Trying to fix a DJM-500 audio mixer that I broke

Discussion in 'The Projects Forum' started by lorenjan, Feb 15, 2016.

  1. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    I have performed several fixes in the past on my Pioneer DJM-500 (resoldered some pots, fixed a temperamental master volume attenuator pot, internal cleaning, tightening loose pots, etc).

    This time, I got too greedy. I took it apart to try and convert the phono-only channel 4 into a line level channel, but I never got that far.

    I had a lot of trouble trying to get one of the ribbon connectors to let go of one of the 15-pin ribbon cables, specifically the one connecting the terminal assembly PCB to the phone assembly PCB. I pulled too hard and frayed the cable, and was worried that I messed it up, so I plugged it in while it was open (my biggest mistake) to make sure it still worked. At first, the mixer did work normally, but the ribbon cable socket looked a little loose, so I pressed down on it with my finger, at which point I heard a half-second BUZZ and then nothing more after that.

    My best guess is that I shorted a couple of the voltage supply lines that were coming into the terminal assembly PCB and probably blew out some components in the process, but that's an uneducated guess. I bought a multimeter and poked around, but wasn't able to see anything super obvious. (Then again, I don't really know what I'm looking for.) I rewired the ribbon cable by using jumper wires and soldering each one directly to the board, but the problem remains.

    Here are the current symptoms:

    1. The master and channel meters go back and forth between illuminating the bottom LED and not. It's not really even a flicker... they stay on for a while and sometimes turn off and back on. Even with signal coming in to any port on the mixer (including either into the terminal PCB jacks or the phone PCB jacks), there's silence and the signal doesn't register on the meters.

    2. The effects board and all the lights and buttons seem to work normally, and respond the way they should. No "error codes" or dodgy behavior or anything like that.

    3. With a multimeter, I get what appears to be signal coming into the places I'd expect (line level resistors and capacitors, phono preamp IC terminals, and so on).

    4. Also with a multimeter, the ground sleeves on the master out and booth monitor terminals measure 1.063 V, which I find surprising. When I flip the terminal PCB over, the ground in that section of the board also measures 1.063V, including what's going into the muting transistors marked Q801-Q804 (part number 2SC2878).

    I am not circuit-savvy but I've always wanted to learn. I figure the mixer is probably toast, but this might be a fun learning experience, so thought I'd give this forum a shot to see what we can find. I'm attaching the two service manuals that I was able to find, both of which contain different types of schematics and PCB diagrams. I can also take photos of whatever would be helpful, so let me know.
     
    Last edited: Feb 15, 2016
  2. MrChips

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    Oct 2, 2009
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    You're off on the right track by locating the service manual. Then you acquired a multi-meter. That's good.

    Next step for us to be able to help you, take an overall photo of what the unit looks like inside.

    Then take a photo of the connector that you pressed on that caused the trouble.

    Try to take photos with a proper camera and not with a smart phone.
     
  3. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Attached are photos of everything that I think might be relevant. (The photos are pretty big, so I'm not pasting them inline.. I will edit this post though if you'd rather I do so.)

    The thing I touched was the right side of the 15-pin ribbon connector marked "CN151" (part number KPE15) on the top of the terminal assy PCB (now visible as a bundle of messily-soldered jumper wires on the lower left side of the terminal assy photo), and I pressed down on top of where pin 2 is.

    I'm also attaching photos of both sides of that connector which is obviously now de-soldered from the board.
     
  4. MrChips

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    Oct 2, 2009
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    Excellent.
    Now the first thing you will do is drag out the DVM and measure the supply voltages at CN151 with respect to GND.

    Pin-1 V+BB +15V
    Pin-2 V-BB -15V
    Pin-3 V+8A +8V
    Pin-4 V-8A -8V

    The metal chassis should be GND.
     
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  5. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    I tried a bunch of times to make sure that these are actually the readings (because they're really far away from what I'd expect given what you listed in your last reply), and the readings at CN151 are...

    pin 1 - V+BB +1.339 V
    pin 2 - V-BB +0.627 V
    pin 3 - V+8A +117 mV
    pin 4 - V+8A +31.5 mV

    Just to make sure the multimeter is working properly, I also tried measuring W111 and W112 on the power assy, which are labeled "V+5" on the board. The readings for those are +5.07 V and +4.99 V, respectively.

    Out of curiosity, are things labeled "V+BB" always +15V? Or did you have to collect more info to find that out?
     
  6. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Mistyped... Pin 4 is V-8A.

    One more data point: There is a jumper on the power assy PCB, W115, also labeled V-8A. That one also measures +31.5 mV.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    Houston, we have a problem!
    Yes, your voltage readings are way off.

    First thing is to make sure that you have a proper ground connection to the black (-ve) lead of your DMM.

    Three things you can do:

    1) With the power off and AC plug unplugged. Set your DMM for lowest resistance range. Measure the resistance between the AC ground plug and the metal chassis.

    2) Measure the resistance between the metal chassis and any one of the outside ring of the RCA jacks (any of the jacks on the back).

    3) With the power back on, measure the above voltages with respect to the outside ring of any of the RCA jacks on the back panel.

    (+BB and -BB are not common labels for power. It can be any voltage. Voltages are specified in the schematics.)
     
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  8. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Ok, here goes!

    1. The resistance between the ground prong and the place on the chassis that i was using as ground (an empty top panel screw hole) measures between 3.7 and 4.3 ohms (it goes back and forth). Also, I had no idea that this thing beeps! The beep stays on while I'm touching those two points.

    2. Any of the input RCA jacks on the back measure between 4-5 ohms (not quite reliably, the beep turns on and off, which I think is just because this probe kind of sucks and has a hard time measuring the rounded jacks...the grounding plates around the screw holes between the jacks do measure reliably). I can't get anything around the four output RCA jack sleeves to beep at all, though.

    3. The four pins measure the same when measured using an input RCA jack sleeve as ground.

    Bonus: The ground sleeves on the output RCA jacks still measure +1.063 V when using the input RCA jack sleeves as ground.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    I think two fuses IC556 and IC557 on the Power Supply Assembly are blown.

    With the power off and the AC cord unplugged, measure the resistance across IC556. Do the same for fuse IC557.

    If you do not get close to 0Ω reading, the fuse is blown.
    (Touch the two DMM leads red and black together and use that as a reference of what a short should look like. A fuse should not be much different from that.)

    You can make a temporary fuse by stripping a piece of stranded cable such as lamp cord and use single strand as a fuse. Solder this across the blown fuse for now. If that brings the unit back alive, you will have to order proper replacement fuses, ROHM ICP-N15 600mA/50V fuse.

    Pioneer DJM-500 PSU.jpg
     
    Last edited: Feb 16, 2016
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  10. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    I am truly impressed that you were able to figure that out so quickly!!

    I removed the power assembly board and tested the fuses. (Had no idea those were fuses.) Sure enough, IC556 and IC557 were blown. To make sure I was testing correctly I also tested IC555 and IC558, and those both registered about 4 ohms.

    I bypassed each of IC556 and IC557 using two single strands from a 16 gauge wire and the mixer works again. Pins 1-4 on CN151 now measure what they should, too. I'll order those fuses right now.

    One more question: I'd like to clean up the CN151 area with something more "long-term". Do you have any suggestions? Is it possible to find a replacement connector that would fit in its place, along with a new ribbon cable? Or should I just remove the ribbon from both boards and solder point-to-point wiring between them?
     
  11. MrChips

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    The right connector might be hard to find.

    If the ribbon cable is long enough, I would just strip fresh ends and solder directly into the board.
    If the ribbon cable doesn't quite give you some room for movement, I would solder extra lengths of flexible wire on the ends as you have done but cover with heat-shrink tubing instead of masking tape.

    I would avoid having to mess around with the other end of the ribbon cable.

    Glad you were able to get the unit working again.
     
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  12. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Ok, this is a surprising turn... The mixer does play audio now, but the faders don't work the way they did before.

    I've plugged one channel of audio into one line input, and all four of the channel volume faders affect the volume of the one channel that's receiving audio. I can achieve the loudest output volume by putting all three of the unused channel faders to anywhere between 3 and 7, and the channel that's receiving input's fader at 10. If I change any of the faders from those settings, the sound gets quieter.

    When the crossfader switch is switched on, the sound gets quieter, but the same channel still plays through to master, no matter where the crossfader is. When the crossfader is in the middle, the sound is louder than when it's on either side, but still not as loud as when the crossfader switch is off.

    I'm going to take it apart again and see if I notice anything obvious.
     
  13. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    I checked to make sure I didn't accidentally solder one of those cables to the wrong spot on CN151, but they all look correct.

    Seems like no matter which input I plug into, fader behavior is the same. The built-in DSP effects seem to work fine, though.

    I'm going to study the circuit diagrams related to the faders ("VR ASSY", not sure what VR stands for) and measure some spots on the board to see if I can find any questionable values. I did some searching on this forum but didn't find anyone else with a similar-looking problem... I'll keep searching though.
     
  14. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    One more piece of data: when I first soldered the single strand jumper wires to IC556 and IC557, I messed up and the wire across IC556 was a cold joint. When I powered it on the first time, all the LEDs on the level meters lit up, so I powered it back off and resoldered, and then it powered on normally the 2nd time (except for this faders issue). Not sure whether that first power-on with positive but no negative voltage could have damaged some of the components.

    I found someplace online that mentioned that channel crosstalk could be caused by a bad ground connection.. I'm not sure whether this counts as a crosstalk problem, but I probed around for grounding problems and everything I could find labeled "ground" has continuity to the chassis.

    A couple more things I noticed are that the master fader works normally, and also that the channel faders deliver almost no volume unless they're near the very top of the range. Before this happened, the volume curve was a lot more smooth on the channel faders (and the crossfader worked normally). Also, the two crossfader assign selectors seem to have no effect on the sound.
     
  15. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    New info! I moved the top part around a bit (the faders and knobs assembly) and the behavior has now changed. I'm guessing that the change was caused by a loose connection somewhere, although nothing seems loose when I look at it.

    Before the change:
    - plugging in to any channel would cause bleed onto all other channels (this bleed was visible on the meters as well as audible)
    - faders had strange behavior (on the channel receiving audio input, unless the fader was all the way up, the sound was very low; on the channels not receiving audio input, they had to be between 3 and 7 (where 1 is the bottom and 10 is the top), or the playing channel's sound would be quieter.
    - the crossfader, when switched on, only made the volume quieter, although it was slightly louder if the slider was in the middle vs at the left or right.

    Now, after the change:
    - With the crossfader switched off, all channel faders behave normally, and there is no channel crosstalk.
    - With the crossfader switched on, the crossfader works normally, but the crossfader assign selectors don't work normally. Mostly they have no effect, but if I put audio into one of the channels, turn the bass pot all the way up to get a really hot signal, and then change one of the crossfader assign selectors back and forth for several seconds, i'll hear crackling, and then the side of the crossfader that corresponds to the selector I'm moving will eventually select the channel receiving input, and it will stay assigned to that side of the crossfader (even across a power cycle) until I repeat that process with a different channel. Right now the crossfader is on channel 2 on the left side and channel 3 on the right side.

    So the behavior is almost back to normal, except for the strange behavior with the crossfader assign switches.

    Pretty bizarre.
     
    Last edited: Feb 17, 2016
  16. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    I tested pins 2 and 13 on IC21, which is the first place that the assign switches connect to after coming in to the DSP assembly PCB. They read normally, so the selector signals are at least getting that far. It looks like IC21 changes those to digital signals (maybe?) and sends them to a unit that's possibly a CPU at IC14. I'm not sure what happens after that, though.... yet...
     
  17. MrChips

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    Oct 2, 2009
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    Based on the symptoms that you have provided, I was thinking that the analog mixer chip, IC1 TC9164AF on Sheet 4 is faulty.
    Before doing anything drastic, I would suggest removing and reinserting connector J114, assuming that the connectors are designed to be easily unplugged.

    When removing any type of connector, do not pull on cables. Always pull on the body of the connector. Check first if there is a retaining clip or tab.
     
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  18. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Since I last posted, things reverted naturally to "state #1" above (lots of channel crosstalk). I removed and reinserted J114 from the DSP ASSY side (labeled CN114). (It's soldered in at the J114 location). That didn't seem to help. I also tried removing the cable entirely and powering back up, and that didn't change much either, except it did disable the crossfader switch and perhaps some other controls. (I'm guessing the "STB" pins on J114 send signals to the analog mixer chip?)

    Looks like TC9164AF is available in a few places online for less than $10. I've never soldered an SMD IC before, so this should be an interesting challenge :)
     
  19. MrChips

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    Oct 2, 2009
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    You ought to do some forward planning. What happens if you go to all the trouble of replacing IC1 TC9164AF and that doesn't resolve the problem?

    IC1 itself could be at fault or it could be the digital signals and data, CLK, DATA and STB1 that could be at fault. If you had an oscilloscope we could check the integrity of the data. I assume that you do not have an oscilloscope.

    I will try to explain how the crossfader and mixer works while referring to the VR ASSY SCH 4 diagram shown below.

    The crossfader slider pot (one section), consists of GND on the center tap. The two arms of the slider control negative voltages going into a pair of inverting opamps (dual opamp IC10). The outputs of the dual opamp, pin-1 and pin-7 would move in opposite direction; as one increases, the other decreases. You can verify this with a DVM. Voltages should read 4V when in center position, 0V and 5V at extremes.


    These two control voltages are fed to two pairs of voltage controlled amplifiers (VCA), IC5 and IC6, M5283P. This is where the level of the signals are actually controlled. Each VCA outputs a pair of signals, one pair for the Left Channel and the other for the Right Channel.

    Left Channel pair is summed in opamp IC11, pin-2 with the output on pin-1.
    Right Channel pair is summed in opamp IC11, pin-6 with the output on pin-7.

    IC1 TC9164AF is an analog switch which selects which channels are to be mixed. The microcontroller unit (MCU) sends out digital information to IC1 based on the channel selection. Fourteen bits of data are sent serially via the DATA signal. This data is synchronized with the CLK signal. After data is sent a strobe signal STB1 is sent to latch and activate the data in the analog switch. All of this can be verified with an oscilloscope.

    If you do decide to remove IC1, there are some tests you ought to perform before soldering the replacement chip.
    (You should have already confirmed the voltages at IC5 and IC6, pin-2 and pin-15, when the crossfader slider is mid-way, fully left and fully right.)

    Apply music source into any channel, e.g. Channel 1. A signal should be available at 1GL and 1GR for testing. Use one of these as a music source.
    Apply this music source into the following nodes, one at a time, either at pins on IC1 or the input capacitor, which ever is more convenient.

    Left Channels shown in green:
    IC1-6, C5
    IC1-9, C6
    IC1-12, C6

    Right Channels shown in red:
    IC1-23, C11
    IC1-17, C12
    IC1-20, C12

    IC1-6 means IC1 pin-6.

    Adjust the crossfader control and experience that the volume level goes up or down as expected. Two channels should behave in opposite fashion.

    Pioneer DJM-500 Mixer marked.jpg
     
    Last edited: Feb 17, 2016
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  20. lorenjan

    Thread Starter New Member

    Feb 15, 2016
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    Thank you so much, the information you provided is absolute gold for my understanding!

    One quick question: when you suggest taking the signal from a channel and applying it directly to the IC on various pins, how would that be done? Would I solder (or maybe just touch) a jumper wire to those two locations on the board?
     
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