A&H GS3000 mpu problem

Discussion in 'Tech Talk - Electronic Repair Modifications DIY' started by Kevin Porter, Mar 16, 2017.

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  1. Kevin Porter

    Kevin Porter Active Member

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    I have a GS 3000 console turn of the century stuff. The Mute mpu board has gone out part 002-347-1 so all the mutes seem stuck off and mute and status leds are stuck on. No LCD. Everything else works.
    Need to find a replacement or figure out how to fix it. The pcb makes use of a P80C32 40 pin chip and what appears to be a 5v power supply.
    Any ideas?
     
  2. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Asking @Boswell seems like the Wise thing to do ;)
     
  3. Boswell

    Boswell Moderator Distinguished Member

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    I've never had to dive inside that particular model of mixer, but I have done servicing on many other A+H consoles, and know that there is a fair degree of commonality between ones of a similar vintage.

    I don't know what type of test equipment you have access to, but the main things to check on that board would be the 5V power being present (multimeter), the crystal attached to processor pins 18 and 19 actually oscillating (oscilloscope), the reset line on pin 9 not being held low (oscilloscope), as well as checking to see if there is any activity on the external EPROM address and data lines (oscilloscope). After that, it's a matter of removing the board from the mixer and carrying out a detailed visual check. This should include applying a light sideways prod to every pin of every component with the sharp end of a scalpel blade to check that the solder joints are still good. Ideally, this needs to be done under an assembly microscope, or at least a reasonably powerful static magnifier, so that even small displacements of the pins under slight pressure can be seen, and also the quality of solder joints inspected.

    Another of the things that can happen with age in this type of gear is that the EPROM device that holds the program instructions can start to fade, causing corrupt data. Erasing and re-writing the part normally gives it a few more years of life, but the problem is that unless you have an alternative source of program data, you can't guarantee that what you read out of the fading one is correct, and you end up reinforcing the flakey data.

    Perhaps you could indicate the sort of actions you feel you would be able to carry out on the board, and I could advise further. It would also be useful if you could post a close-up photo of the board.

    Good luck!
     
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  4. Kevin Porter

    Kevin Porter Active Member

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    Thanks for your thoughts. Much appreciated. I have more test gear than brains. I have a TEK 465 with multimeter built in. It a bit wonky but works. I know very little on how to use it effectively.
    Best i can tell is the 16v +- and 48vpp from the external PS power supply appear inside the console and powers everything up. The MPU feeds off one side of the 16v supply into a 5volt regulator circuit on the MPU. This 5v seems to drive the whole MPU system. The main brain i guess is the P80C32 which is the chip you are talking about.
    OK so i will test for a ripple free 5v and try those oscilliscope tests you mentioned. I have no idea what im looking for but i assume i ground my probe to the chassis and clip the positive on pin 9, 12,13, 18,19, 40 and report back what i find.
    I have a couple of bin files i got from the A&H site v1.11 and v1.12 which appear to be the program info???
    I will get a few pics up tomorrow.
     
  5. Boswell

    Boswell Moderator Distinguished Member

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    I've located the schematics for the GS3000: here. The MPU board I think you are referring to is shown on pp16-17 of the document, with a positional drawing on p18.

    A couple of quick questions for you:
    (1) could you confirm that the 27C128 EPROM chip IC17 is fitted in a socket?
    (2) what voltage is there on the BREAK test point P1? I don't need the exact value; it will be either near 0V or near 5V. Test point P1 is located between pin 1 of the MPU and the transistor Q3.

    As you mentioned, A+H give a pair of .bin files for this mixer on their site. The contents of the files include strings showing they are version 11 and version 12 automation files. Since the MIDI controller is also on this MPU board, it's likely one of these would be correct for re-programming the board's EPROM, if it comes to that.

    If the EPROM chip is fitted in a socket, it may be worth re-seating it when you have the MPU board removed from the mixer. Use a twisting motion of a small flat-bladed screwdriver to pry the two ends of the chip alternately upwards, lifting it a millimetre or so, but don't allow any pins to jump out of their socket contacts. Take care to ensure you insert the blade of the screwdriver between the chip and the socket and not between the socket and the circuit board, as is easily done. After the slight lift of the EPROM, use your thumb to press the chip back into the socket, applying firm downward pressure alternately to the two ends.
     
  6. Kevin Porter

    Kevin Porter Active Member

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    Yes the 27c128 Eprom is on a socket.
    I have P1 Break as being pin 12 or(p3.2 int0) on the P80C32, not pin 1 and going to Q15 instead of Q3. I am guessing Q3 and Q15 are part of a PS monitoring circuit that send a signal to the chip to turn off if there is a PS issue. Is this correct??
    I will try and get a voltage measurement at P1 BREAK later as my VOM is not here right now.
     
  7. Kevin Porter

    Kevin Porter Active Member

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    OK Very interesting stuff,, Bozwell gets a medal.
    So in manipulating the IC in the socket as you suggested the mixer now boots like normal and all Status lights are off and the mutes work. So i consider it a fix thus far.
    I also checked the voltage at pin 12. So when i would power the power up it would go to 4.78 volts for about 2 seconds and then fall to .22 volts. Lights on the front flickered as well for about 2 seconds prior to going into freeze mode.
    After messing with the eprom i get a steady 4.8 volts off pin 12. I think i learned something.
    Bozwell,, would you mind PMing me your address please. I would love to send you a little blessing!!
     
  8. Boswell

    Boswell Moderator Distinguished Member

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    Glad that fixed it, Kevin.

    No need for individual blessings - the reward for any of us here is that we occasionally make a significant improvement to someone's situation.
     
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  9. Kevin Porter

    Kevin Porter Active Member

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    Its been a while. The repair lasted over a year but the other day the same problem happened again. Its amazing what you can forget in a year and i have had to re familiarize myself with this issue. I am going to try the same trick that worked last time but i need a more permanent fix. I can live without the mute system so wondering if there is a way to bypass the whole midi mute system?
     
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  10. Kevin Porter

    Kevin Porter Active Member

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    I find it amazing that the mixer operation depends on the 5v logic being operational and that a circuit board the size of a pack of smokes can render the entire mixer a paperweight. I contacted Allen & Heath again in the hopes of getting a replacement mpu from somewhere or a componet level repair on the mpu i have.
     
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  11. Boswell

    Boswell Moderator Distinguished Member

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    If the fault has the same symptoms as previously, it's worth going through the EPROM re-seating process again. Mild corrosion on the chip legs will probably get worse over time, and, short of fitting a new socket and copying the EPROM content to a new chip, there's not a lot you can do apart from regular re-seating. Applying a small amount of contact cleaner to the chip legs at the start of the re-seating operation could stretch the time between failures.

    Without attempting to find the precise detail of the start-up operation of the MPU, I'm not able to tell whether removing the MIDI board and hard-wiring the pins on the connector is a possible work-around. My guess would be that it would not leave a workable mixer.
     
  12. Kevin Porter

    Kevin Porter Active Member

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    Thanks for the quick response. Well seems the best thing to do would be to have the mpu board serviced and programmed to restore proper function. Who does that kind of work and can it be done without the mixer on hand?
    I have no idea who to approach on that issue. In the meantime i will try the re seating method and hope for the best given it has worked in the past.. Mixer is still not torn down yet. Might get to de wiring it tonight which is the biggest part of the job.
     
  13. Boswell

    Boswell Moderator Distinguished Member

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    If simply re-seating the EPROM restores normal operation (at least for a while), then having the MIDI board serviced would be a relatively straightforward thing to do. You would have to schedule this for a slot in your usage, as you would not be able to operate the mixer while the board was away.

    My suggestion to any local service engineer would be to replace the EPROM socket with a turned-pin type, and also copy the EPROM to a new device of the same type, if one can be found that has been stored vacuum-sealed so that it does not have bad oxidation on its pins. I would offer to do the job for you, but it may not be worth sending the board on a return trip across the Atlantic.
     
  14. Kevin Porter

    Kevin Porter Active Member

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    Thanks Bozwell, you never know i may take you up on it at some point. As it turns out one of the A&H distributors has an MPU so im probably going to buy it asap so at least i have a working one. Seems its prudent to have a couple around. My mixer had this issue brand new out of the box and they replaced the MPU on site. That one has lasted well but the same issue has happened since three times now, so its a weak point in the design. I put a few requests out locally for someone to fix the old one, if i cant find someone i will send it to you. I must add you have been extremely helpful thus far. Thank You!!
     
  15. Boswell

    Boswell Moderator Distinguished Member

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    OK, and good luck! I look forward to hearing how it works out.
     

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