Author Topic: Fluke 78 multimeter  (Read 3817 times)

Offline naffsharpe (Nathan)

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Fluke 78 multimeter
« on: June 29, 2015, 04:14:12 PM »
Can anybody tell me where (uk) can find someone to repair the above or point me to a circuit layout?
I need the Hertz function for a test but the meter seems dead. I've checked the obvious but no joy.

Nathan.

Offline John Swift

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Re: Fluke 78 multimeter
« Reply #1 on: June 29, 2015, 05:36:15 PM »
Hi Nathan

the user and maintenance manuals can be downloaded from Fluke
http://www.fluke.com/Fluke/uken/Support/Manuals/default.htm?ProductId=56130

do you see any thing displayed ? - see service manual page 3-8 for display test

my first test would be to try the low ohms / continuity range to see if it beeps with the input short circuited
if it beeps some of the circuit is working , no beep could indicate a problem with the power supply circuit 
 I've had  old LCD displays fail when stored in over winter in the shed !

   John


« Last Edit: June 29, 2015, 06:05:06 PM by John Swift »

Offline naffsharpe (Nathan)

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Re: Fluke 78 multimeter
« Reply #2 on: June 29, 2015, 06:23:57 PM »
John, many thanks for your reply. Component E1 as identified from the circuit diagram appears to be split/cracked. It looks like a capacitor outwardly but no capacitance is given on the diagram just 1500v/20% . I'll look for it online but any suggestions are always welcome !

Nathan.

Offline naffsharpe (Nathan)

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Re: Fluke 78 multimeter
« Reply #3 on: June 29, 2015, 06:46:05 PM »
My apologies John,  in answer to your question, I have no display at all, no continuity buzzer in fact absolutely zilch! This was an expensive meter when I bought it some time in the 1990s and would love to have it working as it was - like now when I need it ! It was in daily use and then it stopped working without any warning/ apparent damage . Last general use was with the thermocouple to see how effective our heating was (last winter) inside versus outside after much draught proofing/insulating/resealing of fire box and it worked faultlessly.

Nathan.

Offline John Swift

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Re: Fluke 78 multimeter
« Reply #4 on: June 29, 2015, 07:26:57 PM »
Hi Nathan

looking at the circuit diagram E1 is a spark gap (SURGE PROTECTOR,1500 V,+-20%)
some I've seen looked like a  disc ceramic capacitor with a saw cut forming  the spark gap

as it looks like dead -
do you have continuity between the battery connections and each end of diode CR1 ?

then , check you have power to the IC U1
battery  +9V on pin 4 (via a diode CR2) and -9V on pin 40 (via transistor Q7 and switch S1)


   John

Offline David Jupp

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Re: Fluke 78 multimeter
« Reply #5 on: June 30, 2015, 02:43:02 AM »
Fluke does have a calibration/repair facility in UK - though I don't expect it'll be particularly cheap.  I suppose no harm asking them for a quote.

Offline naffsharpe (Nathan)

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Re: Fluke 78 multimeter
« Reply #6 on: June 30, 2015, 05:34:29 PM »
Hi John and David.
Again my thanks for your continued interest and help. I had intended to post earlier but the weather here in Gwynedd was far to good to stay indoors today.
I bought 2 new PP3 9v Alkaline batteries with the intention of sitting here and carrying out the suggested tests , fitted one and the meter came to life instantly !
I metered the original PP3 and measured 9.1v then removed the new PP3 and measured 9.28v both were metered with my AVO 310 . Surely that 0.18 v difference cannot be the cause of the failure? The 9.1v PP3 had been in the meter for only 3 days .
I'm pleased but find it a bit of a puzzle .
Now it's working I can measure the square wave Hz output on my alternator to see if it is the cause of problem with the VW Dynamic Oil Pressure Monitoring system on my van . This is used to give a speed signal to the system, VW dropped the system in 2002 because of the problems with it and now refuse to talk about it !

Many thanks to you both,  Nathan.


Offline philf

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Re: Fluke 78 multimeter
« Reply #7 on: June 30, 2015, 06:03:32 PM »
I metered the original PP3 and measured 9.1v then removed the new PP3 and measured 9.28v both were metered with my AVO 310 . Surely that 0.18 v difference cannot be the cause of the failure? The 9.1v PP3 had been in the meter for only 3 days .

Nathan

Have you tried putting some load on the two batteries before measuring the voltage. I have a cheap digital multimeter with 2v and 9v battery test ranges which put a load on the battery. Testing on the normal voltage range and battery test range can be very different.

Phil.

Phil Fern
Location: Marple, Cheshire

Offline naffsharpe (Nathan)

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Re: Fluke 78 multimeter
« Reply #8 on: June 30, 2015, 06:18:23 PM »
Phil, I'll try them in an old radio I keep in the workshop tomorrow, too hot to go out there now it's still 21.9 C.

Nathan.

Offline John Swift

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Re: Fluke 78 multimeter
« Reply #9 on: June 30, 2015, 07:49:21 PM »
Hi Nathan

yes the weather has been nice here , on a clear day like today I can see north wales

good  to see the meter is  working

it could be as simple as switching on and off several times has cleaned the switch contacts

I've noticed some combinations off battery terminals and battery connector don't make a good connection
either the positive battery terminal is too small a diameter or the inside diameter of positive battery connector
too large

the input resistance of digital multimeter on a DC voltage range is too high to test a battery
 
you should see a more meaningfull  result if you connect a 1K resistor across the 9V
the extra current through the 1K resistor will increase the volt drop across the  batterys internal resistance
which increases as the battery discharges
you may find using your AVO 310 and a 1K resistor the old battery voltage is only 6 or 7 volts

   John

Offline AdeV

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Re: Fluke 78 multimeter
« Reply #10 on: June 30, 2015, 08:35:29 PM »
on a clear day like today I can see north wales

You know what they say about Wales... if you can see the mountain, it's going to rain. If you can't see the mountain, it's raining...  :lol:
Cheers!
Ade.
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Location: Wallasey, Merseyside. A long way from anywhere.
Or: Zhengzhou, China. An even longer way from anywhere...
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