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Old 2nd Dec 2019, 12:27 pm   #1
ajgriff
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Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
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Default Fluke 77 Series II Repair Using Donor

It’s now around thirty years since these meters were in production and it’s often necessary to find a donor if repairs are needed to a cherished example as parts are now in short supply. However, it is not always appreciated that there were some significant design changes towards the end of the production run and that some parts are not readily interchangeable between the two versions. The attached photos show some of the differences. The earlier version is on the left with the later type on the right in each photo.

Photo 1 - On the earlier version the yellow button symbol under the legend ‘PRESS’ is a solid colour whereas on the later example it’s a yellow outline. This represents a simple way to distinguish between the two versions.

Photo 2 - The lugs for the lower fixing screws on the upper half of each case are of a different design.

Photo 3 - The later version has an additional lower case moulding (arrowed) that protrudes through slots in the circuit board. Also, the pillars through which the case screws pass are taller and on the circuit board the components themselves are in different positions or have disappeared altogether.

Photo 4 - The ‘FLUKE 77 SERIES II MULTIMETER’ legend and the analogue numbering are printed on the LCD mask of the late version and the mask is blank on the earlier one.

Photo 5 - The ‘FLUKE 77 SERIES II MULTIMETER’ legend and the analogue numbering is printed on the inside face of the LCD window on the earlier version and the window is completely clear on the later one.

Some consequences and considerations regarding these design changes are highlighted below for the benefit of anyone wishing to swap parts from one version to another:

1. Fluke documentation refers to the circuit board and the attached LCD unit as the ‘PCA’ (Printed Circuit Assembly). An early PCA can’t be fitted to a later lower case because of the additional case moulding (Photo 1). However, a later PCA will fit an earlier lower case.

2. An early upper case can’t be mated to a later lower case because of the changes to the design of the case screw mounting lugs (Photos 2 & 3). In fact if this swap is attempted and the screws are tightened the upper case will break just below the LCD window. However, a late upper case can be fitted to an earlier lower case.

3. If a case half and/or a PCA can be interchanged (as above) it becomes necessary to swap the LCD window or mask in order to avoid a rather disconcerting shadow effect with the legends or their complete absence (Photos 4 & 5). It is better to swap windows rather than masks because the masks crack easily when being removed. Alternatively, the whole LCD assembly can be swapped (4 screws) allowing the contacts to be cleaned at the same time.

4. It is possible to carefully reduce the height of the lug pillars of a late lower case with a small file in order to make an earlier upper case fit. Similarly, and with great care, it should be possible to break out the additional moulding from a late lower case in order to fit an earlier PCA. The latter isn’t something I’ve tried due to the risk of causing peripheral damage to the case.

5. It should be noted that any case swapping, with or without modifications, will have some effect on Fluke’s original efforts to meet safety standards applicable at the time of manufacture. It is a matter of judgement as to whether or not this has significant implications in terms of using the meter being repaired.

6. I can’t comment on the interchangeability of individual components between the different PCAs as I’ve never found any definitive documentation covering the later type. The downloadable manuals I’ve come across on the Fluke website and elsewhere are unhelpful in this regard. As far as a serial number change point is concerned the best guess I can offer is somewhere around 68000000 based upon the relatively few meters I’ve seen.

7. If a donor meter is required it may be best, in the first instance at least, to try and find a suitable candidate of the same type as the meter needing repair.

Finally, if anyone is contemplating trying to find a replacement upper case because of the commonly occurring fault of sheared switch end stops it’s actually quite straightforward to a carry out a satisfactory repair. Simply remove the range control knob, drill a small hole where the end stop used to be and replace the stop with a small piece of hard plastic super-glued in place. When the control knob itself is re-positioned and the meter is re-assembled the repair is invisible.

I hope that some of this is helpful to anyone contemplating the acquisition of a Fluke 77 Series II meter as a donor, either now or at some point in the future. If not, it was fun compiling this write up anyway.

Corrections, general comments or additional information would be most welcome. I don’t know for example whether or not the same issues apply to other meters in the Series II range (75, 73, 70, 23 or 21).

Alan
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Last edited by ajgriff; 2nd Dec 2019 at 12:42 pm. Reason: Correction
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