TL audio M4 psu question

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JAY X

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Jan 9, 2009
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Hi!

A friend of mine has a TL audio M4 console with the original psu. I'm going to visit him by next week, to see what happened. But to advance a bit in diagnostic, he says that nothing in the console lights up. He also says that he hears like a discharging sound when turnning on the psu, besides, the led does not light up. Maybe the caps are bad, but from the pictures they seem to be ok...He has not measured output voltages yet at the 7 pin DC connector.

There is an advertissement: " caution : CONNECT TO MIXING CONSOLE BEFORE SWITCHING ON.

My question is if this psu needs a load to measure voltages or he can measure voltages with the multimeter directly at the pins.

As for the psu pinout is the following:

PC 199, document number cd1199.

PIN 1: +200V DC
PIN 2: 0V
PIN 3: +70V DC
PIN 4: +22V DC
PIN 5: ¿CHASSIS GND?
PIN 6: -22V
PIN 7: ¿?
 

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Khron

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Might the wire going to pin 7 be the wire from the pad very clearly labeled as "PFAIL"?

Or in general, one could follow the wires from the back of the output connector to wherever they come from on the psu board - might give a clue also what sort of values "should" be present.
 

JAY X

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HI!
Thanks for uploading the schematics!

¿What voltage is expected at PFAIL?

I also noticed this psu has no regulators... instead, maybe the regulators (LM317) are in the channels?

Well, as soon I can see the console, we will see what's going on!

Jay x
 

Khron

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If my memory of comparators isn't too rusty, looks like Pfail should be pulled low when things are ok.

Although that's being fed from the AC transformer windings that feed the bridge rectifier for the +/-22V rails.

(Later edit) Finally, found the only(?) place where the Pfail signal connects to - the monitor output relays. So all that does is mute the outputs for a while after power-up, and virtually instantly upon loss of AC.
 
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Khron

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Power LED's driven from the +22V DC rail, so if that isn't coming on either, your issue's either the mains cable, the fuse(s), or worst case, the transformer itself.

What's a "discharging sound", though?
 

chilidawg

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What's a "discharging sound", though?
Someone actually showed that to me when I was still active being an engineer. It sounded like when you play backwards a high frequency sweep test tone. But everyone has a different perception, so I could be wrong.
 

JAY X

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Hi!

Finally i could repair the PSU. A blown fuse at +22v rail. Easy!

My friend also asked me to service some channels, because there are some crackly switches, and some noisy EQ pots. I said yes! of course. But i never seen a picture of a TL Audio M4 opened!!. I make this comment because i don't know if there is a special procedure to open this beast... I mean, ok unscrew these... But what will I find there... All channels are in blocks of 8. Could i pull the channel strips "Block" out... or i need to dismantle the knobs, nuts, etc before pull out the front panel??...
 

amplexus

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Jan 11, 2018
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Hi!

Finally i could repair the PSU. A blown fuse at +22v rail. Easy!

My friend also asked me to service some channels, because there are some crackly switches, and some noisy EQ pots. I said yes! of course. But i never seen a picture of a TL Audio M4 opened!!. I make this comment because i don't know if there is a special procedure to open this beast... I mean, ok unscrew these... But what will I find there... All channels are in blocks of 8. Could i pull the channel strips "Block" out... or i need to dismantle the knobs, nuts, etc before pull out the front panel??...
I'm just in the finishing stages of doing a full refurb on one of these for a client- recapping every channel, psu, cleaning, calibration etc.. right down to sanding out the wood cheeks and giving them a couple coats of oil.
Removing channels is a right pain in the arse. You cannot remove a single channel without first taking off every knob in the bank of 8 modules, then removing the top blue screened panel- at that point you can access the pot nuts holding the modules to the subframe. There is a single 32way ribbon cable connecting to the I/O card which supplies bussing and power rails to the channel strip from the tube backplane, then there is an 8 way ribbon going to the mute/pfl board on the fader bank, and a shielded pair to the fader itself, which is inexplicably soldered and not on a molex or some such like every other connection... Once those are removed and the pot nuts are pulled the cards will shimmy out. Keep in mind that there are preamp circuits, switches, and jacks on the I/O boards and the backplane boards need to be removed (4 screws and gentle wiggling) to get to those.

I just use a residue free solvent cleaner and a needle dropper to flush the pots and switches. Give em a good workout and flush again. I prefer to leave the pots dry- no Deoxit or the like. You can deoxit the switches if you like- though they probably won't need it if they're going to be used and worked out regularly.

The Alps faders DO come out in the whole bank of 8, of course you'll have to desolder the leads from the channel card. Remove them from the plate to clean. They crack open quite nicely with a little metal springy tab at the top end, revealing the rails, and wiper carriage. You can remove the rails and wiper carriage by using a small flat blade screwdriver at the top end- you will see that one end of the rails has a slot milled in it and the other does not, you want to gently pry the small metal finger holding the rail in at the slotted end enough to sanp out the rails and then they'll come out of the other end with a minor wiggle. Don't lose the rubber bumper o rings! Clean them and the carriage holes with some iso, same with the carbon track, and VERY GENTLY with a cotton swab and iso, clean the wiper fingers. Mine were a little rough with corrosion so i used some 3000grit micromesh to polish them back to shiny and smooth. Reassemble in the reverse way, being careful to note that the skinnier rail goes to the bottom, and the thicker rail to the top of the carriage. Before closing up put a very thin coat of light weight silicone oil (i use 30wt or 50wt) or sewing machine oil on the top rail making sure that you just have a light film. As with the pots i prefer to leave the carbon track dry- save the faderlube for conductive plastic. They should be smooth as butter and silent after that.

You are correct that there's no regulation in the PSU it all happens locally on the cards as needed. The desk is very sensitive to ripple on the PSU rails so i would STRONGLY recommend recapping the PSU. Making sure to note the audio rails are 22,000uf. In my experience this desk gets really noisy and the monitor relays get angry if the 22v rails aren't really clean from the PSU. So if those caps are aged and drifted at all you could have a higher noise floor than you'd like. Also the desk runs warm inside due to all the regulators being internal and obviously the tubes being in an easy-bake oven type surrounding very near the caps at the most sensitive audio stages and the stock caps are all 85deg rated- not 105deg. I'm not saying you need to recap the board- i'm just saying that if it's still got it's original caps they've been baking for the better part of 20 years.
 

amplexus

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Here you can see how the blue panels cover the subframe where the nuts are here, as well as how the IO boards are on the rear panel.


IMG_4185.jpeg
 
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