Digitech Studio Quad repair

Problem: the four audio level meters in the LCD display showed irregular full-range signals on power up with no audio input to the unit. Also, the backlight for the large LCD was flickering.

Easy one first: the backlight power comes from a two wire cable connected to header "H4" on the main board. This was loose. A gentle bend to the pins improved the contact.

The main problem was assumed to lie in the analogue side of the power supply, as the logic and functions behaved normally. The Studio Quad requires a 9 volts, 2.2 amp AC (not DC) supply, which uses a 4 pin DIN connector, non-standard. I measured a bit over 10 volts AC here. Input filtering electro caps C87 and C61 had been previously replaced. After these came a diode bridge and four filtering electro caps (470 uF 25 volts). The solder joints were just mildly discoloured from heat, and did not appear cracked. The two +5 volt and one -5 volt regulators measured ok. The +12 volt regulator measured over 13 volts, and the -12 volt regulator measured - 9 volts that gradually increased to over -10 a few minutes after power on. There are associated small electrolytic capacitors  at the outputs of all these regulators (10 uF, 16 volts).
Using an ESR meter, nearly all these caps (except the more recent replacements) measured over 20 ohms, where they should have been around 2 ohms. Similar size and age electrolytics elsewhere on the main board, away from the PSU, measured as they should, so this was presumably the effect of heat stress.
Replacing all electros in the area of the PSU, and reflowing the solder joints to the regulators and diodes, restored a robust +/- 12 volts supply and fixed the problem.
Note: one of the replaced electros was C41, which acts as a timing capacitor for the undervoltage sensor chip next to it, marked MC34064. This was 4.7 uF so I replaced it with the exact value electro.





Below: the front panel needs to be unscrewed from the chassis to release the circuit boards, however their cables can be reconnected without the chassis in place for further testing if needed.


Comments

  1. Hello, I landed on your blog after searching for the internet for this problem, as I have the same exact problem with my digitech studio quad. Do you have, by any chance, a schematic of the board, to help me diagnose ? (I'm looking for measuring points for the voltages). I'm still learning electronics, so this is still tricky without a schematic. Anyway, thanks for sharing your experience in this blog, this is invaluable !

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  2. I don't recall ever seeing any Digitech schematics. I had to just plod along with my meter and work out what was going on. If you're new at this please be very careful around the live voltages, very easy to zap yourself or some unobtainable chip. Maybe get the pin-out diagrams of the voltage regulators (the three 3-pin chips screwed to the metal heat sink) off the internet and carefully measure what's happening on each pin, to start getting an idea of the problem.

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  3. Hi!
    Thank you so much for the post.
    I have the same issue regarding the LCD backlight. But I tried your solution with no success. I also do not have any lights on the buttons.
    Can you please help with this?
    All the best

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  4. Hi Toklant, can you firstly confirm you are using the correct PSU - AC not DC? Secondly, does the unit operate otherwise normally? Audio input signals are being seen on the LCD meters and you can edit the effects and hear effected audio output?

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    Replies
    1. I have this exact problem. Everything is working just fine, just no backlight on the screen or buttons. I am sure that a screw from the screen assembly that came loose and boucned around is the culprit for whatever cause the backlight to go out.
      Please tell me someone knows which component is most likely to be causing this.

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    2. If it's just the backlight gone, and all else works as per normal, then check to see if the white plastic connector with 2 pins marked "H4" is loose.

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    3. Thanks for your quick response, Adam! Well, I've made a visual survey of the main board and there are no visable discrepencies and no loose connectors, only the missing screw of the front display assembly. I will remove the front and take a closer look.
      However, the right ribbon cable is glued to the main board and won' t come off easily; is it safe to use force or should the glue be heated and removed first, do you think?

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  5. Hello, do you know something about "ERR4"?

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  6. Sorry I missed this post earlier. Can you give more details? I can't recall seeing it.

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