Yet another SL2442FX-PRO repair thread

[slobb11]

Hiyas,

I'm looking for any help fixing the main-out on an SL2442FX-PRO please.
It makes a scratchy noise while it's being moved, but is fine otherwise. The rate at which the fader is moved directly affects the level of the noise (fast movements of the fader result in +7db on the outputs. If I'm super slow-mo with the fader I can make it virtually un-noticeable).

When I first noticed a noisy fader, I made the incorrect diagnosis of a dodgy fader. But having replaced the fader I'm now looking for what is ACTUALLY causing the fault.

At Enzo's suggestion, I've checked for DC voltage across the fader, and Lo-and-behold....The 4 pins at the bottom of the fader are carrying +2VDC.
It was very satisfying to discover that, but...I've got no bloody clue what that means
Where's it coming from?

I've also discovered that the 400v 480uF cap in the PSU is bulging a bit. I'm assuming it's probably not related though, as the output voltages from the PSU seem healthy.

Any help or suggestions you can offer would be greatly appreciated.

Thanks in advance,
Mike

[Enzo]

Is that a 480uf or a 180uf?

BUlging really? Are you seeing the plastic end bulging out? Get under the edge of the plastic sleeve, and slit it to free the plastic disc covering that end, see if the plastic is doing the bulging while the cap remains flat on the end. If the cap really is bulging, replace it immediately.

BE CAREFUL: this is a switchmode power supply, and that cap has something like 350vDC on it, and neither end is ground. Plus it is the directly rectified mains, so that 350v has all the current your wall outlet can provide. It can KILL YOU.

But you are right, it likely is not involved in your DC.

Find the schematic, if it is not already on this forum somewhere - use the search - try Elektrotanya. Look at the diagram for the master section. I see the fader wired directly to two ICs, IC45 and IC51. Alsso, IC31 in the MONO out section is wired to the faders. Any one of those three could be infected with DC. Find them and check pins 1 and 7 on each.

[g1]

For those who would like to follow along in the hymnal, main out: http://music-electronics-forum.com/a...-mix-fader.pdf
smps: http://music-electronics-forum.com/a...spsu2_revf.pdf
SL2442 complete: BEHRINGER SL2442 Service Manual free download, schematics, eeprom, repair info for electronics

[slobb11]

Is that a 480uf or a 180uf?

BUlging really? Are you seeing the plastic end bulging out? Get under the edge of the plastic sleeve, and slit it to free the plastic disc covering that end, see if the plastic is doing the bulging while the cap remains flat on the end. If the cap really is bulging, replace it immediately.

BE CAREFUL: this is a switchmode power supply, and that cap has something like 350vDC on it, and neither end is ground. Plus it is the directly rectified mains, so that 350v has all the current your wall outlet can provide. It can KILL YOU.

But you are right, it likely is not involved in your DC.

Find the schematic, if it is not already on this forum somewhere - use the search - try Elektrotanya. Look at the diagram for the master section. I see the fader wired directly to two ICs, IC45 and IC51. Alsso, IC31 in the MONO out section is wired to the faders. Any one of those three could be infected with DC. Find them and check pins 1 and 7 on each.

Thanks for your response Enzo!
You're absolutely right, the cap is 180uF. My fish-brain forgot the numbers in the short time it took me to walk inside from the garage
I've already got the schematic (although initially it was only half of what I needed), and did some testing. What I originally had didn't show the mono-out so I never looked at IC31.
What I couldn't figure out from the schematic, is which end of the fader is which. According to the schematic, one end of the fader is grounded.
Is that at the end with 4 pins?
Also....what should I be referencing the IC pins against? Ground? Or themselves? (eg, input - against input +).

Looking at the full schematic now, I can't see IC31 in the mono section. Did you mean IC34?

[slobb11]

Hmmm.
Pin2 of IC34 shows -7.5VDC against ground. That doesn't seem healthy to me.
Think that could be our culprit?

[slobb11]

For those who would like to follow along in the hymnal, main out: http://music-electronics-forum.com/a...-mix-fader.pdf
smps: http://music-electronics-forum.com/a...spsu2_revf.pdf
SL2442 complete: BEHRINGER SL2442 Service Manual free download, schematics, eeprom, repair info for electronics

Cheers G-one. The relevant pages are 12 & 14 of the third link.
The capacitor is fine...just the plastic that was bulging.
Goddam alarmist New Zealander over-reacting again

[Enzo]

Yes, IC34, I typo'd.

If pin 2 has -7v, what is on pin 1? The pin 2 voltage could be coming from pin 1. But considering you have 7v at the chip and only 2v at the fader (the othe end of R281, R299) I have to think IC34 is where it is coming from, regardless of the bad pin. Remove IC34 from the board and see if your fader voltage goes away.

All circuit voltage readings should be assumed in reference to ground unles otherwise specified.

[slobb11]

Yes, IC34, I typo'd.

If pin 2 has -7v, what is on pin 1? The pin 2 voltage could be coming from pin 1. But considering you have 7v at the chip and only 2v at the fader (the othe end of R281, R299) I have to think IC34 is where it is coming from, regardless of the bad pin. Remove IC34 from the board and see if your fader voltage goes away.

All circuit voltage readings should be assumed in reference to ground unles otherwise specified.

Cool cool.
My toddler woke up, so that's slowed things down a lot.
I had a chance to check out IC34 a little better....
-7VDC on pin 2
-12.5VDC on pin 1
-12.5VDC on pin 5
-12.6VDC on pin 6
-12.6VDC on pin 7

I checked IC39 and it's all healthy, so it's definitely looking like IC34 is to fault.
One interesting point though..the power source for IC34 is a bit fubar. Pin 4 is a happy -14.6v, but pin 8 is -13.2v?!
On the schematic, pin 8 of both IC34 and IC39 are connected together to the same source, but pin 8 on IC39 is at +14.6v as it should be.
IC35 which I can't even find on the schematic, (although I'm probably just missing it because some small hands are tugging at the paper while I'm reading it), also has -13.2VDC on pin8.

EDIT: I've just found IC35, it's the talkback mic. So it's not directly in circuit with the main or mono faders. So I'm sticking to the IC34 theory. I'll remove it tomorrow when the little-one's in pre-school.


Thanks for all your help!
Mike

[Enzo]

Before you do anything, find out where the missing +15 went. on pins 8. Very difficult for even a good op amp to put zero DC on its output pin when both the power rails are at -14v.

You have -14 on the + pin because the supply that should bve ther isn't getting that far. so the negative rail just fills the IC. The IC may be fine with proper power.


Do the ICs associated with the fader have both good supplies? ICs 45, 51 if I recall. If those are good, I;d say the IC34 is probably the source of the voltage even if not the cause. Meaning that removing IC34 will probably eliminate the problem with the main faders. But that doesnl;t mean IC34 is bad. You need to find where the voltage went, it is probably just a broken connection from another section of the board.

On some of the pages you wil see a row of IC power connections. Most of the ICs are wired direct to the +/-15v rails. Some though are wired with low value resistors, like 22 ohm, to isolate them somewhat. But your suspect ICs all seem to be direct wired. SO I have to think a copper trace is cracked, or a jumper wire missing or broke free. ANy other ICs missing their + rail in the area?

Do you have any really fine wire? If nothing else, you could probably just tack solder a wire to pin 8 of the IC34 and solder the wire to some other nearby +15v point. For example some other nearby IC pin 8 that does have +15. DO this before removing or replacing the ICs. You may not need anything other than the jumper wire.

Of course it is also possible a shorted IC burnt the trace to the power rail open.

[slobb11]

Before you do anything, find out where the missing +15 went. on pins 8. Very difficult for even a good op amp to put zero DC on its output pin when both the power rails are at -14v.

You have -14 on the + pin because the supply that should bve ther isn't getting that far. so the negative rail just fills the IC. The IC may be fine with proper power.


Do the ICs associated with the fader have both good supplies? ICs 45, 51 if I recall. If those are good, I;d say the IC34 is probably the source of the voltage even if not the cause. Meaning that removing IC34 will probably eliminate the problem with the main faders. But that doesnl;t mean IC34 is bad. You need to find where the voltage went, it is probably just a broken connection from another section of the board.

On some of the pages you wil see a row of IC power connections. Most of the ICs are wired direct to the +/-15v rails. Some though are wired with low value resistors, like 22 ohm, to isolate them somewhat. But your suspect ICs all seem to be direct wired. SO I have to think a copper trace is cracked, or a jumper wire missing or broke free. ANy other ICs missing their + rail in the area?

Do you have any really fine wire? If nothing else, you could probably just tack solder a wire to pin 8 of the IC34 and solder the wire to some other nearby +15v point. For example some other nearby IC pin 8 that does have +15. DO this before removing or replacing the ICs. You may not need anything other than the jumper wire.

Of course it is also possible a shorted IC burnt the trace to the power rail open.

Don't you hate doing something and then checking your messages and finding one from someone telling you not to do what you've just finished doing?
So in other news, I've removed IC34 and the Main-out is now working beautifully.
IC35 (about an inch and a half away from IC34) had the -13.2VDC on pin 8 as well. It controls the talkback mic, which doesn't work and makes a big POP when the talk button is pressed. So you're probably right about the broken track somewhere in that area.
All other ICs in that general area of the PCB were powered correctly.

That said...I fired up IC34 after removal (punched -15.5VDC into pin 4, and +15.5VDC into pin 8), and then checked the voltage between pin 4 and all other pins. They all had at least +6VDC on them, with a couple running up to 12VDC. Surely that shouldn't be the case?

I'm going to order twenty NJM4580Ms (because 20 is cheaper than 1 believe it or not), but fast shipping to NZ will cost five times as much as the chips. So I'll get them via uber-slow-post which takes the better part of a month and a half to arrive, but is free.
I'll replace the Mono-Out op-amp when they get here, and investigate the talkback op-amp and the possibility of a broken power-rail at the same time.
For the moment, this desk will now serve its purpose (which is to see my new company through its first year until I can afford an X32 or a 24.4.2). The mono-out isn't important to me at present, because I'll only be using it to run two SRM450s and two SX300s. If I need to sub-hire in some bass-bins, they come with a Crown XTI series amp, which has a built in DSP and cross-over, so I don't need a sub-out of the desk.

Thanks so much for all your help Enzo. You're a bloody star!
I'll keep tabs on these forums from now on, just in case I ever have the opportunity to help someone in turn.

[bkahuna]

That said...I fired up IC34 after removal (punched -15.5VDC into pin 4, and +15.5VDC into pin 8), and then checked the voltage between pin 4 and all other pins. They all had at least +6VDC on them, with a couple running up to 12VDC. Surely that shouldn't be the case?

You're making measurements relative to the negative rail, which is somewhat unorthodox. I would recommend measuring relative to ground. In your situation I would expect to see about 15volts from pin 4 to all others except pin 8, which should be around 30volts.

If you measure relative to ground, most pins of an opamp (set up to pass audio, and using bipolar supplies) should read close to zero. The exception would be pin 8, which should be +15v, and pin 4 would be -15v (assuming +- 15 volt supplies)

[Enzo]

Take most any dual op amp from your stock, and apply the same test to it, just to see if your procedure works.

And if you have or have local acces to other dual op amps, that IC34 and the talkback chip can be pretty much any dual op amp you have. WHo cares about slew rates and stuff for a talkback mic? And really, the mono sum output, especially if you don;t use it, won;t suffer much if you use a TL072, a 4560, or what have you. But while it is open, and the chips already removed, now would be the best time to explore the missing +15 rail at those chip locations.

[slobb11]

Oh ya, but the trouble is it's not really very "open" any more.
It's fully reassembled, complete with all the knobs fitted. When my twenty new chips arrive (I don't have stocks of anything lying around unfortunately), I'll re-open it and check that +15 rail.

Bkahuna, I'm not sure what "ground" is when the chip is just sitting on my desk

[Enzo]

If you powered it on your desk from +15 and -15, you had to have the two supplies connected together, and that common point would be "ground". Otherwise you are just putting 30v across the IC. Generally, an IC needs to have its feedback specified or there is no telling where the outputs will go.

[slobb11]

Goddam you're good at this game!
I got impatient, as my replacement op-amps weren't going to arrive for weeks, so I pulled the console apart again to investigate the power situation.
After making a jumper from a healthy +15v point to pin 8 of IC35 everything was looking good in that area.
So....I replaced IC34 again, and wonder-of-wonders....it's all good
Only issue left with this desk now is a dry-joint on the pot for the talk-back level. (The audio cuts in and out as I apply pressure to the knob.)
An easy fix which is by no means urgent.

You're a proper legend Enzo. Thanks for everything!


Edit: I forgot to mention...I looked for a broken track, but couldn't see anything obvious. I just had a lamp blow in my workshop though, so the lighting isn't great at the moment. So I'm not sure why those two ICs were missing their +15v power.