Soundcraft Folio Rac Pac • PFL permanently active

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hotfingers

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Feb 17, 2021
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I know that this unit has been discussed here previously and am hoping for a little information –

My Rac Pac has worked flawlessly for years but now, the pfl is permanently active.
There are NO individual pfl channel leds on and the switches all seem fine. As both the pfl active led and the audio pfl monitoring are activated, I'm assuming that there is a fault between the enable input of the Output 3 board and the input pin of the ASCN3-7 connector to the sub-board.

I have schematics but no voltages shown. I was wondering whether anyone might have access to the expected voltages around Q3/4/5/6 and IC39?
For reference, my voltages on IC39-A are:

Pin 1: 6.4v (hence the activation)
Pins 2 & 3: both c. 0v
So far as I can measure, there is 1mv between pins 2 & 3, if the meter is to be believed.

I'm not enthusiastic about having to take out the PCB, nor change an IC. If that is the problem, I might just snip the wire to the 'enable in' (ASCN3-7) and pull it down towards the '-' rail, simply disabling the pfl which I don't now need to use.

Any pointers would be hugely appreciated, especially the voltages around this part of the circuit.
 

abbey road d enfer

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Pressing the channel PFL button puts a positive voltage on the EN bus, which in turns makes:
Q3 emitter positive
Q5 collector negative
IC39 pin 1 positive, which is transmitted to IC43 pins 9, 10 & 11
 

hotfingers

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Feb 17, 2021
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abbey road d enfer said:
Pressing the channel PFL button puts a positive voltage on the EN bus, which in turns makes:
Q3 emitter positive
Q5 collector negative
IC39 pin 1 positive, which is transmitted to IC43 pins 9, 10 & 11

Many thanks for this.

Did you mean Q3 emitter going positive? I think it's hardwired to the +16v rail.

The EN bus is static at 0.1v until a pfl switch is enabled when it rises to 0.22v.

Do you happen to know whether the pfl EN bus should be at 0.1v quiescent or is this too high?

The collector of Q4 remains at +16v regardless of any pfl switch.

Although the board is still in place I'm fairly sure I've correctly identified the transistors. The legend of the schematic is very low res!
 

radardoug

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Fixing it is better than chopping a wire and finding out later that you DO need the pfl. You are presumably using a digital meter. It will default to +/- its lowest reading if a signal is present. It may be less than .1 volts.
Normally such a bus would go to a transistor base, and so you would expect about .5 volts when activated. If its permanently on, maybe that first transistor is shorted.
 

abbey road d enfer

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hotfingers said:
Many thanks for this.

Did you mean Q3 emitter going positive? I think it's hardwired to the +16v rail.
  I meant Q3 collector.

The EN bus is static at 0.1v until a pfl switch is enabled when it rises to 0.22v.
0.1V is ok, but when enagged, it should be close to 0.6-0.7V in order to turn on Q4.

Do you happen to know whether the pfl EN bus should be at 0.1v quiescent or is this too high?
Anything that's low enough for not turning on Q4 is OK. 0.1V is perfect.

The collector of Q4 remains at +16v regardless of any pfl switch.
Correct. It shouldn't change much since it's defined by the rail voltage and the Vbe of Q3.

Although the board is still in place I'm fairly sure I've correctly identified the transistors. The legend of the schematic is very low res!
Indeed!
 

hotfingers

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Feb 17, 2021
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Many thanks for the feedback. I have the following readings:

Q3
B 16.13v
E 16.84v
C 16.79v

Q4
B 0.093v | 0.217v (when a channel pfl is activated)
E 0.00v
C 16.11v

Q5
B -16.33v
E -16.93v
C -16.94v

Q6
B -0.66v
E 0.01v
C 0.11v

I'm fairly sure that the issue is Q4 which is also pulling down the EN buss.
I would appreciate reassurance of my thinking before taking the unit apart. Could I be missing anything else which is causing the pfl to be permanently active?

Many thanks.
 
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