Fluke 75 piezo buzzer repair - Page 1
Forum section: electronics / repair

author: albert001
author_rank: Regular Contributor
timestamp: December 12, 2018, 11:45:46 pm
content: Hello,I have a Fluke 75 where the piezo buzzer doesn't produce a loud enough sound. If the dmm is placed directly on my ear in a certain position I can here a very faint buzzing sound. So the piezo buzzer is still working. However I'm not really certain how it functions as they're no wires attached and uncertain what sort of contact it makes with the dmm's pcb.Below are some pics. As you can see the piezo buzzer has some sort of holder with a plastic tab to hold the piezo in place. Also there are some small fasteners. two are some sort of permanently mounted hard plastic for the plastic tab. While other two (one larger than the other) are pieces of rubber appear to be attached to inner and outer part of the piezo disc.I can't see any points on the pcb where the piezo buzzer makes contact? Is there some sort of frequency modulator on the pcb that works with the piezo to produce sound?Anyway I'm trying to determine the best approach to repair the piezo buzzer.I found different diameter Piezo Transducer Sound Discs on eBay. I'm wondering if replacing the piezo disc will fix and increase the piezo buzzer audio output enough where it will become functional again.

author: retiredcaps
author_rank: Super Contributor
timestamp: December 13, 2018, 01:47:21 am
content: Have you looked at the Fluke 70 series service manual? There are troubleshooting steps for the piezo buzzer.

author: albert001
author_rank: Regular Contributor
timestamp: December 13, 2018, 03:27:04 am
content: Sort of looked through a service manualhttp://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%2077,%2075,%2073,%2070,%2023,%2021%20Series%20II%20Service.pdfFound something about:Checking beeper drive signalPut the unit in the Diode Test Mode and short the input leads. Using an oscilloscope, measure the signal at pin 3 of UI. The signal should be 4V p-p symmetrical square wave at at about 4.1 kHz.I'm currently trying to figure out how the buzzer operates and what can be done to fix it's problem with output volume.I do have an older Dual Channel 10MHz Oscilloscope in storage, (one of the channels has problems, the last time used the primary channel was working ok).I'm doing some remodeling so currently don't have any workspace to take out of storage and setup.I'm wondering if it's a simple fix?I've been looking at a few older Fluke 70 series both Series II and III. I've been looking at a Fluke 79 Series III and also a Series II, one difference is the Series III (cost more) is able test frequency. I'm uncertain of other differences between Series II and III. There are Series IIs that are rated as a true-rms same as the Series III.From what I can determine some 70 series didn't come with a buzzer?It's mainly when I working on something and need to know continuity I need to find one of my other DMMs where the buzzer works. The DMM's I own with working buzzers can also be difficult to here. I don't always know where I've put them.The Fluke 75 buzzer is so faint I'm only able to hear it if I hold it in a certain position directly on my ear.

author: ModemHead
author_rank: Frequent Contributor
timestamp: December 13, 2018, 03:50:13 am
content: The piezo element is connected to the board by the two little black buttons, which are conductive. On yours, the one on the left is damaged and probably is not making much, if any, contact.

author: albert001
author_rank: Regular Contributor
timestamp: December 13, 2018, 07:50:23 am
content: I'm wondering if I would be able to repair or replace the Piezo Transducer Sound Disc?I found these ones on eBay with and without leads, however without dismantling the existing disc I'm uncertain how those conductive buttons are attached. I remember them feeling rubbery and not made of metal at least on exposed part.When I have the time, I can remove the disc and dismantle to see how it's built. However I'm not certain about being able to install the correct type of contacts on a new disc. I'm uncertain where these make contact to the pcb? (will need to take a closer look)Link to Piezo Transducer Sound Discshttps://www.ebay.com/itm/5-Piezo-Transducer-Sound-Discs-YOU-CHOOSE-SIZE-Drum-Trigger-Guitar-Pickup/121253903935?var=420223347563

author: albert001
author_rank: Regular Contributor
timestamp: December 13, 2018, 10:23:15 am
content: Ok I dismantled the piezo, and took some pics. If my assumptions are correct I can see how the piezo buzzer functions, connecting to two contact points from the pcb.When I removed the piezo disc the little black conductive buttons that feel to be made from rubber. One fell off and the other stayed attached to the piece of plastic. Perhaps the black conductive rubberized buttons have metal inside making them conductive?. (Similar to how the LCD connects to the PCB?)I can see how the little rubberized buttons would be an area where solder contacts of the pcb would connect to the piezo disc through the little black buttons (which I assume must be conductive, I would need to try to test them with a DMM). however I'm unable to determine exactly what on the pcb can be considered to be actual piezo disc contacts?How I'm suppose to replace what appears to be rubberized conductive buttons with new ones I'm uncertain.I took a pic of an area of the pcb which appears ti line up with the two black buttons. After comparing measurements between two contact points on the pcb and two holes in the piece of plastic, both have a measurement of approx. ~13.5mm. (between each other)Below is a pic of what I suspect are potential pcb piezo buzzer contact points, that appear to be normal soldering points that are rounded and not sharp or pointed.pics of piezo disc, plastic holder and what appear as rubber mounts (black buttons).

author: ElecSeb
author_rank: Regular Contributor
timestamp: December 13, 2018, 10:32:27 am
content: Easiest/cheapest solution:1. Solder your piezo with 2 leads/wires.2. Test function3. Solder wires directly to PCB

author: albert001
author_rank: Regular Contributor
timestamp: December 13, 2018, 10:54:20 am
content: Ok that's a idea just solder using a replacement 20mm disc with leads.However the dmm and case are designed to completely separate from each other, with no wires or attachments other than piezo disc contacts. So it would be better if I'm able to find some sort of material that works the same as the existing rubberized black conductive buttons.There are materials referred to as conductive rubber.So far I've found a three types listed. 1. Multicon (oriented wire in either sponge or solid silicone) 2. Conductive Silicone (metallized filled silicones), and 3. Radthin (wire screen imbedded into silicone).From eBay found a conductive silicon heatsink pad.https://www.ebay.com/itm/Blue-100x100mm-x-0-5mm-GPU-CPU-Heatsink-Cooling-Thermal-Conductive-Silicone-Pad/301510641687?epid=1746887231&hash=item46336f4817:rk:1:pf:0https://www.ebay.com/itm/1pc-Blue-GPU-CPU-Cooling-Heatsink-Silicone-Thermal-Conductive-Pad/153237579803?hash=item23adabe81b:rk:5:pf:0However these pads are designed for thermal conductivity and not electrical so I'm uncertain if would have enough conductivity for a piezo buzzer?

author: ElecSeb
author_rank: Regular Contributor
timestamp: December 13, 2018, 11:06:24 am
content: 1. Is your buzzer defect. Did you test? (solder leads to it and test, only real way of knowing)Furthermore, nice to know your body can be completely seperated, however how often do you do this

author: ModemHead
author_rank: Frequent Contributor
timestamp: December 13, 2018, 12:34:02 pm
content: Soldering to the piezo element is not recommended. It's very easy to destroy the metal film coating on the center portion.Find a spring that will take solder. Clearance between the piezo and the PCB is limited, so cut two very short lengths of the spring. Solder these to the PCB contact points, so that when the PCB is installed, the springs compress and contact the piezo.

author: albert001
author_rank: Regular Contributor
timestamp: December 13, 2018, 01:20:07 pm
content: Ok I'll give that a try. I think I have some light springs somewhere that can be cut to a certain length to produce contact from the PCB to the piezo disc.

author: ElecSeb
author_rank: Regular Contributor
timestamp: December 13, 2018, 02:02:01 pm
content: Did not know this,if I am correct this is the reason not to solder?http://www.noliac.com/tutorials/technical-support-actuators/soldering-procedure/(Special tin etc)

author: ModemHead
author_rank: Frequent Contributor
timestamp: December 13, 2018, 03:49:07 pm
content: Quote from: ElecSeb on December 13, 2018, 02:02:01 pmThat sounds exactly right. I won't pretend to understand the chemistry aspects of it, I learned the hard way. The electrode practically evaporates if the temp is too high or you stay on it more than a second.Springs are a common way of making contact with off-board piezo elements. Fluke's little black buttons are probably silicone, laced with carbon. Like the material on the back of flexible membrane keypads.

author: albert001
author_rank: Regular Contributor
timestamp: December 16, 2018, 01:54:07 am
content: One thing I found odd is yesterday while testing some 1.5V AA, AAA and C batteries the Fluke 75 started to behave strangely, not displaying accurate readings such as ~3V for a 1.5V battery. Other times just kept flashing "OL" and wouldn't display any reading.I tried again today to test batteries and the Fluke 75 started to do the same thing as before. So I shut it off waited a while and now it's working ok providing what I assume are accurate dc voltage for 1.5V batteries. AC is also working testing an 110/120 AC outlet.

author: retiredcaps
author_rank: Super Contributor
timestamp: December 16, 2018, 04:05:57 am
content: Just to get the obvious out of the way, did you try using a fresh 9V battery?

author: albert001
author_rank: Regular Contributor
timestamp: December 16, 2018, 05:08:18 am
content: That's a possibility, however I replace the 9V battery not very long ago with a new one, and have run out of 9V batteries.Most of my other DMMs also use 9V and don't have a battery installed. I have one that uses two AAs and I will look for it tomorrow. I remember sticking it somewhere and haven't used it for some time.