Author Topic: Ancient 5 1/2 digit DMM - voltage with floating inputs  (Read 4980 times)

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Offline Dave92F1

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Ancient 5 1/2 digit DMM - voltage with floating inputs
« on: January 30, 2015, 05:20:40 pm »
I picked up an ancient (~1975) 5 1/2 digit DMM on eBay (Systron Donner 7205).

What should a DMM like this read for voltage with the inputs floating? After disconnecting from a test 5 volts, the reading gradually drops toward zero, but incredibly slowly (~300 microvolts/second). It's like there's a capacitor in there that's slowly discharging.

Is that normal on a DMM of this vintage? The input impedance is speced at "> 1000 MOhm".

It also gives crazy voltage readings when it's first powered up (no leads attached). But it goes close to 0 when I short the leads (needs a calibration).

Other than that it seems to mostly work, altho it's a bit "flaky" - sometimes it works great, other times does odd things. I think the giant multi-pole button switches in it need cleaning, but haven't figured out how to get at the wipers. I tried 100% ethanol (drops from a syringe) + vigorous working of the wipers, but it didn't seem to help much.
 

Online Vgkid

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #1 on: January 30, 2015, 06:06:26 pm »
That is normal for any high input impedance multimeter.
post up some pics.
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Offline Dave92F1

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #2 on: January 30, 2015, 06:18:41 pm »
Thanks; I suspected as much but no other DMM I have does that.

If you want to see what it looks like, there are good pics on this page: http://www.amplifier.cd/Test_Equipment/other/Systron_7205.html

Mine looks the same except that the digits are neon orange. It uses a Panaplex gas discharge display (the real reason I bought it - if I can get it working as a reliable DMM, that's a bonus).

Tell me what you want to see and I'll post some pix of my own (this weekend).
 

Offline codeboy2k

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #3 on: January 30, 2015, 07:45:33 pm »
I picked up an ancient (~1975) 5 1/2 digit DMM on eBay (Systron Donner 7205).

Oh, you got that one! I was watching it for days too, undecided if I should buy it or not, then I thought... nah.. I don't need another 5-1/2 digit multimeter, I wanted my next multimeter to have more resolution.  So I passed on it, but I just love anything Panaplex, it was what caught my eye.  That, and the fact that the seller had the manual with the schematics, too, made it less risky.   However, I wanted more resolution on my next meter so I ultimately let it go to someone else.. that was you. Nice to see someone here got it. So I'm definitely curious how this comes along for you.  Do keep us informed!

Regarding the schematics, there are no 7205 schematics on the net. Do you have the ability (and time) to scan the manual with schematics?  It would be a great service to the net-at-large to have it at the BAMA site and /or at ko4bb.com

Quote
What should a DMM like this read for voltage with the inputs floating? After disconnecting from a test 5 volts, the reading gradually drops toward zero, but incredibly slowly (~300 microvolts/second). It's like there's a capacitor in there that's slowly discharging.

Is that normal on a DMM of this vintage? The input impedance is speced at "> 1000 MOhm".

As answered already, yes, the slowly counting down numbers in the DC range with leads floating is normal for any high-impedance input DMM. As long as it's stable with a real-voltage, there's nothing to worry about (aside from accuracy!)  You can bring your hands closer to the leads and it will pick up a charge from your hand, and bump the numbers up :)

Quote
It also gives crazy voltage readings when it's first powered up (no leads attached). But it goes close to 0 when I short the leads (needs a calibration).

Other than that it seems to mostly work, altho it's a bit "flaky" - sometimes it works great, other times does odd things. I think the giant multi-pole button switches in it need cleaning, but haven't figured out how to get at the wipers. I tried 100% ethanol (drops from a syringe) + vigorous working of the wipers, but it didn't seem to help much.

It's probably going to need a good cleaning, and check the power supply rails and caps ESR if you can. They will be old by now, might need replacement.
I've seen the other site, but do post some pics of yours, please.

Congratulations, it looks like a great meter to own!
 

Offline Dave92F1

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #4 on: March 19, 2016, 02:46:07 am »
Sorry, somehow I only saw your response now (15 months later).

It took a bit of work, but I got it working nicely; after calibration it's quite accurate. I did have to replace one of the Panaplex modules. I also modified the power switch (that knob wasn't a good idea) and wired it so the switch only controls power to the display - the rest of the electronics are powered all the time now (so at thermal equilibrium).

Re the manual, because of the large foldouts, the only practical way I have to scan it is by taking photos with a camera. One page of schematics (page 6-11) is missing - looks like it only got printed on one side of the paper back in 1975.

If there's a interest I don't mind taking photos of the manual.

« Last Edit: March 19, 2016, 02:53:38 am by Dave92F1 »
 

Offline radhaz

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #5 on: March 19, 2016, 04:47:17 am »
Please post all of it to one of the above mentioned site, for internet posterity.
 

Offline lowimpedance

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #6 on: March 19, 2016, 06:58:16 am »
I too have a 7205 with issues and the schematics would be very handy as would the adjustment procedure. If you get the time to upload that would be much appreciated.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline Dave92F1

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #7 on: July 30, 2016, 05:35:41 pm »
Better late than never? I hope.

The manual is here, on my website: http://nerdfever.com/files/Systron-Donner%207205%205.5%20Digit%20Digital%20Multimeter%20(1974).pdf

About 100 MBytes.

You're welcome (indeed, invited) to repost to manual archive sites.

FWIW, the original manual was missing pages 5-21 and 5-22 (parts list, won't likely be missed) and page 6-11 was blank. (It's included in the PDF.)

Other than that, it's all there.
« Last Edit: July 30, 2016, 05:42:59 pm by Dave92F1 »
 
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Offline TAMHAN

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #8 on: July 30, 2016, 09:56:27 pm »
Hello,
may I ask how you digitalized the manual?

I think I have the only surviving copy of the SM for the Systron Donner 6153...and no way to digitalize it ;(

Tam
Feel like some additional tamile wisdom? Visit my YouTube channel -> https://www.youtube.com/user/MrTamhan for 10min tid-bits!
 

Offline blueskull

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #9 on: July 31, 2016, 12:58:16 am »
Nowadays standard input resistance for 2V or 200mV range on benchtop DMMs is 10GOhm.
 

Online Vgkid

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #10 on: July 31, 2016, 03:26:15 am »
That 3814 is a rather interesting ic.
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Offline David Hess

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #11 on: July 31, 2016, 07:03:36 pm »
That 3814 is a rather interesting ic.

Tektronix used the 3814 in their early 4.5 digit multimeters and some of their embedded 4.5 digit voltmeters.  My guess is that it was pretty common during that era before fully integrated MOS solutions became available.
 

Offline Dave92F1

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #12 on: July 31, 2016, 11:54:22 pm »
may I ask how you digitalized the manual?

I ran the standard-sized sheets thru my scanner/printer. The foldouts I spread out and photographed. Then merged everything into one PDF (various free online tools merge JPGs and PDFs).

I tried to get enough detail to see everything clearly (esp. the schematics).
 

Offline lowimpedance

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #13 on: August 01, 2016, 01:21:05 am »
Better late than never? I hope.

The manual is here, on my website: http://nerdfever.com/files/Systron-Donner%207205%205.5%20Digit%20Digital%20Multimeter%20(1974).pdf

About 100 MBytes.

You're welcome (indeed, invited) to repost to manual archive sites.

FWIW, the original manual was missing pages 5-21 and 5-22 (parts list, won't likely be missed) and page 6-11 was blank. (It's included in the PDF.)

Other than that, it's all there.

All good things come to those who wait  :).
Thanks for your effort, this will be very helpful.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline trobbins

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Re: Ancient 5 1/2 digit DMM - voltage with floating inputs
« Reply #14 on: June 06, 2018, 08:35:18 am »
I also have a 7205 and manual, so I've scanned the schematic pages and dovetailed with Dave92F1's scan and posted to:
https://www.dalmura.com.au/static/Systron-Donner%207205%205.5%20Digit%20Digital%20Multimeter.pdf
I had to repair blown fuses F1 and F4 (paralleled new fuses and inconvenient to pull out).  The displayed value sometimes just slowly ramps up, with no change in value from range switching (except for decimal point) and it is pushed to zero if TP1 is grounded, so appears to be related to U1 area - the displayed value then returns to normal with a few more button pushes.

I also scanned the earlier 1971 vintage 4+ digit Systron-Donner 7004 meter manual.  Unfortunately my meter has two 5750-S nixie displays with bad sections, and sadly those aren't easy or cheap to come by.  Pity there are no niche pcb modules on ebay with a 74LS47 BCD to 7-segment driver and pads for a single digit display - I reckon four of those could be fairly easily retrofitted for the digits, and any leftovers used for spares for the last bit.
https://www.dalmura.com.au/static/Systron-Donner%207004%20Multimeter.pdf

Ciao, Tim
 
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