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Thread: Buzzing RL02 - Need Help Troubleshooting

  1. #1
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    Question Buzzing RL02 - Need Help Troubleshooting

    Hello Guys,
    Tradde generously gave me a 19" rack and two RL02's. I don't have a PDP yet, so I can't spin them up, but we did plug them in to AC at his house just to see what they did. One appeared to power up okay, while the other one buzzed fairly loud. Any ideas where to start checking? I got the tech manual from bitsavers, but don't really know where to start as I've never opened one of these up before.

    I spun the rear fan manually and it's free, as well as the spindle motor inside. Could it be something in the power supply? I haven't powered it up again, as I'm afraid doing so might damage something if there's a short somewhere.
    Any thoughts would be appreciated...
    -Jon

  2. #2
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    Were any of the front panel lamps lit?

    Lou

  3. #3
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    Hey Lou,
    I didn't get to see the front panel, as it was pointed away from me when we turned it on. We only ran it for about 2 or 3 seconds. It made it very apparent that there was something wrong.
    -Jon

  4. #4
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    I powered it up today, and the buzzing didn't seem as loud as I remembered in the past. The rear cooling fan spins fine, and the fault light comes on (no controller). After a short time, the load light turns on too. I'm trying to use my ears to figure out where the buzz is coming from, but it's hard to localize.

    -Jon

  5. #5
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    Okay, discovered where the buzzing is coming from. It's coming from the head assembly. When I opened the cover, the buzzing was MUCH louder, and when I touched my finger to the plastic shroud around the heads, I could feel an *intense* vibration. It feels as if the head seek motor is trying to move the heads further in, and they can't go any further, thus it is buzzing / vibrating. Does this make any sense?
    What is the best way to open it up and investigate further?

    -Jon

  6. #6

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    Quote Originally Posted by Vercus View Post
    Okay, discovered where the buzzing is coming from. It's coming from the head assembly. When I opened the cover, the buzzing was MUCH louder, and when I touched my finger to the plastic shroud around the heads, I could feel an *intense* vibration. It feels as if the head seek motor is trying to move the heads further in, and they can't go any further, thus it is buzzing / vibrating. Does this make any sense?
    What is the best way to open it up and investigate further?

    -Jon
    Can you move the heads manually in and out? Maybe they are locked down? I don't
    remember if an RL02 has a head lock. I know the RK05s do.

  7. #7
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    Hey Tim,
    The heads do move in and out. In fact, I moved them out a bit by accident. When I did, the heads clicked together and touched. I gently slid them back, and they opened up again. Did this ruin the heads?

    I'm going through the tech manual now, and printed out a list of voltages to check on the DC Servo board. I'll use my DMM to see what they're at. I don't have an oscilloscope, so I can't check some of the more advanced parameters unfortunately.

    -Jon

  8. #8
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    Question

    Hello again,
    I lifted the DC Servo board, and disconnected the 6-pin cable that goes to the servo motor so I could check voltages without it buzzing.

    Sure enough, when I turned it on, there was no buzz. I checked the 5 test points on the servo board listed in the Tech manual and here are the results:

    Voltage
    +V unregulated: +15.0 Volts (+14V to +18 V acceptable)
    -V unregulated: -10.5 Volts (-14V to -18V acceptable) * OUT OF SPEC*
    +5V: +5.15 (+4.85V to +5.35V acceptable)
    +8V: +8.0V (+7.7 V to +8.3V acceptable)
    -8V: -6.1V (7.7V to -8.3V acceptable) *OUT OF SPEC*

    As you can see, two voltages are out of spec. According to the manual, the voltages are not adjustable. If a voltage is out of spec, the faulty "FRU" should be replaced.

    I'm assuming FRU means "Field Replaceable Unit". In which case, DEC is simply saying replace the DC Servo board? I don't exactly have spares lying around..lol. I'm guessing it's probably one or more of the Fixed Voltage Regulators on the underside of the board.

    At least I feel like I've made some progress in determining the problem.

    Any advice on how to proceed would be much appreciated!

    Thanks,
    Jon
    Last edited by Vercus; August 1st, 2009 at 01:30 PM.

  9. #9
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    Jon,


    Check the power supply board and all of the voltages on there. I have two old RL01s in my garage, and one of them has a bad power supply board. RL01 and RL02 use the same power supply. I remember that it was a linear supply. The bad PS I have has a bad 7808 (8V) in a TO5 case.

    Are those voltages low with and without the servo board attached? (Is there somethng wrong on there pulling it down?)

    In addition to finding a pdp-11, priority two should be a scope. A Tek 465 would do you well. Seen at hamfests in good shape for as little as $100....


    Lou

  10. #10
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    Hey Lou,
    So far, I haven't tried measuring voltages with the servo board attached, as I was afraid of damaging it. I have a revision 2 power supply board which, like you mentioned, would work in either an RL01 or RL02. However, the original RL01's with a Rev. 1 board won't work in an RL02.

    I'm thinking of ordering a few 7808's just in case. My other concern is this: the -15V says "unregulated", which I am assuming means it is not controlled by the 7808 fixed voltage regulator. If that is the case, than what IS it controlled by? I noticed that the test point for -15Vu and -8V are directly under the same 7808. This leads me to theorize that the -15Vu is indirectly affected by the same 7808. However, what confuses me more is that the MC7808CK is a POSITIVE voltage regulator, and it is my NEGATIVE voltage that is out of spec. Odd...

    -Jon

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