Allen and Heath GL2 repair

[JoeSmoak]

I recently purchased a GL2 to use in my studio but it is having some issues. Certain channels will stop working until I turn the gain up then back down, and my stereo channels just wont work at all. Is this a repair that a beginner could fix at home? Or should I bite the bullet and take it to a shop?

[James Perrett]

That sounds a bit like a dirty connection. The first thing I'd check is the insert sockets. Try plugging and unplugging a connector in each of them and see if you then get any sound. With gear that has lived in a smoky atmosphere I often find that I have to leave TRS connectors with the tip and ring shorted together permanently in each insert that isn't currently being used with outboard gear.

[Lostgallifreyan]

With the mixer unplugged from any power source, try blowing with compressed air from a can, through a small tube supplied with it, to blow stuff out of the faders. Then drip some isopropyl alcohol (NOT some other solvent, this must be very specific or you'll risk serious damage to something.) While the innards of the fader are slightly wet with it, run them from end to end, fast but with minimal possible force, at least twenty times each, but take it gently for the first move in case it won't run smooth, in which case slight vertical pressure on its lever can help.

If a fader withstands this, you should get a lot of extra life out of it. If it doesn't, it was probably in terminal decline already so at least you know it needs replacing.

Make sure you leave time for the isopropyl alcohol to evaporate before reconnecting power. Its flash point is fairly high so it's safe enough, but don't temp fate. It should take between half an hour and one hour to dry fully. It may appear to take less but you need to avoid a buildup of vapour in the case.

Whatever it costs to do this, it will be a lot less that the cost of a full set of faders and the time to install them.

EDIT: What James said.. It does sound like connectors need checking. A wipe of TRS jacks with isopropanol-soaked toilet paper can help a lot, especially if you aren't careful to avoid touching the contacts when handling them in normal use. Then again, the odds are that faders might want attention too, and you'll know if they do.

[ajay_m]

Be very careful never to get IPA on perspex , eg displays etc, it can cause catastrophic cracking.

[Lostgallifreyan]

Be very careful never to get IPA on perspex , eg displays etc, it can cause catastrophic cracking.

I never knew that, but it may be dependent on quantity, dilution, and how it's used. I never saw this happen, though polycarbonate and most solvents is known to be bad (polycarb cracks with time even if merely stressed by long term pressure by a fastening, unlike acrylic...)

This page https://www.twowaymirrors.com/how-to-clean-acrylic/ suggests that a 50% IPA solution with water is safe, and that's advice coming from a maker of acrylics.

If wiping a surface I'd dampen a bit of toilet paper and do it fast. I think evaporation might be faster than absorbtion so I got lucky and never saw cracking. It's also possible that evaporation causes cooling, water vapour deposition, thus rapid dilution on the surface.

It may also be that I got lucky because my usual surface-cleaning mix is about 50% water, 35% white spirit and 15% isopropanol (doesn't need to be exact) with a few drops of washing up liquid, shaken to emulsify it then thinly applied on soft paper as a wipe. I've never seen anything get harmed by that. It's good for general grime, and leaves no smell or residue if wiped clean before it can dry. I've seen it remove paint but only when the paint was already chemically compromised by grime, especially from proximity to long term cigarette smoking. (One of the uglier 'perks' of doing repairs on other people's stuff. It cured me of smoking early, seeing what it does to stuff when people don't quit..).

For pots and switches, they're usually designed to withstand solvent washes in assembled gear when new (like most board mounted parts) to allow cleaning of resin flux residue, so isopropanol is likely safe. I apply it by hand at the point of need, dripped from a thin tube. Dip tube into solvent, put thumb on other end, carry tube to pot or switch, let go tube end so the solvent drips directly out of the far end where needed, no-where else. I never let the stuff go splashing about because I don't want to worry about every material that could get hit by it.

[Lostgallifreyan]

Be very careful never to get IPA on perspex , eg displays etc, it can cause catastrophic cracking.

Ok, that really baked my noodle so I had a bit of late night fun with it...

Assuming that perspex meant acrylic, I tried four samples, 3mm thick sheet from a sign maker's warehouse. 8mm thick from some Ebay source. 12mm thick from RS Components (bar offcut from sheet), and a 10mm clear LED made from PMMA.

I dunked one end of each in 99.9% pure isopropyl alochol, about 15mm deep in a clean glass, and left them for an hour. None of these showed any sign of staining, warping, softening, clouding, crazing or cracking after that hour.

I tried a 10mm thick sheet of polycarbonate too (from RS Components). It was a damaged bit I keep around for whatever use I can find. I rubbed it clean, laid it flat with a thick wad of soft paper on it, and kept the wad swilling with IPA for about half an hour. Even that did not show any damage, and an impact test showed that it was as tough as before.

I did not expect damage from IPA, and didn't see any. I did expect at least a bit on the polycarb, but didn't see any. Whatever the reported damage is, I think it is either from some contaminant in someone's isopropanol, or a mis-identified plastic. There are reports of it all over the web, but I tried to replicate it with known grades and sources of material, and I couldn't do so.

I tried a CD jewel case for good measure, and that was fine, but looking up those suggests it's not acrylic, but injection-moulded polystyrene. That suggests that three common types of clear plastic are unaffected by isopropanol.

[ajay_m]

Well I spilt a few drops on the plastic plate of a 3d printer (not the build plate but it is underneath that) and I assume it's perspex about 5mm thick. It cracked catastrophically within 24 hours so I wonder if thickness and material stress are factors.
It's definitely a known issue, see here https://chemistry.stackexchange.com/que ... plexiglass

[ajay_m]

I also destroyed a cheap handheld green laser pointer trying to clean the pushbutton switch which was a bit dodgy. I think the adhesive holding the crystal assembly together was weakened. So yes you're wise to be cautious. I have a hypodermic syringe or two repurposed from when I had to inject anti clotting agent after an operation, now used for isopropyl alcohol and lubricant.
I still have a can of PTFE spray from the old Maplins which is amazing stuff, I wouldn't spray it on pots (I have the proper electrolube stuff for that) but for situations where you'd reach for wd40 this works much better and leaves a very thin lubricating film that lasts for a long time.

[Lostgallifreyan]

Well I spilt a few drops on the plastic plate of a 3d printer (not the build plate but it is underneath that) and I assume it's perspex about 5mm thick. It cracked catastrophically within 24 hours so I wonder if thickness and material stress are factors.
It's definitely a known issue, see here https://chemistry.stackexchange.com/que ... plexiglass

Sure, I don't doubt it exists as a 'thing', I'm just not sure what the context is. One page showed a big acrylic tube-shaped reservoir (some water-cooling widget). It had thick walls and a heavy base at one end, and I'm sure there'd be stresses in that. Cast acrylic may need annealing, and the odds are that it's not always done well. Sheet may not need it at all unless it's very thick. Formed bends made later might make it more likely to react badly to isopropanol.

One thing I noticed was several mentions that didn't specify what the alcohol was, so concentrations, adulterants, are unknown. One person had used ethanol or methanol, thinking it was IPA.

If I can find some chunks of cast acrylic, or a heat-formed bend, that fails with IPA contact, I'll post, if only to say I've seen it for myself, for once..

When cleaning electronics, the best thing would be to test on an unimportant sample with a few drops first, as usually suggested for any cleaning product, and to make sure the stuff is 99.9% pure because we can always dilute that to some percentage of water, we can't control what's in anything less pure.

Re crystals in green lasers, I heard about that before. Mind you, I read that some of them could fail on their own as the frequency doubling KTP crystal is inefficient and mounted directly on a vanadate crystal that makes 1064nm from the 808nm diode pump's light, and thermal stresses could make them come loose unless mechanically anchored together by spring pressure.

The syringe is a nice touch, those can be accurate. RS sell 2ml types, and needles without the sharp angled end cut. Still, I like the simple capillary tube because it's extremely convenient.

(I just reread your post, saying 'wouldn't' spray it on pots, but I left my paragraph below intact because it might be useful.)

With pots, I never add a lubricant film. Maybe if the bushing needed grease, a bit of kilopoise might work, but buying that stuff in tiny quantities is difficult. The problem with thin films of lubricant in pots, is that microscopic dust always gets in, and forms a grinding paste! It works for a while but accelerates the eventual decline. IPA is nice there because rapid low-force turns (or slides in faders) can sweep the track clear like vehicles forcing slush off a road. When it dries, the dust tends to stick in place, but not on the track where the wiper contact runs. It lengthens the time before it needs doing again.

[Lostgallifreyan]

I said I'd post again if I saw the damage for myself. I just did..

I found a red acrylic display stand, small thing, used to display leaflets at a dentist or chemist (based on what the white print on it suggests). It's red, formed like the slopes of an A, and has always strongly resisted pressure on the bend. I kept it because the stuff was clean and strong, and the red was in the substrate, not just a surface paint.

The outer surface of the bend looked like a candidate for internal stresses, so I laid a single sheet of folded toilet paper, soaked in 99.9% pure isopropanol, along the top of the bend. When I checked about an hour later it had a single crack I'd not seen before, and pressure to close each side of the bend caused a clean crack to run along the bend, splitting the thing in two with little force.

When I tested the sheets and offcuts earlier I tested them with strong force, even impact on the heavier stuff, and it was fine, but it's clear that if there are internal stresses in the plastic from moulding and shaping followed by little or no annealing, it's very vulnerable, and may well not show its weakness until force is applied.

Fortunately acrylic is rare in pots or switches. I've never seen it there, though some designs may use it in a dust cap.

EDIT:
I tried again on some of that red acrylic, on a flat area well away from the bend, near a corner so I had a chance of snapping it. It did not fail, it stays very tough and elastic. This seems to confirm that existing stresses make it vulnerable to IPA. I found another test piece too, from some display, it was 4mm thick and had a 1mm flange around it on all sides. This was an obvious casting rather than a rolled sheet, so I expected it might be more vulnerable. It wasn't. After more than an hour in 99.9% IPA I couldn't break it with any force I could exert by hand without hurting myself. It looks like stuff with bends and corners is the big risk.

[jimjazzdad]

IPA is fine for cleaning the heads on tape recorder heads, but for faders and all other potentiometers, I use Caig FaderLube. For switches, jacks and contacts in general, I use Caig Deoxit. I have tried a lot of electronic cleaning products over the years. MG Chemical is also good, but Caig, though expensive, is top of the heap. I am not affiliated with any of these products; take my recommendation as worth what you paid for it... I'm surprised SOS has not done an evaluation/comparison of various professional contact cleaners. (perhaps I missed it)

[ajay_m]

I believe flame polished acrylic is particularly vulnerable to attack by IPA due to internal stresses. But as you say it does seem to depend. I'm just cautious these days just in case.

[forumuser840717]

I'm with jimjazzdad on this.

I learned the hard way never to risk IPA in faders or other potentiometers. Depending on type and age, a lot of those with conductive plastic tracks will have sticky/gooey conductive plastic tracks, or no tracks at all, just a mess/stain on the board, after contact with higher purity IPA. And dilute IPA runs the risk of leaving deposits of impurities from any water used to dilute it unless very pure, preferably distilled water is used and it should still be properly rinsed with a suitable clean rinsing solution after use. IPA can also effectively 'wash' carbon tracks off their board so it needs watching - perhaps carefully test a small unused area before dripping it all over the place.

So when cleaning faders internally, it's definitely best to use something designed for the purpose or at least which doesn't contain solvents capable of melting the tracks or damaging any soft plastic components (though there aren't usually many/any of those in a fader good enough to have conductive plastic tracks).

And before dripping things into the faders, have a good look at them under good lighting. They sometimes get blobs/smears of muck or contaminants on the traacks from people tipping things into them, which can insulate them in spots along their length and cause them to drop out.

But as the original question didn't mention faders, some other relatively easy things to look for which can cause the symptoms described by the OP include dry joints (try tapping the affected channel in various places along its length and listening to whether the audio clicks/crackles/cuts in or out - if so, it's time to get a bright light, magnifying glass, and something insulated with which to do a more detailed tap around inside and start hunting), dried out or failing capacitors (increasing level will sometimes bring them back to life for a varying amount of time), really dirty switches or connectors mucky enough to be partially insulated (check switches and things like insert jacks are clean and making good contact, and unplug and re-seat any ribbon connectors and/or Molex connectors and check that they're not loose), also check any directly soldered wires, particularly ribbon cables, haven't got any dry joints and haven't suffered corrosion from insufficiently washed off flux. Power supply issues like rails intermittently failing or going too low can also cause level related audio cut outs, as described. As can other things but they might need investigation by someone more experienced in electronics.