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Thread: Bezzera BZ-99 OPV screech

  1. #1
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    Bezzera BZ-99 OPV screech

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Hi all

    I picked up an old (early 2000s) Bezzera BZ99 from a fellow CSer who had done some work on it to ensure everything was in good order. It pulls great shots but it's been producing a fairly loud screeching noise once pressure builds up. I pulled the machine apart and I've isolated the noise to the OPV, which explains why the screech only starts at full pressure.

    I pulled the OPV out, took it apart and descaled every part (except the rubber gasket... I had a laugh) with a strong citric acid solution. Put it all back together and the screech was gone temporarily, but within a day it was back.

    Soooo... I can get a new OPV from the Australian Bezzera distributor for about $60 but I'm just wondering whether there's a real performance or machine wear & tear issue that I need to resolve or whether I can just ignore the noise and keep going about my latte-making? The noise doesn't really bother me that much, but if I'm putting the machine at risk I'll drop the $60 on a new part.

  2. #2
    Mal Dimal's Avatar
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    G'day mate...

    Given that you've removed, cleaned and replaced the OPV, I doubt there is anything really wrong.
    The 'screeching' you can hear is due to a confluence of physical properties between the OPV spring, the Pump pulse frequency, various pipe lengths and diameters and the incompressibility of water. Basically, it is just a resonant vibration that you can hear when the OPV starts to release water through its innards.

    You could try replacing the spring with a similarly rated one, then reset the OPV accordingly, and this might get rid of the resonance. Not going to cause any damage to your machine though so if it doesn't worry you too much, it'll be fine to leave as-is...

    Mal.
    P.S.
    If you would prefer a more detailed explanation, then this isn't a bad link...
    Last edited by Dimal; 9th February 2016 at 06:18 PM.
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  3. #3
    Senior Member artman's Avatar
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    As above. Or maybe try tweaking the pressure up or down half a bar and see if changes/stops? Or tee in a bit of blanked off hose between the pump and OPV. Or increase the length of the hose between the pump and OPV. The change in volume of water that is vibrating should alter the resonant frequency and may solve it.

    Cheers
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  4. #4
    Member b430m's Avatar
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    You could also try not tamping so heavy or make your grind not so fine.

  5. #5
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    Thanks for the replies. The weird thing is that when I disassembled and reassembled the machine was nice and quiet for a few cycles... no screech at all. Is this just because the components haven't re-settled yet?

    The screeching isn't "normal" (I've listened to other BZ-99s on Youtube and they're nice and quiet apart from pump noise) but what I'm hearing from you guys is that it's probably just very minor miscalibration/wear that's resulted in a resonant frequency under pressure?

    I'd have a go at adjusting the pressure if someone could tell me how... I'm new to this machine but quite comfortable opening it up.

  6. #6
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    No screeching from either of my BZ99s - occasional quiet whistling from the boiler relief valve when heating up / cooling down but the OPVs are ok.

    Edit: Ignore - wrong advice - To adjust pressure, just turn the screw on the top of the pressostat (little can at right of boiler at top, you'll see a microswitch hanging off it.). From memory it has a + and - on it indicating direction to turn. Microswitch has 240V going through it, so of course, make sure you're comfortable with this sort of thing.
    Last edited by simonko; 10th February 2016 at 03:38 PM.

  7. #7
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    Quote Originally Posted by simonko View Post

    To adjust pressure, just turn the screw on the top of the pressostat (little can at right of boiler at top, you'll see a microswitch hanging off it.). From memory it has a + and - on it indicating direction to turn. Microswitch has 240V going through it, so of course, make sure you're comfortable with this sort of thing.
    This will not adjust brew pressure, only boiler pressure. Brew pressure is limited by the OPV.
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  8. #8
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    Where can I hide? Sorry OP, answered before thinking.

  9. #9
    Super Moderator Javaphile's Avatar
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    On some machines screeching can occur when the pump pressure is to high and water passes through a properly adjusted/operating OPV or when the pump pressure is correct and the OPV is showing its age and is opening at a lower pressure than designed for or, on models so designed, is set to open at to low of a pressure. In such cases the OPV needs to be adjusted or replaced.


    Java "Oh how Cimba SCREAMED!" phile
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