Pioneer DT-510 Display Flickering

Discussion in 'Pioneer Audio' started by 1mike161, Nov 10, 2017.

  1. 1mike161

    1mike161 New Member

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    Illinois near St. Louis MO
    My Pioneer DT-510 timer has developed a problem where the display segments flicker between the actual time and 12:00 am. I read several posts that blame a failing zener diode (diode 8 - MZ150) for this phenomenon. I found a good substitute (15 volt .5 watt) for the diode and installed it, but the problem persists. Does anyone know what else could be causing this?
     
  2. vman96

    vman96 Member

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    79
    Location:
    Boston, MA
    I got DT-530 with exact symptom you're describing. Never got deep into it but would like to find out the source of the problem..
     
  3. redk9258

    redk9258 Super Member

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    1,161
    Location:
    Illinois, close to St. Louis.
    Is the alarm set for 12:00 AM? Maybe a dirty switch causing it to display the alarm time.
     
  4. parman

    parman Vinyl Addiction Subscriber

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    Can't help with your problem but I see your using the Dr. Stangelove avatar. There was a famous AK member who was the Pioneer expert that used that same avatar. Unfortunatly he no longer on the board
     
  5. 1mike161

    1mike161 New Member

    Messages:
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    Location:
    Illinois near St. Louis MO
    There is no alarm on this unit. However the timer "on" is set at 11:00 am and the timer "off" is set a 2:00 pm. The problem is intermittent -- the unit will run perfectly for days, then it starts flickering...then it stops. I do think the problem has diminished since I replaced the diode, but something else is causing the on-going intermittent malfunction. I will probably look at the power supply and voltage regulator next. I was hoping someone in the forum might have the answer and save me a little troubleshooting time.

    Yes, I've seen the gentlemen with the Dr. Strangelove avatar. I didn't mean to steal his identify -- I've been using that avatar for dozens of years. I'm a big fan of Stanley Kubrick and his epic works.

    By the way, I'm also a big Pioneer fan! I have most of the blue florescent and spec series and love the stuff! I don't think I've ever heard any system that sounds as good!
     
    parman likes this.
  6. redk9258

    redk9258 Super Member

    Messages:
    1,161
    Location:
    Illinois, close to St. Louis.
    Yeah, I meant timer time, not alarm. Was thinking like a clock. Anyway, are the switch contacts clean? Does anything happen if you wiggle them?
     
  7. avionic

    avionic " Black Knights " Subscriber

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    Periodic power switch arcing is a possibility.
     
  8. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    Folks, this problem is generally, if not a dirty mode switch, from the failure of the clock chip's internal pull-ups/downs on its control line inputs. You can permanently correct it by dead-bugging external 10K, 1/8W pull-ups or pull-downs on the control lines. What happens is the input lines are supposed to be definitively pulled to a default state if the lines are left open (either by not connecting them at all, or via mode switch settings. When the chip's internal pull-up/down resistors fail, the input floats into the indeterminate range of the input, hence the undecided nature of the display mode.

    Now, before you willy-nilly start attaching resistors, you will need to get the data sheet for the chip, and the schematic for the timer. The implementation is weird, like using a negative voltage to power the chip and ground for a high. Go see and you will understand.

    Maybe I will post a thread in the Pioneer Restorations sub-forum, later.

    In any case, don't pitch the unit. It is fixable, once you understand what is needed.

    Enjoy,
    Rich P
     
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  9. 1mike161

    1mike161 New Member

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    14
    Location:
    Illinois near St. Louis MO
    Thanks Rich! I think I understand what your saying. I will do the suggested research before making the modification. Thanks for the help!
     
  10. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    Be careful with polarities and desired default states, etc.

    Enjoy,
    Rich P
     
  11. qprhooligan

    qprhooligan AK Subscriber Subscriber

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    My DT-510 just started flickering this morning. I would love to hear how to fix this too.
     
  12. qprhooligan

    qprhooligan AK Subscriber Subscriber

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    Could I just replace the MN6076 IC?
     
  13. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    As far as I know, the clock chips are no longer being made, not for a long time. If you took one from a parts unit, it could develop the same problem. I have never de-capped one to see what is going on with the pull-ups/pull-downs. What I have seen is that the rest of the internal connections work well. External resistors generally fix issues like this.

    Enjoy,
    Rich P
     
  14. 1mike161

    1mike161 New Member

    Messages:
    14
    Location:
    Illinois near St. Louis MO
    I am not an electrical engineer -- just a hobbyist. That being said, I have reviewed the clock chip MN6076 data sheet and the schematic for the Pioneer DT-500/510. I "think" I understand the solution Rich P. has described. Basically, the chip's internal resistors on the 3 mode select inputs are failing and causing the clock's operating mode to randomly change. Adding 3 external 10k 1/8 watt resistors to replace the failing internal resistors should solve the problem. Each of the 3 mode select inputs should be tied to ground with 3 10k resistors (1 for each mode select input). This will force all 3 mode select inputs low and hold the clock chip in "clock" mode. The ability to set the clock time and change the timer's ON & OFF times should not be effected.

    I have illustrated the modification with the attached diagram. The red trace is positive and the green is ground. The purple, blue, and orange traces are the 3 mode select inputs. The new resistors (shown in bright red) pull the mode selects down until their pulled up (by operating the mechanical rotary switch).

    I have not implemented this, so I don't know if it will work. This is my best guess as to how the modification should be made. Again, I'm just a hobbyist.

    Also, Rich P. warned that the clock chip is weird due to it's negative voltage power requirement. I noticed this too, and I'm not sure how the chip works in the Pioneer circuit which seems to power the chip with a positive voltage.

     
  15. 1mike161

    1mike161 New Member

    Messages:
    14
    Location:
    Illinois near St. Louis MO
  16. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    The chip is powered with a negative voltage, to be tied to pin 27 (Vdd) of the MN6076 chip. The chip's reference voltage is tied to pin 28 of the chip (Vss), which is the reference for the positive voltage supply. The inverse powering is due to the fact that the chip is designed to drive electro-fluorescent displays directly. In any case, What you need is to tie 4 10K resistors, right at the chip. all 4 resistors have one end tied together, and then tied to pin 27 of the chip. Then, the other end of the resistors are to be tied, individually, to the the following pins: 29, 30, 31, and 36. That should solve most issues with these units.

    What is shown above looks correct, but needs one more pull-down, the one to pin 36. This cures the issue with the unit tending to run fast, thinking that it is set for 50 Hz, when the switch is set to 60 Hz, allowing the pin to float to an indeterminate level, due to failing internal pull-down..

    Be careful, and if you do not understand my post, do nothing until you do understand.

    Good luck,
    Rich P
     
  17. 1mike161

    1mike161 New Member

    Messages:
    14
    Location:
    Illinois near St. Louis MO
    Wow! Thanks Rich! Do the chip pins (29, 30, 31, and 36) need to be lifted off the board and out of the circuit?
     
  18. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    No. What you show looks right. Just need to add one more. Actually, I like the way you show it better. Easier to do, but a bit harder for future techs to trace out. 6 of one, 1/2 dozen of another.

    If you create a new thread, titled "Correcting Display Instability and Speed Issues: DT Timers," I'll tweak it and move it into our "Pioneer Restorations" sub-forum. This should benefit alot of folks, and I should have done it years ago.

    Enjoy,
    Rich P
     
  19. 1mike161

    1mike161 New Member

    Messages:
    14
    Location:
    Illinois near St. Louis MO
    Rich, I created the new thread. However, I failed to correct the typo in the thread title -- it should be "Speed" not Seed. Thanks.

    Mike
     
  20. slayer44

    slayer44 AK Subscriber Subscriber

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    Location:
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    Just an FYI, I had this same symptom with mine and found that applying a little pressure to the top of the circuit board above the mode switch would straighten it out. Determined that warping the mode switch minutely cured it. I think the rotary switch has a design flaw. I never really fixed it, but I tried rebuilding the switch to no avail. Ended up wedging a piece of plastic between the board and faceplate and has been working correctly for over a year. I dont know if your prob is the same, but this redneck fix may rule out an IC or circuitry problem.
     

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