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| Repair Line6 Spider III Guitar Amp |
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| obiwanjacobi:
I am trying to repair a Line6 Spider III (30 watt) guitar amp (schematic is attached). It is a digital 'frontend' with an analog power amp (TDA2050). When I started the whole digital part lacked a 3.3V power rail and so I replaced the LM317T. That resulted in a 3.1V rail but the digital part still didn't start up. I scoped out the MASTER_CLK and it did wobble a bit but was not making a full 0V-3.3V swing. The MASTER_RESET remained low. I think U3 (is an MCU) will only release the reset when the clock is stable? Anyway my question is what could make the 3.3V rail be so low (3.1V) and does that have an effect on the oscillator circuit not working properly. - or what (other things) could cause the oscillator circuit to fail. Another thing I noticed is that the power amp IC (TDA2050) is getting hot quickly when running the amp with all volumes at 0 and loading the output with a head phone. Could a malfunctioning TDA2050 cause problems on the digital section? (I can not see it would) |
| Vicus:
Something maybe is loading the rail. |
| pcprogrammer:
Tricky one. The master clock is not super high frequency (12MHz) and not being full swing should not matter that much as long as it reaches the specs of the MCU and DSP clock input. The DSP is from the 56000 series, bit of an oldie. Did you look into the data sheet of that DSP? The master reset not being released can have several causes. Best test might be to see if the FLASH (U4) is being addressed. Easiest with a logic analyzer (cheap 8 bit 24MHz one will do). That one can contain the code for the DSP, but it might also be that the MCU uses it for its own code to. Depends on what type it is of course, but did not see it in the schematic. Might be a 8051 based MCU. There are quite a lot of components loading the 3.3V rail, which makes it hard to figure out what might be the problem. Do you have some good pictures of the main board? |
| obiwanjacobi:
--- Quote from: Vicus on June 23, 2024, 06:40:12 pm ---Something maybe is loading the rail. --- End quote --- Yes, that was my initial thought as well. I desoldered the two main supply smoothing caps to see if that would bring up the rail voltage - it did not. The rest is all SMD and I lack the tools to work that effectively (I only have a soldering iron). (I do have some experience soldering SMD but not so much desoldering -without destroying the part) --- Quote from: pcprogrammer on June 23, 2024, 07:30:36 pm ---The master clock is not super high frequency (12MHz) and not being full swing should not matter that much as long as it reaches the specs of the MCU and DSP clock input. --- End quote --- Yeah - but now it sort of oscillates in a +/-500 mV range somewhere around the 2V level... So no way any logic IC is gonna see that as a clock. --- Quote from: pcprogrammer on June 23, 2024, 07:30:36 pm ---Did you look into the data sheet of that DSP? --- End quote --- No. I figured it did not matter much as long as there is no clock and the reset stays active... --- Quote from: pcprogrammer on June 23, 2024, 07:30:36 pm ---The master reset not being released can have several causes. Best test might be to see if the FLASH (U4) is being addressed. Easiest with a logic analyzer (cheap 8 bit 24MHz one will do). That one can contain the code for the DSP, but it might also be that the MCU uses it for its own code to. Depends on what type it is of course, but did not see it in the schematic. --- End quote --- I interpreted it as the MCU being fed the board clock and having startup code that releases the master_reset (that's how I would implement it)... I'm pretty sure nothing is accessing the FLASH (U4) when there is no clock and reset is active - but I haven't measured it. It's all SMD so I'd have to solder on some wires to be able to measure that. --- Quote from: pcprogrammer on June 23, 2024, 07:30:36 pm ---Do you have some good pictures of the main board? --- End quote --- No - I couldn't find any online - so I will make some (when I get home). What do you hope to learn from the photo's? BTW: I also desoldered the power amp IC (TDA2050) that was getting warm without any load. I figured I could tackle that problem later (I have some in stock). |
| pcprogrammer:
Maybe there is a part number on the MCU that can provide some insight. And pictures are always nice. :) But with the master clock not being up to par, you are right that not much will work. Did you scope the power supply voltage itself. Is it stable at this 3.1V or is there a large ripple on it? I noticed in the schematic that it is possible to desolder a jumper that brings the master clock to the MCU. Does the clock improve if you take this jumper out? The MCU might be able to start up with an internal clock and have a boot loader that needs to load the main code from the flash first, and then check if the master clock is operational and release the master reset if so. There are many scenarios to think of, but without knowing the actual parts used it is just guessing. Is there a way to easily separate the analog amplifier part from the digital part, supply and signal wise. That way the problems could be investigated separately. |
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