Kurzweil PC3LE Blinking problem – Ultimate solution

I was fixing many Kurzweil PC3LE’s with a blinking problem, which looks pretty much like this:

This fault is probably caused by long power cables, together with shit Molex connectors, at which voltage drop occurs. During power on, the boot loader stresses the power supply by turning ON all the LEDs. This causes voltage drop at the reset circuit U6 (MCP120T) located at the connector board (or U7 on older models). Therefore U6 performs reset, and the story begins again.

There is some service bulletin available to all Kurzweil’s authorised service centres, but the steps described there does not always work. I have applied the mods in about 20 cases, but the instruments were occasionally coming back with the same problem. You can download the service bulletin below.

It seems like the only ultimate solution is to remove the U6 entirely and put a low ESR 2G2/10V capacitor (or higher) into the U7 holes between 5V and GND (see the picture above). This may sound funny, but no instrument came back after this modification and no one has complained so far to any malfunction caused by missing reset circuit.

The only side effect I found is the OFF – ON cycle must take at least 2 seconds, otherwise the instrument wont boot up.

Jan 2020 Update: Before you remove the U6, always check the MOLEX connectors (Big white body with thick wires). The contact springs are usually tired and / or oxidized. Pull out the pins, clean them, stretch the springs and inset them back. If this does not help then proceed with U6 removal


18 thoughts on “Kurzweil PC3LE Blinking problem – Ultimate solution

  1. Hello,
    I have this problem now. Please tell me if I firstly act according to tips given by Service Bulletin I can still follow your solution? In other words could those two solution works independly? Maybe it’s a stupid question, but I really like to avoid to brick this device.
    Thanks in advance!

    1. Yes, they work independently. Make sure you check the MOLEX power connectors first!

  2. Is that 2.2 micro farad low ESR electrolytic? Mine has been working sporadically after reseating all the connectors. Also, do you just remove the U6 altogether, or do you need to lay a bridge across the contacts when it’s removed?

    1. Yes it does work for a while after disturbing the cables. That is typical.
      The capacitor is 2200 micro Farad, (2200uF/10V, Low ESR is better) and it goes to place where U7 originally sits (as shown at the picture)
      U6 or U7 must be removed, no bridge there. Just leave the pads empty

  3. Hey Jirin

    Thank you for your blog.
    I’m going to repair my PC3LE6.

    Do you have a service manuel.

    1. Hey Jirin.
      After 3 hours of work, we have now made 2 mods on my Kurzweil PC3LE6.
      First the one from Kurzweil Service Bulletin from 2014. Followed the instructions in the service bulletin.
      And after that was done we followed your “modification” removed UC6, and placed a 2200uf / 10 V capacitor in the empty UC7. As you have described.

      Now it seems to work fine.
      We had it on turned on for 10-15 minutes, and didn’t start to blink or reboot by it self.

      Thank you for your blog.
      Especially because it seems to be the UC6 part that makes the problem keep coming back to previous Kurzweil PC3LE users.

  4. Hi Jirin,
    The Synthesizer is a second hand bought from Germany via eBay. The warranty has expired. I had a flashing problem since January 2017 when I took PC3LE7, installed OS 2.26 and the problem was rare. In July, after booting up the operating system, there was a 4-5 noise case, but the synthesizer worked after a restart. Finally, the reboot did not help, and there was no sound anymore at the three output L, R and headphones.We did the service bulletin and your modification but there is no signal. Do you have any idea? If possible write on my e-mal.
    Best regards

    1. Hi Krasimir,
      the synth is very complex device therefore I cannot say what is the problem without seeing it. Sorry.
      If it makes constant noise, no matter of volume, then check the 7805 voltage regulators at the main board (I think it is SOIC8 case).
      In fact, always measure all supply voltages first. All dual opamps should have +- 12V (or similar), the logic is usually 5V, 3.3V, 1,6V etc.
      Also, use probe (oscilloscope) to check whether the signal goes from main board to IO board (the grey shielded cable)


  5. Hi again and thanks for the answer,
    The noise is not constant, after loading the OS for about 5 seconds there is a strong crackling noise that is not affected by volume. Then the noise stops, and there is no signal from the outputs. Please write to my e-mail: eclipse69@abv.bg if you are comfortable to explain and other related to Kurzweil HQ dep.
    Best regards

    1. Man, I’m sorry but it could be anything. This sounds like dry joint somewhere, but it’s the hardest to find. Freeze the board with a cooling spray, part after part, then heat it up with hot air gun (gently !!! not over 100°C). Listen to the output signal for any changes. Good luck.

    1. Krasimir, I was not talking about 5V going from power supply. I was talking about 5V voltage regulator at the mainboard. It is U16, a SOIC8 device. When it fails, it makes noise like in your video.

  6. Good afternoon. I have Kurzweil pc3le8. It turns on, works one minute, or two minutes, and then starts to flash and restart. Is this a problem as you described in this article or another?Thank you.

    1. I don’t know. It seems like it. Always try the cables for bad connection first!

  7. Just wanted to say thanks! It took me a while to get to because I had to repair the lower register keyboard cable as well, and didn’t want to do either repair until I could do them at the same time. Works great!

  8. Hi, thanks a lot for the hope your webpge provides… I was kind of depressed when I met the issue yesterday and I’m 10 days away from a gig (local music event)… I know well the problem because i faced it 5 years ago while my pc3le6 unit was still under warranty and after several repair attempts by the local kurzweil repair center they finally gave me another unit… that last until yesterday.

    Question : how hard is it to open the unit as I plan to do it myself ? Do you have the service manual for the pc3le series ? I could only find the pc3X on the web.

    Thanks a lot.

    1. It’s not hard, you must turn the synth upside down, remove all M4 screws from bottom and rear side, (do not remove keybed screws, they are much longer), remove side panels, turn it back (upside up?) and carefully open the piano. Be careful not to damage any cables as you open it.

      If you’re not sure, give it to some technician.

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