Author Topic: Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)  (Read 59641 times)

0 Members and 1 Guest are viewing this topic.

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00
@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!
« Last Edit: March 05, 2021, 09:30:05 pm by SilverSolder »
 

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
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
 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 12277
  • Country: us
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..

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).   

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
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.
« Last Edit: March 06, 2021, 03:35:25 pm by dietert1 »
 

Offline mansaxel

  • Super Contributor
  • ***
  • Posts: 3559
  • Country: se
  • SA0XLR
    • My very static home page
The rear connector is an ELCO 8016- 20 Pin socket - you can find both sockets and connectors on eBay.....

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.

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

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.

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00
The rear connector is an ELCO 8016- 20 Pin socket - you can find both sockets and connectors on eBay.....

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.

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

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.

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

 

Online joeqsmith

  • Super Contributor
  • ***
  • Posts: 12277
  • Country: us
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.

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00

This instrument has enough grounds to start a medium size farm!  :D
 
The following users thanked this post: dietert1

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
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
 

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
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
« Last Edit: March 10, 2021, 10:14:43 pm by dietert1 »
 
The following users thanked this post: SilverSolder

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00

@Dietert, that is amazing.  I haven't got to first base with the high speed measurements yet - time to get something going!

One thing I've been thinking about, regarding the ADC:  Maybe we can run it one more time?  -  currently, it runs 5 times and gets to 21 bits resolution.  What about modding the software in the controller to run it "deeper" and get more bits - e.g. "real" 7.5 or more resolution?  This might require lowering noise elsewhere in the system, but...  it would be a pretty cool mod!




 

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
That's a nice proposal, shouldn't be to difficult. Yet the 8080A cannot efficiently add two 32 bit quantities. Maybe the extra bits could be joined in on the GPIB host. Or fit in an Arm MCU that supports in-circuit development and debugging like we use nowadays.

My own like to have is automated ADC calibration. Apparently ADC fine tuning with those low-ohms series trimmers isn't very stable. Guess it suffers from contact resistance in the trimmers. Don't know yet how to solve that. There should be a module to generate the test voltages like 10.1 V, -10.1 V, 0.5 V, 5.1 V ... with a multiplexer. And something that replaces the trimmers and that can be tuned by implementing one of the unused commands in the firmware. Most part of automation would be on the GPIB host. Ideally one could perform ADC calibration at normal operation temperature - with all covers closed.

By the way, that fast mode test ran for 8.22 hours until now with an average speed of 333.35 samples per second. Much more stable now since i modified GPIB operations. Seems the 8502A doesn't like premature UNTALK without having sent its data yet.

Regards, Dieter
 

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00
That's a nice proposal, shouldn't be to difficult. Yet the 8080A cannot efficiently add two 32 bit quantities. Maybe the extra bits could be joined in on the GPIB host. Or fit in an Arm MCU that supports in-circuit development and debugging like we use nowadays.

My own like to have is automated ADC calibration. Apparently ADC fine tuning with those low-ohms series trimmers isn't very stable. Guess it suffers from contact resistance in the trimmers. Don't know yet how to solve that. There should be a module to generate the test voltages like 10.1 V, -10.1 V, 0.5 V, 5.1 V ... with a multiplexer. And something that replaces the trimmers and that can be tuned by implementing one of the unused commands in the firmware. Most part of automation would be on the GPIB host. Ideally one could perform ADC calibration at normal operation temperature - with all covers closed.

By the way, that fast mode test ran for 8.22 hours until now with an average speed of 333.35 samples per second. Much more stable now since i modified GPIB operations. Seems the 8502A doesn't like premature UNTALK without having sent its data yet.

Regards, Dieter

If we have all the individual ADC readings, maybe it is possible to make up for the flawed ADC trim after the fact?  - so the problem would be reduced to feeding the meter a sequence of known voltages before running a critical test, and then compensate all the following readings accordingly?  That way we could also remove errors introduced by the active filter, input modules, attenuators, etc.

Taking advantage of the modular design by replacing the whole controller board with a modern MCU would of course also be fun, but now we are REALLY taking it to an extreme!

The A/D module could possibly be replaced by one based on the 8.5 digit A/D discussed elsewhere on the EEVblog.  Would that be cool, or what?  Like putting a supercharged V-12 into a Ford Anglia!  :D

 

Offline m k

  • Super Contributor
  • ***
  • Posts: 2648
  • Country: fi
Yet the 8080A cannot efficiently add two 32 bit quantities.

Don't know its efficiency but pretty simple it is.

Code: [Select]
      lxi  sp, 4010h
      lxi  h, 7aaah  ; 7aaacccc + 99998888
      push h
      lxi  h, cccch
      push h
      lxi  h, 9999h
      push h
      lxi  h, 8888h
      push h
      lxi  h, 0
      pop  b         ; start
      pop  d
      xthl
      dad  b
      xthl
      inx  sp
      inx  sp
      xthl
      jnc  over
      inx  h
over: dad  d
      xthl
      dcx  sp
      dcx  sp        ; end
      pop b
      pop d
      hlt
Advance-Aneng-Appa-AVO-Beckman-Danbridge-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-OR-X-REO-Simpson-Sinclair-Tektronix-Tokyo Rikosha-Topward-Triplett-Tritron-YFE
(plus lesser brands from the work shop of the world)
 

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
There are 7 byte registers in the 8080A. The ADC code in the 8502A disassembly is register based and operates on two 24 bit quantities. They use A for shifting in ADC bits and H to address the backplane, so it's really very tight. Guess that was the reason why they left it like it was in the 8505/6A.
And when you look at the ADC code, they first collect 12 bits of data with control codes 0C and 00, then another 18 Bits with control codes 08, 00 and 00. So it isn't about making a loop six rounds instead of five.

Meanwhile i have a data logging run of 16 hours with a total of about 20 million samples, so that seems to work now. With 30 second logging interval i have 10 000 samples to average and the stdev of those averages gets down to 0.03 ppm. Of course systematic errors add in on top of statistics, like misaligned ADC.

Regards, Dieter
 

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00
There are 7 byte registers in the 8080A. The ADC code in the 8502A disassembly is register based and operates on two 24 bit quantities. They use A for shifting in ADC bits and H to address the backplane, so it's really very tight. Guess that was the reason why they left it like it was in the 8505/6A.
And when you look at the ADC code, they first collect 12 bits of data with control codes 0C and 00, then another 18 Bits with control codes 08, 00 and 00. So it isn't about making a loop six rounds instead of five.

Meanwhile i have a data logging run of 16 hours with a total of about 20 million samples, so that seems to work now. With 30 second logging interval i have 10 000 samples to average and the stdev of those averages gets down to 0.03 ppm. Of course systematic errors add in on top of statistics, like misaligned ADC.

Regards, Dieter

Sounds like we're good for another two digits, LOL


Maybe it is possible to read the ADC with 6 rounds, then "spit it out" on GPIB all from inside the same tight loop...   doesn't seem like it would be much code?

We could load that code into RAM and run it from there...


Would be cool with a mod where the controller could simply load some external code and execute on boot...


« Last Edit: March 12, 2021, 09:42:03 pm by SilverSolder »
 

Offline 1audio

  • Frequent Contributor
  • **
  • Posts: 317
  • Country: us
These efforts seem to have just stopped. I have an 8506A and just acquired a less than working 8506A/CT. I would really like a pointer to the GPIB software that is getting so much data from these.

My 8506a and my 732A seem to agree to .2 PPM which seems like pure chance. The AC performance seems quite good. The specs actually seem to match what the reference thermal converters were capable of at the time.
 

Offline dietert1

  • Super Contributor
  • ***
  • Posts: 2473
  • Country: br
    • CADT Homepage
Yes i stopped working on that when i got an Advantest R6581T as a "present" of M. Reps.
The fast GPIB communication mentioned above was using a proprietary GPIB to USB interface that we made in 2009. I guess you will reach similar speed with other adapters, e.g. a PCI card. What made a difference was  blanking the meter display and using binary ADC data - doing the floating point conversion to physical units on the host. The poor 8080A in the meter is slow with floating point.

Regards, Dieter
 

Offline trobbins

  • Frequent Contributor
  • **
  • Posts: 826
  • Country: au
With respect to the first post in this thread, another factor influencing comparison accuracy would be the 50 ohm termination used.

Silversolder, what would be interesting is with the measurement made at the 8506A frequency extremes.  One hassle with cross-comparing sine amplitude for the 3325A is that few 5-6 digit meters have frequency response below 1Hz and above a few hundred kHz (well not the meters I have, so I have to resort to a 3400A for anything approaching a MHz, and beyond).
 

Offline 1audio

  • Frequent Contributor
  • **
  • Posts: 317
  • Country: us
I would be happy to make some measurements at the frequency extremes. Which extremes are you interested it? 10 Hz and 1 MHz?

However the problem I have been contending with is confirming accuracy at those extremes. I have a 540B, 931B a collection of thermal converters and still have limited confidence in any of this. The accuracy and uncertainty of the converters is a real limit to any AC measurement.

Ballantine converter spec: https://static1.squarespace.com/static/560bfd14e4b01fdcd8c45b5f/t/5a0a092f24a6942e842dd165/1510607151655/1395B.pdf

Fluke 540B spec

A55 specs

They are pretty close to the 8506A spec.

This is from NIST on an effort to make a JJ DAC to improve accuracy on AC measurements: https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=32363

8506A low frequency measurements are limited by the time constants of the thermal converter. HF by all the frequency effects from amplifiers to skin effect which could influence the accuracy. Its a real and complicated problem.
 

Offline SilverSolderTopic starter

  • Super Contributor
  • ***
  • Posts: 6126
  • Country: 00
With respect to the first post in this thread, another factor influencing comparison accuracy would be the 50 ohm termination used.

Silversolder, what would be interesting is with the measurement made at the 8506A frequency extremes.  One hassle with cross-comparing sine amplitude for the 3325A is that few 5-6 digit meters have frequency response below 1Hz and above a few hundred kHz (well not the meters I have, so I have to resort to a 3400A for anything approaching a MHz, and beyond).

I'm in the middle of reorganizing my work area, when it is back up again I'll give that a try...
 

Offline 1audio

  • Frequent Contributor
  • **
  • Posts: 317
  • Country: us
A short note re AC stability-
I have a Fluke 510A at 2400 Hz connected to my 8506A for 24 hours now. I had carefully adjusted the 510A to match my 732A with the 540B. (This sounds like some strange secret code.) The 8506A has read from 9.99999 to 9.99960 during that period in an unheated garage. I think that is well within the specs for both instruments. The 8506A was last calibrated in 2012.
I'll check its response at extremes in the next few days.
 

Offline 1audio

  • Frequent Contributor
  • **
  • Posts: 317
  • Country: us
I checked the AC accuracy of my setup. I am using a vintage Optimation calibrator and have accumulated some of the essentials to calibrate it and etc. They all disagree to some level, which is to be expected. I chose to standardized on a Fluke 10V A55 thermal voltage converter since it has the least that can go wrong.

I measured everything at 7V because measuring at 10V has the risk of an overvoltage pulse when switching ranges and switching down range and back will require a long wait for the converter to stabilize. What is encouraging is that everything is in tolerance except the 1 MHz range of the calibrator. This is actually quite a relief and I can move on to other endeavors (like calibrating the 100V range of the calibrator). . .

 

Offline trobbins

  • Frequent Contributor
  • **
  • Posts: 826
  • Country: au
Re: Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
« Reply #423 on: November 01, 2021, 03:09:17 am »
Sounds like it was worth the effort to check to 1MHz limit end  :-+
 

Offline Neptuni

  • Contributor
  • Posts: 12
  • Country: se
Re: Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
« Reply #424 on: November 12, 2021, 08:05:00 pm »
Hi folks,

I now have a 8506A and a 8505A kind of working.
The 8505 with RMS, ohms and current, the 8506 with ohms. Both instruments with GPIB.
I also bought some spare cards for the future.
The 8505 has software version 5.01 and the 8506 has 6.05
Both instruments have the same issue, the reset procedure at boot restarts continously. If I press and hold the reset button on the frontpanel the instruments will come out of the reset cycle and work normaly.
I have started to recap the instruments, but still work to do.

Mikael, Sweden
« Last Edit: November 12, 2021, 08:07:45 pm by Neptuni »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf