 is in ON state?

Obviously there's a mistake regarding Q3's collector connection. It should go to R183.

 

[moamps]

Obviously there's a mistake regarding Q3's collector connection.

I don't think so.  A resistor in base or emitter of Q3 is missing, as I already stated.

 

[abbey road d enfer]

I don't think so.  A resistor in base or emitter of Q3 is missing, as I already stated.

You're correct, I didn't see that Q5 is taking care of the LED.

 

[JohnRoberts]

I am repeating myself but it is pretty common to find transcription errors in hand drawn schematics (like all were decades ago).  Also there may be circuitry not shown off that schematic page limiting base current.

As usual don't ASSume that every schematic is accurate (or reported measurements for that matter).

JR