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

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bdunham7:
So since my bench is now occupied with large Fluke meters anyway, I went out and retrieved the old 8500A I had purchased for parts in case I needed any for the 8505A.  Of course almost no parts are compatible and I didn't need any anyway--and the 8500A worked perfectly, so I just left it alone.  At one point I adjusted Ibias and input zero so that I could use it as a null in place of an electrometer that I don't have.

So after a warmup, it seems to work just fine and since we were discussing input impedance and capacitors, I set up a test.  I put a 0.47uF mylar cap on the inputs and then put a 10V source on it, then removed the source.  Two hours later, the measured voltage has just dropped below 8.8V, giving an apparent input impedance of about 150G. 

SilverSolder:
How do you actually adjust the input zero and bias - is there actually an adjustment, or does it mean changing fixed components?

The 8502A, 8505A, and 8506A seem to share a number of compatible parts.

bdunham7:

--- Quote from: SilverSolder on January 29, 2021, 11:01:19 pm ---How do you actually adjust the input zero and bias - is there actually an adjustment, or does it mean changing fixed components?

The 8502A, 8505A, and 8506A seem to share a number of compatible parts.

--- End quote ---

Under the cover of the DC Signal Conditioner module there are two trimpots.  You set it to 100mV range and connect a shorting plug and adjust the zero. Then you put a 1M resistor + small cap (I think I'm using 2.2nF) across it and adjust the Ibias trimpot to zero.  At 1M, 1uV implies 1pA bias current.  It's quite fiddly, especially the bias.  If you sneeze it will display HHH.HHHH H.


--- Quote ---The 8502A, 8505A, and 8506A seem to share a number of compatible parts.

--- End quote ---

AFAIK, the 8505A and 8506A are essentially the same, with the mainframe and most of the modules sharing almost all components except some special internal connections for AC and of course, the front panel lettering and indications.  I haven't actually seen an 8506A, they seem to be valued because of the novel thermal converter.  Personally I'm glad to have the 8505A because it can do AC amps, you don't have to choose between ohms and current converters and the AC TRMS converter actually works pretty well--accurate down to near zero % of range and reasonably accurate to 1MHz.

The 8502A I'm not sure about.  The 8500A has almost no compatible large parts.  Of course, I'm sure many of the individual board components are the same, but the most of the modules are different p/n, the power supply is not the same and won't fit, the module retention clips (the ones that break 100% of the time) aren't used and even the case top is not ventilated.  The 8500A actually has a very nice dual-bobbin transformer that I was hoping to use to quiet my humming 8505A, but that's also a no-go.  The 8502A looks a lot more like an 8500A with the additional buttons than anything else.

joeqsmith:
After 24 hours, the capacitor discharged to 6.8V.  LSD is basically flat.   



--- Quote from: SilverSolder on January 29, 2021, 10:10:22 pm ---
--- Quote from: guenthert on January 29, 2021, 06:05:07 pm ---
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---[..]
With that 10Gohm input impedance my little cap is not discharging very fast.  After 15 hours its down to 7.4 volts.  The LSD bins continue to look fairly even.

--- End quote ---

     I haven't read the specifications of the 8505/6A, but generally those long(ish) scale DMMs specify an input resistance of >10GOhm (in some ranges, which should include 10VDC).  That's only a nominal U/I ratio, not to be understood as plain resistor.  There will be some bias current of the FETs at the input leaking out and the AZ circuit's capacitance will want to be fed, etc. .   At 10V you should expect an average current of less than 1nA (it might be less than 100pA, but peak values might very well be considerably higher), hence the nominal >10GOhm.

    The bias current means, that a low-leakage capacitance on the input will actually get charged (not necessarily towards +10V), which can be used to determine the average current.

--- End quote ---

The specification is max 5pA bias current.  I measured mine to around 3pA with an electrometer.  I've tried to look at the circuit diagram to figure out how they get it so low, but I'm not understanding it...  analog black magic overload with stacked dual FETs etc.

--- End quote ---

I used a new 9V transistor battery in series to try and get the noise low enough to make some sort of ballparkish measurement.   With my old electrometer placed in the lowest range (20pA), standing back and let it settle looks like around 1.5pA.    Any sudden movements, over 2pA.  Walk away and don't twitch, 1.33pA.   I can believe it's <5pA. 

SilverSolder:

--- Quote from: bdunham7 on January 30, 2021, 12:12:44 am ---[...]
The 8502A I'm not sure about.  The 8500A has almost no compatible large parts.  Of course, I'm sure many of the individual board components are the same, but the most of the modules are different p/n, the power supply is not the same and won't fit, the module retention clips (the ones that break 100% of the time) aren't used and even the case top is not ventilated.  The 8500A actually has a very nice dual-bobbin transformer that I was hoping to use to quiet my humming 8505A, but that's also a no-go.  The 8502A looks a lot more like an 8500A with the additional buttons than anything else.

--- End quote ---

I have successfully swapped modules between 8502A, 8505A, and 8506A.   Of course, just because the modules "work" doesn't mean the parts can't be made to different tolerances etc. -  I haven't tested them that thoroughly (yet!... now that I had the idea...)  -  but some cards, like the GPIB card etc., should be a no-brainer.

The transformers hum loudly in every single unit I've ever laid hands on - it is safe to say this is a "feature", not a bug...   I guess it might be fixable by rubber mounting it - I have a theory that it is perhaps  too stiffly coupled with the chassis, mechanically? - but the hum is hardly the worst noise offender in the old test gear collection...  :D

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