Author Topic: How I repaired my Fluke 8050A 's (Power Supply & rebuild of the Hybrid Ceramic)  (Read 1351 times)

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Offline Satbeginner

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  • Dutch, early retired, living in Spain
Hi all,

As a summer project I bought 2 "for parts, as is, broken, not working, etc." Fluke 8050A's on Ebay.

They both came from the US, and since I live in Spain, I was prepared to face some Power Supply issues.

When they arrived, they both turned out to be Battery Versions.
Of course, all the batteries were 'dead', so I opted to remove them and convert the meters into 'Mains' versions.
 
In short: it turns out that inside the Battery Versions, the transformers used by Fluke do have a 240V winding, where as the "Non battery (both EU and US) versions" use a fixed -one voltage only!-  transformer.
So in that respect I was lucky, I did not need to look for other transformers, I just needed to change the position of this white wire inside the unit.

Several people already suggested a mod to change the LM317 current regulator into a fixed voltage power supply, I choose to do something else.
I replaced the LM317 with one of these small,  cheap LM2596 based Switch Mode DC to DC Step Down converters.

After this modification, both meters came alive, one just needed cleaning and calibration, with the other one the AC circuitry was not working.......  :-//
(And yes, both display's were OK   8) )

After some troubleshooting and checking the signals from the input to the Ceramic RMS converter I found the following parts broken:

U22, the 4-way analog switch CD4016, and U31, a dual comparator LM393.
After replacing these, the meter (AC-part) was still not working, so there was a problem in the RMS converter too.

I tried to replace some of the parts on the ceramic, but soon it turned out that soldering on this part did not really work.

So, I decided to rebuild it. ( "We have the technology!!"   ;D )

I think the engineers at Fluke in those days tried "to confuse the Russians", because they did not only put the RMS converter on the ceramic, but also quite a lot resistors that are part of dividers and amplifier circuits on the meter's mainboard.

I designed and build a small prototype board containing the resistors to replace the ceramic, but the RMS converter I did not rebuild.
I simply replaced it by using an AD536 Analog Devices True RMS converter, that is also placed on the ceramic 'replacement'.
So now it is a sort of DIY Hybrid.

Important: because the gain of the new RMS converter is higher than the original ceramic, U33 must be jumper-ed completely. (Pin1 to Pin4)

As connector I soldered a female, inline connector on the mainboard, and a 90-degree male pin connector on my replacement 'ceramic'.
This way it became a sort of plug-in.
I intentionally kept the right side of the board bigger, so it is impossible to accidentally reverse the board.

In the attachments some pictures and the schematic of things.


Un saludo from Spain,

Satbeginner




Some remarks on the calibration:

R7 (AC gain) is now combined with the gain on the new RMS converter.
R29 (Offset) is no longer used, and replaced by the offset on the new RMS converter.

So, first calibrate the DC settings.
Now put R7 centered.
Then set the meter on 200mV AC, and short the inputs.
Now adjust the new RMS offset for 0V when on 200mV.
Then, when inputting a 2V PP 200HZ AC adjust the new RMS gain for 0,707V DC on TP11.
Now adjust R7 for 0,707VAC on the display.

The rest is like mentioned in the Section 4 Maintenance, of the Service Manual.
Only when the R29 is mentioned, use the offset on the new RMS converter.


« Last Edit: July 20, 2016, 12:34:15 pm by Satbeginner »
You need a scope to repair a scope, and you need many multimeters to repair another multimeter!
*Tek 2467B, Tek 2465B, Tek 2465B, Tek 485, Tek 475A,  Keithley 175A, Keithley 2000, HP 3468B, HP 3457A, HP 34401A, PM 6671, PM 5716, Fluke 45, Fluke 75, Fluke 77, Fluke 79, AFX 9660BL, KPS 605D, etc. *
 
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Offline retiredcaps

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Re: How I repaired my Fluke 8050A 's (Power Supply & AC measurement)
« Reply #1 on: July 19, 2016, 04:19:52 pm »
Welcome to the forums and thanks for sharing your modifications on your very first post.   :-+
 
The following users thanked this post: Satbeginner


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