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

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SilverSolder:

--- Quote from: mansaxel on March 06, 2021, 08:40:39 am ---
--- 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.

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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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Interesting! -  I found it by accident, browsing eBay for some other connector, I recognized it when the picture showed up!

Yeah,  Fluke doesn't mention anywhere in the manual what the connector is - they just gave it their own part number...   apart from it probably costing $200 per connector when bought from Fluke, they likely haven't carried this part for 20 years or more!  :D

joeqsmith:
This service manual for the 8502A may be newer than what you have.  It contains the serial interface. 

http://ftb.ko4bb.com/getsimple/index.php?id=manuals&dir=Fluke

608 talks about the isolator.  608-9 covers the IC0-4.     

Table 1-1 talks about the need for the isolator for remote operations.   

2-8 has a warning about electrical shock and grounding.  They take about reversing the input leads (input HI grounded and high voltage applied to LO).  The don't mention the isolator. 

4-71 warns again about common mode voltage when using the interconnect in place of the isolator. 

I would imagine the 20V you mention would be with the interconnect installed but so far I have not found any such note.   Even with the interconnect, I would not expect to connect the meter to a PC with a common ground, then attach a power source to the inputs with the LO also tied to the same common ground and having it short out some internal 20V supply.    That's just scary.   I would expect something like this to be on the cover page with large bold letters.

It's difficult for me to follow the schematics in this manual as they split them across multiple pages.   On page 313 we can see the input transformers.   On page 314 we can see the regulated supplies.  Not how the isolated logic supply VCC is referenced to the analog -15 VA2.    So from analog return to -15 then -15 to VCC and VCC to logic return, we have 20V.  I wonder if this is what the note was talking about.    Still this is all on the back side of the isolator.

I guess like anything, when you start probing around, it's best to understand what you are doing before diving in.  I could see a beginner assuming all the grounds are common and damaging the meter or other equipment.

SilverSolder:

This instrument has enough grounds to start a medium size farm!  :D

dietert1:
In the supplies schematic p8-11 the guarded section has a 5 V logic supply U5. Its +5 V output is connected to the -15 V analog supply from U4. So when i use the guarded common ground (TP1 of active filter module) as reference, guarded logic swings between -20 V and -15 V. The screen dump shows that with the active filter Ack test point TP4.

If you wire in something on the guarded logic and pick some logic Gnd there for the logic analyzer signals, you can make a short by connecting the front panel input Low to PE.

Regards, Dieter

dietert1:
Now i have another benchmark for fast mode (3 byte readings) at 330 Hz. About half of the 3 msec period is GPIB processing. The ADC operation takes 1.47 msec. Appended i have a MSO capture that shows the GPIB board Ack signal together with IC0..IC6 backplane address. Cursors indicate the 3 msec measurement period from trigger to next trigger. The zoom shows the first run of the ADC  (backplane operations 1C 06 06 06 06 06 06), the other four ADC runs are labeled above. If we drop the '?' GPIB trigger and pulse the trigger input instead, the sampling rate could probably reach 400 Hz. If one could avoid the 500 usec delay in the GPIB host (USB adapter with MS Windows), we could get to 500 Hz.

Meanwhile i found that this 8502A contains a Thermal RMS module and the more recent Ohms module with the GAL circuit. In retrospective i think Fluke could have made these into something like the IBM PC, if they had reserved some backplane addresses for custom modules and provided a public API, maybe like Silversolder proposed. Others could have provided special modules, for example to make it into a very high resolution differential multimeter or whatever else.

Regards, Dieter

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