What's wrong with my rackmount effect box?
August 7, 2014 9:59 PM   Subscribe

My Digitech Studio Quad 1u rackmount box gets unhappy over time (hours). As it warms up, the input levels drop and it becomes unusable. Turned off for a few hours, it comes back to life. Is this terminal or repairable? Should I get something new? Is there a comparable product (multiple ins/outs, flexible routing)?

I have a Digitech Studio Quad, 4 in 4 out rackmount unit. Purchased new approx 15 years ago. Has been in storage for 10 of those 15 years. It is connected via send/return to a Mackie 1402 mixer.

It works perfectly for a few hours when turned on cold. Once warmed up the inputs go silent unless given a very high volume signal which seems to 'shock' the gain back to the correct stage. When use continues, the input levels will slowly drop until shocked open again. Some other glitching occurs but may be because of overload inputs.

Is there an explanation for this behavior? Is it repairable? Am I using incorrect cabling/gain staging?

Thank you!
posted by erebora to Technology (5 answers total)
I'm not familiar with the Digitech Studio Quad, but 15 years is the right time for the power supply capacitors to need replacing. The high-volume signal may be refilling a week capacitor, which then drains back down.

If you have done DIY electronics project before AND YOU KNOW HOW TO SAFELY WORK WITH POWER SUPPLIES WHICH LIKE TO KILL UNWARY PEOPLE, it is pretty easy to do the repair yourself. If not, it is a quick and easy repair for a synth tech to do.

Keep in mind that one of the ways that capacitors are graded is by the number of hours they are expected to last. There should be only a handful of caps in the power supply, so it is worth asking for the higher quality caps that may cost dollars rather than cents so that you don't need to have them replaced again in 5 years.
posted by b1tr0t at 10:09 PM on August 7, 2014

I'd say it's more likely to be caps in the signal chain than in the power supply. Bad power supply caps would probably manifest as noise or rebooting/resetting.

Note that some capacitors in the power supply will be charged to voltages substantially higher than line voltage (380v is common). Also it's fairly likely they will remain charged for quite a while after the device is disconnected from the outlet. So when people say you should know what you're doing, they mean you should really know a bit about power electronics, not just that you spent an hour on google and wikipedia.
posted by ryanrs at 10:44 PM on August 7, 2014 [1 favorite]

honestly i've never seen a bad cap in something that new. i'd bet on a cold solder joint, possibly even on one of the IO jacks. i have audio stuff from the 70s that hasn't been recapped that's still fine.

i also think everyone is overstating the scaryness of power supplies in stuff like this. it would shock me(heh) if it didn't just have a bleed resistor on any of the big power caps. try and turn it on when it's unplugged, and then just leave it for a few hours. it shouldn't be some dangerous person-killer. it's actually unusual for stuff to not have bleed resistors unless it's really old or weird.

The annoying part here though is finding the bad solder joint, if all the caps are fine. i've experienced this exact problem though with more than one piece of gear(the "loud signal cuts through" thing piqued my interest(, and it was a shittily soldered stuff, or in one case a blown out transistor.
posted by emptythought at 11:09 PM on August 7, 2014

Hang on a sec! This device doesn't have an internal power supply. It has an external brick, right? This is good news.

Normally I'd say try replacing the power supply, but that power supply is weird and it's probably difficult to find a replacement. Also, since the output is AC, I bet it's just a simple transformer. In other words, I doubt it has any capacitors inside to go bad. Your power brick is probably fine.

So the likely culprits are caps in the signal chain or the low voltage (safe) power regulator inside the 1U chassis.

If you're competent at soldering (and especially desoldering, which is harder), and you want to do this on your own, I'd be happy to help you. You will need a decent digital camera that can take good macro photos (your cell phone is probably not good enough). If you are good at soldering and taking things apart (and reassembling them), then it will take a maybe an evening or two of work and $30 to refurbish the caps in your device.

Send me a mefi mail if you're interested.

On preview: emptythought, I've definitely come across switching supplies without bleeder resistors on the PFC bus caps. First gen Apple Time Capsules, for one.
posted by ryanrs at 11:12 PM on August 7, 2014 [1 favorite]

Thanks all! Yes to the external brick question, and yes to it's custom nature (odd connector plug). I'm not too skilled at soldering so I'm leaning toward passing this to a pro for evaluation. I greatly appreciate the offer to guide me ryanrs but I think I'll build up my solder skills on something less expensive!
posted by erebora at 10:16 AM on August 8, 2014

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