Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
joeqsmith:
I had removed the serial interface to change the cal switch. 4-68 Line 6, R64 will not come in. I pulled the covers from the Fast ADC and had a look. Problem appears to be the resistance module. Once removed, I was able to make it to 4-77. That's a job for another day.
Shown adjusting R64.
joeqsmith:
Looking at this Fluke 8505A, they have a good photo of the resistance board. It has a date of 1991 and uses a GAL. The schematic in my manual shows no GAL and a different layout. The board I have is dated 1983 and does not look like their photo or match the manuals schematic.
https://xdevs.com/article/f8505a/
Their photo appears to match the schematic in this manual:
http://ftb.ko4bb.com/getsimple/index.php?id=download&file=Fluke/Fluke_8505A/Fluke_8505A_6.5_Digit_Multimeter_Service_Manual.pdf
Mine appears to be something between the schematic in my manual and this one above. The one in the manual I have uses a single coil for K1. Where the board uses a dual coil.
SilverSolder:
Nice, she's looking happier! - The circuitry in these meters are full of JFETs acting as switches... they can go leaky and cause issues. I had a problem with a Filter module, where the switching between different filter sections was not working well - causing the Average reading to be significantly lower than the plain reading (because Average mode automatically turns on the "heavy" filter section). I can easily imagine a leaky FET in a module loading the analog bus and causing false readings, if the module cannot properly disconnect from the bus.
I made some progress with my Unit 4 with the noisy ohms: I looked at the sense current with another meter and an oscilloscope, comparing the noisy unit to a good one. The current turned out to be equally noisy on both the "near perfect" unit and the broken one - so there is something else wrong than the current generator circuit. More work for another day!
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SilverSolder:
--- Quote from: joeqsmith on January 24, 2021, 04:21:13 am ---Looking at this Fluke 8505A, they have a good photo of the resistance board. It has a date of 1991 and uses a GAL. The schematic in my manual shows no GAL and a different layout. The board I have is dated 1983 and does not look like their photo or match the manuals schematic.
https://xdevs.com/article/f8505a/
Their photo appears to match the schematic in this manual:
http://ftb.ko4bb.com/getsimple/index.php?id=download&file=Fluke/Fluke_8505A/Fluke_8505A_6.5_Digit_Multimeter_Service_Manual.pdf
Mine appears to be something between the schematic in my manual and this one above. The one in the manual I have uses a single coil for K1. Where the board uses a dual coil.
--- End quote ---
There is a revision letter on each board, if you take off the cover and look at the PCB. I have encountered MANY different revisions of boards...
I have encountered revisions not covered by the above table too!
SilverSolder:
The newest manual I've seen is also on the Xdevs site:
https://xdevs.com/doc/Fluke/8505A/Manuals/fluke_8505a_multimeter_sm.pdf
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