Products > Test Equipment

Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

<< < (6/91) > >>

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!
Modify message

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

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod