Static/distortion/crosstalk from effect send jacks

Discussion in '2488 and DP-24/32 Digital Portastudios' started by shaddart, Jan 21, 2019.

  1. shaddart

    shaddart New Member

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    Hi, I just bought a used DP-24 - needed midi or I would have bought a new one.

    I only have one problem with it (besides dodgy transport buttons)

    when I send a signal out of the effect send to another device there is intermittent static / distortion.

    I adjust the send levels but it doesn't help . It may be fine for a minute or two but then the weird static
    distortion starts happening ( and it's intensity seems to be dependent on the volume of the signal)

    Also, I get a ghost signal coming out of effect send 2 when I am only sending to effect send 1.

    Weirdly when I send out of effect send 2 I also get the distortion but no signal out of send 1.

    When I monitor the effect sends with the "Monitor select " button, there is no distortion or static=- it
    only happens when I plug a cable (yes i tried other cables :) ) into either of the send jacks.

    OK thanks, I love this machine - I am not looking forward to opening it up and not sure what I would look for.

    thanks.

    Paul
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  2. -mjk-

    -mjk- Moderator Staff Member

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    I bought a used DP-32 for the same reason. I recommend spraying each control (except the encoders) and every connector jack on the entire machine with a good contact cleaner. The crosstalk stumps me, but all the other issues are most likely oxidation related.

    Try a forum search on the team "Deoxit" for more information. Be sure and select the "site results" tab.
  3. shaddart

    shaddart New Member

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    thanks for the tip. Ironically, my current work around wouldn’t be possible without the midi jack - i synced up my old, almost discarded fostex vf80 ex via midi and sent the track i wanted to vocode to it and recorded it- now i can send it to my kong ms2000 for vocoding with (why not) some distortion effects from the fostex - hilarious work around and yes i have too much time on my hands. Thanks to my now ex- girlfriend ha ha.

    so can that contact cleaner evaporate ok spraying it in there - oh never mind i will read the thread
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  4. -mjk-

    -mjk- Moderator Staff Member

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    Deoxit is specifically designed for that purpose.
  5. shaddart

    shaddart New Member

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    Bump- I tried the deoxit, through the 1/4" jacks, using swabs and using coated 1/4 inch plugs.

    The noise continues so at some point I am going to have to open up the unit.

    Does anyone have any hints of what I should look for in there?

    I believe this is an early unit, judging by the serial number, it appears to be one of the
    first 400 that were produced.

    maybe something like a bad or bridged solder joint?

    ok thanks.

    Paul
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  6. -mjk-

    -mjk- Moderator Staff Member

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    1. Is the distortion present only when using external devices?
    2. Send 1 goes to the internal FX. If you use Send 1 on the internal FX, do you hear the distortion, say in a reverb return?
    3. When you hear the distortion on Send 1 on your external device, can you hear the distortion on the monitor bus for Send 1?
    If your answers are Y, N, N, then there could be either a problem with the cable from the output jack of Send 1, or a problem with the jack itself.
  7. shaddart

    shaddart New Member

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    No, it only appears from the physical jack- not in the monitor bus (if I select the send) nor on an internal effect.

    I have used several cables that are good, with exact same result- and really it's not a consistent distortion, it varies in intensity. Also, like I said before, even with effect send 2 turned off, I can hear the distorted effect send one signal from another cable plugged into the effect send 2 jack.

    thanks!

    i'm wondering if there is something inside between the 2 jacks or their signal paths on the board. I will find out when I get the guts to open her up.

    Paul
    -mjk- likes this.
  8. Mark Richards

    Mark Richards Well-Known Member

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    Edited:
    This post had wrong info re the Send knobs. Thanks -mjk- for pointing that out. Leaving this post so the following 3 posts make sense.
    Last edited: Feb 2, 2019
    -mjk- likes this.
  9. -mjk-

    -mjk- Moderator Staff Member

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    Wait! Aren't those encoders, and not potentiometers?
  10. -mjk-

    -mjk- Moderator Staff Member

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    I think the only two things in common with Sends 1 and 2, is the ground path.
  11. Mark Richards

    Mark Richards Well-Known Member

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    You're right mj. Thanks.
    I wasn't fully awake on that one. Apologies to the OP. I edited that post to remove my suggestion.
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  12. -mjk-

    -mjk- Moderator Staff Member

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    Paul, I wonder if the D/A converter chip is going bad. Send 1 is routed to the internal FX in the digital domain. Just before the Send jacks are D/A converters. The block diagram shows separate blocks, but they could be on the same chip. It seems your issues exist on the analog side of the converters.
  13. Arjan P

    Arjan P Well-Known Member

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    It's a long shot, but maybe this issue is related to a possible ground loop? Are the effects units you send to on the same (grounded) power supply? Doubtful since you said the distortion is also related to FX send volume, but who knows..
  14. shaddart

    shaddart New Member

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    that will definitely be one to check/ i’ve heard rumors about bootleg chips..... thanks.

  15. shaddart

    shaddart New Member

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    OK, I still haven't opened up the unit- I ordered some better solder tips and a replacement DAC.

    I found the schematic online and identified the DAC that is at the effect send outputs. It's a 2 channel DAC (AK4384) that handles both sends. I looked up the data sheet for the DAC and
    discovered that the chip can be used in parallel (2 channel) or serial mode-

    Since I am hearing both sends coming from a single jack I am suspecting that the chip is
    in serial mode when it should be in parallel mode. I guess this could be a problem with

    1. the chip itself - internal defect
    2. shorts/ bad solder joints/ etc on the board interfering with the signal to the pin that sets
    it to parallel

    3. a software bug in my firmware which doesn't seem possible ( I have the latest)

    Anyway I repost again when I learn more. I'll try cleaning and examining the board(s), then
    installing a new chip.

    Paul
  16. -mjk-

    -mjk- Moderator Staff Member

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    Thanks for the update Paul. Looking forward to the next one.
  17. shaddart

    shaddart New Member

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    DAC chip with pins dremeled off.jpg

    SUCCESS!

    I am pleased to report the replacing the DAC chip solved my problem!

    I can't believe I didn't screw up anything!

    I used a dremel tool with sanding disc to cut the pins of the tiny surface mount 16 pin
    DAC chip- I could just barely weasel in in there between the tall capacitors.

    Then after scraping the pin fragments out I managed to solder that thing on there - took
    a few passes and had to fix solder bridges a couple times.

    I watched a lot of youtube videos to give me confidence and bought some chisel and beveled
    soldering tips and good solder , and a flux pen which was key. - I wish I'd had a microscope- had to make do with varied magnification lenses- A checked it out real good with a lupe to fix errors.

    Any way I'm as happy as the time I fixed the sewing machine noise on my ex wife's old subaru by replacing a tiny o-ring in the oil pump, which was of course behind the timing belts. Put the
    same old oil pump back in and after revving up the motor the noise went away.

    OK maybe this post will help someone else.

    Now I can get back to where I was- feeding my banjo track out the effect send into my
    Korg MS2000 for vocoding!

    No more static, distortion, or crosstalk - 100 percent clean hot signal.
    Phil Tipping, Arjan P and -mjk- like this.
  18. -mjk-

    -mjk- Moderator Staff Member

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    Wow Paul, that is fantastic! I really admire how you dove into such a daunting replacement project and meticulously worked it until you achieved success! Congratulations!

    So, it really was the DAC after all.... Wow....
  19. shaddart

    shaddart New Member

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    Yup, I forgot to thank you for that tip - You were right on the money on that one. Not sure I
    would have thought of that !

    It started to make sense when I read that the chip which handles both sends can be in series or parallel mode- accidentally being in series would explain the cross talk and distortion.
    -mjk- likes this.
  20. -mjk-

    -mjk- Moderator Staff Member

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    It was just an educated guess Paul. I don't even want to think about you going through all that work only to find that the chip wasn't the problem, lol. But, with your very detailed explanation of the issues, and your research, the chip was most definitely the place to begin. Good work!