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24.4.2 Teardown for Fader and Preamp Repair (DIY, How To)
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soundguybob
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Hi Gang!

So, my "Version 1" SL24.4.2 (part of the original shipment) started having some major issues, and since it was well out of warranty I decided to dig in and see if I could sort it myself before shipping it off for service. Specifically, I was looking to address these 2 issues:

1. CH 23, 24, 4 Subgroups, and the Main fader would randomly jump in level anywhere from 3-6 dB higher than where they were set. The offset was obvious in Universal Control when it would happen - I could watch the faders "jump" on screen. This behavior was completely random, and basically rendered the console useless in my situation, since it's used as a FOH and MON console at our church - FOH functionality was killed, since there were no reliable outputs for the main mix. I was glad to have a computer connected up running UC, and an iPad already setup and ready to go. I put the console in "locate" mode, so it would ignore the faders, and mixed from the iPad. Wouldn't have made it through our Sunday services otherwise!

Since all 7 channels would jump in level at the same time, I figured that the fader bank had an issue that would be common across all 7 faders. From reading other posts here on the forums, I knew that the fader banks were usually in sets of 7. I found a cold solder joint on one of the leads common to the bank of faders, which resulted in the issue. The console is now on the bench for 48 hrs of testing before I consider returning it to service, but seems to be fixed.


2. Talkback input had a major hum in it from day 1, bad enough to render the TB input useless (and kill SMAART functions, etc.) Having heard this type of hum before, I figured that the issue was likely either a failed DC blocking cap, or an issue in the opamp. Since I had the parts on hand, I went ahead and replaced the two 47uf 63V DC blocking caps, and the MC33079 opamp. I only had a TL084 IC on hand, so I went for it, since the input was already non useable, and at worst, the TL084 wouldn't work. However, I've had enough experience substituting IC's in the past, that I felt the TL084 would most likely work fine in the preamp circuit, after comparing data sheets and diagramming the preamp circuit. I'm happy to say that the TB input in now nice and quiet, and is finally useable! After doing some research on the available opamps out there these days, I may eventually sub a few OPA4134UA's or LME49740MA/NOPB's on a few input channels, and give them a run against the stock MC33079's. I'll likely give the TL084's a test run as well if I'm in there, especially considering the cost offset VS. either of the other two. That'll likely happen after I have a replacement in place of this 24.4.2, just in case...

OK, so now, for the teardown: (Sorry, I only have a few pics below, but this should get you into the console w/o them!) NOTE: Any references to top/ bottom/ side/ front/ rear are made in reference to the console as it would be if you were standing in front of it in a normal desktop setting. I HIGHLY suggest that you read through the entire disassembly process before trying this yourself!

1. Remove the 3 screws from the bottom of the plastic trim panels, using a phillips #2. The panels slide forward maybe 1/4" or so, and will then come right off. (Thanks to the old thread with the teardown of the SL16.4.2 for the tip on these needing to slide forward!) I used the palm of my hand and gave each panel a smack or two to get them to slide forward. Set the trim panels off to the side, and out of your way.

2. Remove the two metal side panels, using a phillips #1. You need to remove all of the screws EXCEPT for the 3 on each side that are holding the brackets for the plastic trim panels. Set the side panels off to the side and out of your way.

3. Remove the five Phillips screws from the top edge of the back panel, above the jack field, using a phillips #2. Be sure to use a screwdriver with a good tip, these can strip easily. Keep these separate from the other screws, as AFAIK they are not used anywhere else on the console. I keep a stock of old tic-tac boxes on my bench, and put the different screws in them as I go.

4. Remove the knobs from all of the trim controls. I was able to pull them off by hand without any problems. Use a cup or something to keep the knobs from getting lost or rolling around on your workspace!

5. Stand the mixer up on it's side. It will stay balanced there as long as your workspace is level and stable.

6. Remove the five TORX #8 screws that are in line with the trim controls.

7. Remove the five screw along the bottom edge of the armrest. Phillips #1. NOTE: The control surface will be free from the rest of the console when these screws are removed, be careful, as the control surface will want to fall away from the bottom!

8. Separate the control surface from the bottom section. NOTE: LOOK into the side before doing this! There are five connections between the control surface and the bottom section. I suggest separating the top edge (along the trim controls) of the control surface first, BUT only just enough to clear the shafts, then using that point as a pivot, moving the bottom of the control surface away from the bottom section to gain access to the connections. You'll need to unplug the light socket, headphone jack, ground cable, power cable, and a ribbon cable.

If your issue is in the fader section, go to step #9, power supply or DAC, #13, analog section, Digital I/O, or Firewire, #14

Fader Section:
9. On the bottom side of the control surface, unplug any connections going to the bank of faders you need to service. IIRC, There is only 1 connection per bank, and the banks are grouped as such: CH 1-8, 9-15, 16-22, 23-main.

10. Remove the fader caps from the group of faders that you need to work on. You'll need to pull off all of the caps for any faders that are in the same bank as the ones you need to service. They should easily come off by hand.

11. Remove the two screws holding each fader to the face of the control surface, and remove the fader bank. Phillips #1.

12. Service the fader bank as needed. You may need to simply re-solder a few connections as it was in my case, or you may need to replace the faders. In my console the faders were B10K's made by ALPHA. This tells me that it is most likely a linear taper, 10Kohm pot. No other info was stamped or printed anywhere on the faders.

Power Supply and DAC:
13. Remove and service power supply or DAC section as needed. The Power supply is the single board on the bottom of the chassis. The DAC section is the two boards on the back of the chassis. Unplug any connections from the modules, remove any screws holding them in place. I did not need to remove or service either section, so I do not have the exact details. If you got this far, you should be able to sort it out!

Analog Section, Digital I/O, Firewire:
14. Remove five screws from the bottom of the console at the seam near the back. Phillips #1. You only need to remove the 5 screws that are toward the front side, the other five can stay.

15. Pivot the connector pod away from the larger part of the bottom, using the bottom side as the "hinge". There are several connections that will need to be disconnected, including 2 that pass through to the power supply that will need zip ties cut in order to free up.

16. You'll notice that there are a few boards that are stacked on the rear panel. The small one is the preamp section for CH 23 & 24, the large layer below is the preamps for CH 1-22, and the outside layer is the jack field, with the preamp for the talkback input. The small PCB for CH23-24 preamp is attached from the other side of the larger preamp board, so you need to remove the larger preamp board to get the small one loose. NOTE: There are several header connections that connect the larger preamp boards to the jack field board below, so use caution while separating the two so you do not bend or otherwise damage the pins. Speaking from previous experience, they are extremely difficult to realign once bent. There are also plastic screws attaching the preamp boards to the jack field boards, use a proper fitting #2 screwdriver and use very little force so you don't break them off! If you need to remove the jack field, all of the screws next to the connectors, plus all of the nuts on the jacks will need to be removed. Unless you need to service the preamp for the Talkback section or physically replace a connector, you most likely will not need to remove the jack field boards from the chassis.

17. Digital I/O, FireWire, and Direct Out boards are in an isolated area below the jack field. I did not have to access this area to resolve my issues, but it look like it's just removing another series of screws to gain access.


A NOTE ON REASSEMBLY: When reinstalling the metal side panels, put the screws into the section that's aluminum at the hand rest and faders first, then you can tweak the side panel as needed to align it the rest of the way. You'll likely strip the screw holes in the aluminum otherwise!

Sorry I don't have more pics!








This message was edited 6 times. Last update was at 03/09/2013 17:32:50

Graniet
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Hi,
And thanks for sharing this.
It may come in handy one day. Just hope not to soon.
Unclejambo
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Hi,
bumping an old thread to find a little more info on this. I thought the faders were non generic and could only be sourced by presonus?

Did you simply have to re-flow the solder to solve the issue with yours? Has the repair lasted or has the issue re-surfaced/worsened?
Jamie-Lee Warlow - Front Of House/Tour Manager/Driver

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soundguybob
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@Unclejambo- The faders are definitely not a custom item for Presonus. I haven't bothered (have been way too busy) to find the exact part number of the Alpha fader, but it is absolutely something that could be ordered through an electronics supply company. I have Alpha's order specification sheets, just haven't had the time necessary to narrow down the exact PN. In my case, the solder re-flow completely corrected the fader issues, and I've not had any further fader issues since. I've since had 2 more SL24's come through my shop with fader issues, and a proper cleaning of the offending faders corrected the issue.

What set me off on the teardown in the first place was that Presonus was completely unwilling to sell me replacement parts for my out of warranty console, so they were going to force me to do a mail in, flat rate repair. I would've paid something like $185.00 to have a solder joint fixed. Whoever I spoke to in the office about just getting a replacement fader bank was extremely rude, confrontational, and short with me, and didn't even offer to connect me with a dealer that could supply a replacement. It was a BIG FAIL for Presonus's customer service in my particular instance. I've worked in the AV business for over 25 years now, and have NEVER had a company flat out refuse to sell me a replacement part, either directly or through a dealer/ distribution channel. It's the kind of thing that will make me think twice before I ever suggest another Presonus product.
Unclejambo
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Thanks for the reply.

To be honest, if I can find conventional, more traditionally servicable (i.e. compressed air and lube) parts with the same values and solder lug location then I'd be tempted to re-fit it with them.

Jamie-Lee Warlow - Front Of House/Tour Manager/Driver

email:info@jamie-foh.com
web: www.jamie-foh.com

Head Engineer - Hobos Music Venue
House Engineer - Clwb Ifor Bach

UK/EU/US Tour and Festival Experience with:
Man Without Country
Royal Canoe
Karin Park
Drenge
The Boy Royals
Caesars Rome
Monolithent
Supreme Baconator
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Thanks for clearing that first part up Bob. I was always under the inpression that the group of faders was the only way you could get them. Nice to know there's a way for those who are out of warranty and want to chance it on their own.

I can solder with the best of them but I'm not gonna risk losing any chance of Presonus fixing my mixers for me until they are no longer supported at all.
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My advice and suggestions should never be considered advice or suggestions. These are mostly insane ramblings of a poor aircraft mechanic who can, strangely enough, still hear.

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salvadoredelle
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I would hope Presonus gets some service centers areound the nation and world to serve thier customers better... to have to send it all back to LA from California or wherever is just not right... there should be some tech centers that are qualified to do repairs for Presonus products... this is not rocket science... they just need to put out the service manuals and parts and support for the service centers... WTF???
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fredy2
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Faders are readily available on Ebay as part number ALPS RSAON11

Always try lubing them before replacing them. DO NOT use contact cleaner, only lube designed for the faders.

The faders only handle DC and are sampled via CH051B Cmos multiplexors into an A-D from a refernce voltage impressed upon them.
fredy2
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Faders PN RSAON11 10KB are available in singles from "onlinecomponents" at $5.74 each... don't know what shipping would be. In 50's they are less. I have a few rough ones on one of my SL's... tried Caig Fader lube spray... helped a little, but CRC226 works better... I have some Caig Fader Grease coming... will try that and report back.
djrobbaron
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I realize this is an old post... but WTG Bob! I wish I'd seen this before I sent mine back for repair. I had your #1 issue along with selection pads above the sub/main fader section not illuminating properly.

This post is absolutely going in my bookmarks! To think I could've warmed up the Hakko and saved myself $200+ and 2 weeks+...
fredy2
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I am working on a video about lubing the faders using the CRC2-26 and the Caig Fader Grease. Should have it up in a couple weeks.
fredy2
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A little further information on progress of refurbing.the 24.4.2. I have found one of the pins that connect the preamp board to the back panel connector boards had intermittent contact so I had to take the connector board loose. While doing that I uncovered the DSP board to clean that area. They use an Analog Devices DSP chip and they have a helper chip for the Firewire as well as the firewire driver chip. It appears the TI Firewire chip drives the jacks directly so one had better not hot plug the Firewire cable as there is no protection for the TI chip. In addition, one other thing I did not like to see was the DSP board mounts only on two screw standoffs and the SPDIF jack and Firewire jack escutcheons. There is one heavy electrolytic capacitor that given a hard bump on the bottom of the case could flex the DSP board... Word to the wise, don't set your board down hard!. Given how well most things are mounted in the Presonus, the suspension of the DSP board was an unwelcome surprise. I have just found the Control Room outputs and the headphone output to be dead. It appears that one PCM4104 D-A chip which is common to these has gone bad. Noticed it was very hot at one time. It is a fine pitched one and a bear to replace but I have the tools to do it.
fredy2
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Update: I have successfully replaced the bad PCM4104 and now the headphones and control room outputs work. The PCM4104 is a 48 pin chip with leads spaced at 0.5 mm. There were three circuit traces blown apparently due to the chip shorting. To attach replacement leads to these, the gull-wing leads were bent up with a sewing needle and about a #36 bare wire was used to attach these "dead bug" leads to their original destinations. Special equipment and materials were used to remove and replace the chip.
djrobbaron
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Special equipment and materials were used to remove and replace the chip.


Did you happen to use any ChipQuik stuff? I love it
fredy2
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Used some equivalent products... by Zephyrtronix. Low melt alloying solder.
 
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