CTL - Control - is a bus. All the various PFL buttons on each channel or section tap onto it. Bottom center. of last page of mixer schematic: IC33b converts CTL into SOLO_FET. Page 8 bottom center, SOLO_FET controls a few JFETs which route signal to the phones amplifier.
If your page numbers differ, IC33b i9s on the page with the VU meter LEDs and the seven segment display. The phones amp is on whatever page it is on.
Any PFL switch grounds the bus with a 1k resistor. The LED control on the channel is local. A couple transistors. Since those LEDs also serve as peak lights, there is a direct connection to thoswe transistors from an op amp output pin.
The PFL switch opens a ground connection to those transistors, normally closed. Frankly, my first thought here is that you have dirty switches. If that contact doesn't make, the LED will stay on. Doesn;t affect operation of the channel.
Try this: pop the button cap off the PFL switch on the affected channels. Squirt a little Deoxit down alongside the switch shaft post, then exercise the switch a few cycles. ANy help? Or be scientific, and go underneath, and measure continuity on the switch positions from its pins.
I am trying hard here to save you from having to pull the whole board from the chassis.
Clreaning the push switches on mixers like this is a common repair. I did a MX3282A recently and had to pull ALL the channel assigns, the PFLs and the MUTE switch caps and clean the switches. They were all intermittant and dirty, and every one cleaned up fine.
If your page numbers differ, IC33b i9s on the page with the VU meter LEDs and the seven segment display. The phones amp is on whatever page it is on.
Any PFL switch grounds the bus with a 1k resistor. The LED control on the channel is local. A couple transistors. Since those LEDs also serve as peak lights, there is a direct connection to thoswe transistors from an op amp output pin.
The PFL switch opens a ground connection to those transistors, normally closed. Frankly, my first thought here is that you have dirty switches. If that contact doesn't make, the LED will stay on. Doesn;t affect operation of the channel.
Try this: pop the button cap off the PFL switch on the affected channels. Squirt a little Deoxit down alongside the switch shaft post, then exercise the switch a few cycles. ANy help? Or be scientific, and go underneath, and measure continuity on the switch positions from its pins.
I am trying hard here to save you from having to pull the whole board from the chassis.
Clreaning the push switches on mixers like this is a common repair. I did a MX3282A recently and had to pull ALL the channel assigns, the PFLs and the MUTE switch caps and clean the switches. They were all intermittant and dirty, and every one cleaned up fine.
Enzo thanks again mate.
I understand the CTL part, I measured all the chan. PFL switches (from the bottom of the board) against the 4 good chan., no difference, both parts of each swich are correct.
Looking at the schematics it seems like each PFL transistor/LED setup is on it's own.
So how come 4 chan. suddenly have the same problem?
You mentioned the peak level function of the PFL LED, maybe it has something to do with the 4 op-amp's clipping. (power failure on one side?)
As I write this I come to realize I have not done power measurements on the chan. yet.
I have to investigate if there is a common part / line used for these 4 chan.
Mate, thanks again for thinking with me, I'm of to the shed to check things.
Cheers!
I understand the CTL part, I measured all the chan. PFL switches (from the bottom of the board) against the 4 good chan., no difference, both parts of each swich are correct.
Looking at the schematics it seems like each PFL transistor/LED setup is on it's own.
So how come 4 chan. suddenly have the same problem?
You mentioned the peak level function of the PFL LED, maybe it has something to do with the 4 op-amp's clipping. (power failure on one side?)
As I write this I come to realize I have not done power measurements on the chan. yet.
I have to investigate if there is a common part / line used for these 4 chan.
Mate, thanks again for thinking with me, I'm of to the shed to check things.
Cheers!
Thanks again Enzo,
I just wrote a quick reply, but it seems to have disappeard?!
Man, your remark on the PFL led also being used as peak indicator did the trick.
I had to look for some part that was common to all 4 chan's.
It appeared that all were clipped to -15v. because the +15v is missing!
So now I have to find out if one of the op-amp's has a short, if not, just make the connection and GO!
I'll let you know, cheers!
I just wrote a quick reply, but it seems to have disappeard?!
Man, your remark on the PFL led also being used as peak indicator did the trick.
I had to look for some part that was common to all 4 chan's.
It appeared that all were clipped to -15v. because the +15v is missing!
So now I have to find out if one of the op-amp's has a short, if not, just make the connection and GO!
I'll let you know, cheers!
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