Digitech Footboard - no workee (mentor needed! - have schematic!) - diyAudio
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Old 13th February 2009, 12:50 PM   #1
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Join Date: May 2008
Default Digitech Footboard - no workee (mentor needed! - have schematic!)

Hello there,

I've got a Digitech guitar floorboard effects unit that doesn't work!

The unit lights up, but the dual 7 segment LED show nothing & the footswitch selector LEDS are all on (& they don't change when the associated footswitch is pressed).

I've tested the mainboard onboard power...all the regulators are outputting as they should be (12V, +12V & +5V)....in short, I don't think this is a power problem.

Alas, I have very little troubleshooting experience with digital chips. Last night I was poking around trying to glean what I could...I thought the LED driver chips would be a good place to start (on account both the 7 segment LEDs aren't showing anything). From what I can gather , there doesn't appear ro be a clock on any of the multitude of 74HC574 chips that are in that vicinty. That said, I'm no expert, but when I scoped the clock pin, I would have expected to see a high frequency square wave. There was nothing on the clock pins except VCC.

I've since got the diagram (phew),...

http://freepdfhosting.com/64512fb9b0.pdf

but I need a bit of help! (I have experience fixing analogue type circuits, have a scope & multimeter etc)

On the circuit entitled sheet 9 of 10 (has the footnote Analog Control/LEDS), running from top to bottom for the clock pins of U26A, U27A, U28A, U13A, U30A, U32A have the inputs OUTEN1, OUTEN3 OUTEN0 OUTEN6 OUTEN2 & OUTEN7 ...there are no clocks present on those pins.

Edit: I've just found where those clocks are derived (sheet 7 MCU/EPROM/SRAM) U6A, which in itself is fed from a Z84 chip. Are there any common faults with Z84s?!

I willl be giving it another go this evening & will obviously check the Z84 clock, ground & VCC (likewise U6A)....anything else I should keep in mind for approaching this part of the circuit? I see the Z84 has a reset input, which is fed from the lower right part of the same sheet...what's going on there, because the lower right reset circuit seems to have no input as such?!!!
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Old 13th February 2009, 11:42 PM   #2
mjf is online now mjf  Austria
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Join Date: Jan 2009
Location: Austria
hello.
floor board effect.
perhaps it is a bad solder point or a " hair rent".
if a ground or power supply contact is lost perhaps all leds light up or not..........
i would look after this .
after that searching for a damaged opamp.
perhaps it can help you.
greetings.................
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Old 14th February 2009, 01:33 AM   #3
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Join Date: May 2008
Thanks for the feedback.

Ok, managed to fix it this evening (after about 3.5 hours)... about 6 *different* faults!

1. Onboard 12AX7 valve faulty - top cracked (fortunately this is a common valve & my mate has one going spare!)

2. VCC on some chips should have been 5V - however only 2V present. Wired a jumper from the regulated 5V VCC rail to those places where it was too low.

3. & 4. (related) ... two clock signals missing from the EPROM chip (pin 20 & 22)...once again, jumpered some wires!

5. A few dry solder joints around the wall wart PSU mainboard connector.

6. Finally, there were some pins shorted on the main codec chip....removed short!

Unit is working now! (even without the aformentioned valve, as that's only used on certain programs)

Wouldn't have been able to fix it without the schematic though (which I managed to lay my hands on this morning)

Cheers,
Hank.
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Old 14th February 2009, 05:08 AM   #4
SQLGuy is offline SQLGuy  United States
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How on earth was this thing ever working? Did you get it from someone else that had been attempting to repair it?

Congratulations on fixing it, by the way!
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Old 14th February 2009, 09:21 AM   #5
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Join Date: May 2008
I bought it as faulty on Ebay - with the intention of using the chassis & footswitches for a totally different project (ie housing a different mainboard & reutilising the footswitches to interface with the new board)....however I was annoyed with myself for not realising that the 10 footswitches would be the micro type built into the main circuit board, rendering it fairly useless for its new intended purpose!

I therefore thought, I'd have an attempt at fixing it. (I reckoned I'd need a bit of help on the digital aspect of the board - but fortunately, in the end I was ok 'flying solo' with the schematic)

The unit's external condition (surprisingly) is as new, it's just inside that had problems.

It's one of those multilayered PCBs, & I guess one of the inner layers has taken some form of 'hit' (hence the need for jumper wires).

Before I had the diagram, I thought the VCC of 2.5V was correct (the datasheets for the chips suggested that low level was in spec). But when I got the schematic it showed that it was meant to be 5V.

I couldn't actually find/see the break (like I say it's multilayered), so I soldered a 5V VCC jumper wire to one chip (SMT too!) & they all sprung to life (which makes sense as they all have a common VCC). This still didn't sort it, so therefore has a poke around making sure the clocks & derivatives of the clocks were available on all chips that needed it - they weren't. There's an EPROM...it had no clocks - two more jumper wires! And so on.

I'm quite pleased with myself as I'm not 'match fit' when it comes to fault finding (in other words it's been a long time since I last did this stuff)
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