Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
SilverSolder:
@Dietert1, I think both the schematics you are looking for are in the 8505A manual, there is a quite recent one on Xdev's site,
https://xdevs.com/doc/Fluke/8505A/Manuals/fluke_8505a_multimeter_sm.pdf
Thanks for the hint about the 8502A manual covering hardware/firmware, I don't recall seeing anything about that in the 8505A / 8506A manuals - will take a look now!
Edit: Looks like the explanation (Theory of Operation) is the same in both manuals. Well worth reading, surprisingly detailed!
dietert1:
Found the schematics, thanks.
Here i have a measurement of the Fluke 8502A backplane, addressing the active filter module. Since addressing is somewhat ambiguous i want to measure which addresses are actually used by the firmware. Also i found there may be a problem with the isolation interface on that instrument: The signals on IC5 and IC6 look a bit dirty.
Regards Dieter
joeqsmith:
--- Quote from: dietert1 on March 05, 2021, 08:37:06 pm ---When wiring up programmers and analyzers to the isolated logic in the DVM, please be aware of a 20 V offset between guard and logic ground. Better leave guard on the front panel unconnected. Otherwise all this may be just a sophisticated way to kill a working instrument..
--- End quote ---
Is this stated in the manual somewhere, or is it something you measured with your meter? I would expect with the isolator, assuming the front switch is not engauge to select the internal guard, that it would be basically isolated. The manual shows an internal 150K between LO and guard (or shorted depending on the switch position). I measure about 140K with my meter. Looking at the AC and DC voltages between the digital ground of my meter and LO as well as the guard, I get under a volts. Just some leakage.
You scope shot looks like it's just AC coupled. Almost like the isolated side wasn't referenced back to the digital side (strapping the two grounds together).
dietert1:
I read a remark in the manual about that 20 V level difference, yet i think it was between isolated digital ground and analog ground. This way they saved on the level converters for n-channel JFET switches. Yes, unrelated to front panel guard, yet related to front panel low.
My DSO has Gnd connection via the logic analyzer inputs. The Ack signal (analog channel 1) looks perfect, while bus lines I6 and IC5 seem to suffer from capacitive crosstalk and lack of drive. Will check again.
Regards, Dieter
PS: According to the isolator schematic it doesn't drive IC5 and IC6 on the isolated bus. So the isolated bus has five address bits IC0 .. IC4 and in order to learn which addresses the firmware actually sends i need to look at the unguarded bus.
mansaxel:
--- Quote from: SilverSolder on January 28, 2021, 05:58:08 pm ---
--- Quote from: bdunham7 on January 28, 2021, 05:43:53 pm ---
--- Quote from: SilverSolder on January 28, 2021, 03:16:02 am ---The rear connector is an ELCO 8016- 20 Pin socket - you can find both sockets and connectors on eBay.....
--- End quote ---
You made my day! I was absolutely convinced it was Amphenol for some reason. That cross-pin or tuning fork design is pretty interesting--the male and female ends using the same pins. Now I'll be able to make a 4W Ohms harness and test the X-REF circuits to make sure they're not blown up by whatever burned the 39K resistors on the input board.
--- End quote ---
It took me something like 2 years to figure out what that connector is... it is one of the best kept secrets on the Web! :D
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I saw it immediately. Very common in UK-sourced broadcast gear. My first interaction with one was back in 1988 or so, at which time I'd not yet learnt about the importance of pair twisting in multicore audio cables. That was a memorable experience in crosstalk.
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