Boss Tu-2 Repair  [SOLVED]

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munkeyboy
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Post by munkeyboy »

So I got this pedal in a used package buy and would like to save it if possible. This pedal isn't showing the notes. It turns on and passes audio, but that's just it. If it's turned on it should mute the audio. I've gone thru the testing procedures outlined in the service manual and everything returns good including the eeprom.

Any clue where I start to look for bad components? I figure a clue is that the audio doesn't mute, I just don't understand the switch logic where that occurs. If it happens inside the CPU, then I guess this is a lost cause. But hopefully it is one of the transistors or the M5218AL unit.

Thanks in advance!!
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stolen
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Post by stolen »

Hi!

never saw the schematic for that one, thanks for sharing! Yeah, the audio mute is a good clue: When the pedal is in bypass, the signal to the tuning circuit is muted, so there's a good chance that the digital section is fine and that there's something up with the control signal for the FET switches. You used the output jack, not the bypass jack, right? Signal is always present on the bypass jack. If you have a multimeter, it's easy to go through some points to see where the failure occurs:

The FXC line is the general switching signal. It's one of the 5 signals between the MAIN PCB ASSY and the CPU PWB ASSY boards. Find a convenient connected component to grab it from and see if it switches high and low correctly. If it doesn't, it might be that it's shorted somewhere; this is usually a pain to search for, but disconnecting the line from the main board and measuring directly on IC3 might be a good start.

If FXC switches correctly, the collectors of Q3 and Q2 are the next suspects: Q3 should switch opposed to FXC, Q2 in unison, just with the HIGH signal at 8.4V. If that is behaving properly, check the gates of Q8-10 if they follow their respective control signals accordingly. Lastly, Q4 switches power for the preamp circuit; with FXC low, the collector of Q4 should be at 5V.

If you have a scope or audio probe you could just see if the ZC line carries audio when the tuner is active and there's signal at the input. If it does and the microcontroller still won't recognise audio, it might get quite messy.

Good luck!

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munkeyboy
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Post by munkeyboy »

Awesome instructions @stolen, thank you.

So I had already done a quick and dirty test of that ribbon cable. I did not hear audio coming thru on ZC, but then I didn't really know what I was looking for. Let me run thru your instructions and I'll post back.

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munkeyboy
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Post by munkeyboy »

So FXC had no voltage when switched on and Q3 emitter has positive voltage instead of being grounded. Looks like someone accidently cut the ground trace by "1" wire. jumping that cut however, isn't fixing the overall issue. more to come once I get a more permanent jumper over that cut.

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munkeyboy
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Post by munkeyboy »

That was it. Just needed that cut trace fix. Wish there was a clean way to fix that instead of a wire soldered on the back, but hey it works awesome now.

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Post by stolen »

glad it worked out :D!

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