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Ballantine 3205B Microhm meter

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Vgkid:
-----Ballantine 3205B Teardown-----
All images are linked to larger versions.
So I always wanted a milliohmmeter, ever since Robbenzs teardown of his LOM-510.  The commonly available milliohm meters on ebay that are relatively compact include the Keithley 580(possibly the most high specked milliohm meter, also several multiples more expensive than the other options). Next up is the Simpson 444(these go for a couple hundred, these use an ultra low current,5-ma). Various Valhalla models, each one did something different, look into Valhalla site for more info. The Ballantine 3205B, these are rather uncommon(I thought that it looks very much like a Valhalla instrument), then lastly the awesome LOM-510. Then there are the much newer hioki's that are on the internet. To bad more companies did not make microhmmeters. Then you have the newer Tonghui's/Applents/Extechs, and those nifty microhm meter based on the hx-711 chip, eg DannyF's.
Back to the meter.

Now we can see the front of the unit. If you have ever seen any of Valhalla's test gear, you will see why I thought that this was a rebadge. The measure button is only needed for non pulse resistance measurements. The Zero trimmer is used to null out the thermal EMF's. The slightly newer, and much more exspensive hp3456A actually does this by taking a measurement with no current, then with current, and subtracts the thermal emf's in processor.
   On top of the unit is nothing but discolored beige plastic, with a ring of epoxy on it. :(

Flipping the meter over we are greeted with a  placard detailing the user instructions, and the accuracy specs. I wish these were still included, heck my Digibridge feature a bunch of info relating to impedance calculations :D .

Taking off the top of the unit we can look at the display board. The to-18 transistors are bc109's. I did not make note of the to-92 transistorsThe solitary ic is a DM7447, a BCD to 7-segment decoder. The only difference between the A-B is the additional digit of resolution.

Looking at the mainboard we see a rather full board.
The large filter capacitor is a Uptron branded capacitor. The only information I can find is that this was an Indian firm that went out of business in 2002. The bulk of the electrolytic caps are Indian branded Websen produced by license from Rubycon. The non elco caps are made by Desai Electronics PVT.

Taking a look at the back half of the mainboard reveals an interesting dip-8 ic. The icm755 is a high performance 555 timer. I think this is for the pulse mode. The intersil 7103 is a 4. digit a/d
The gold lidded cerdip is another 4.5 digit chip, Not sure what that one is for. I have only had time to do some basic testing/ teardown of the unit.

Looking at the front of the unit, we are greeted by my LEAST favorite type of switch...
The dip-16 by the input is a ICL7652 Az opamp. At least that is really nice.  The red resistors are wirewound resistors by the indian firm PEC, datasheet here: http://peccomponents.com/psp.htm
Every range has its own current set by the resistors.
I will add some more tomorrow.





Vgkid:
Now that the unit is home, I will do a quick test before going to bed. Milliohms will be tested with the L&N 4385, Ohms with the ESI-655.
Milliohm     Recorded     Error
2000            1999.5           +0.025
1000            1000.2           -0.02
500               500.0              0.00
100                99.98             0.02
050                 49.99            0.02
020              19.986             0.07
010              9.997               0.03
Ohms          Recorded        Error
2000             1999.7            0.015
200                199.21            0.395
20                    19.104           -5.47
Unfortunately I'm not sure if there is a problem with the meter, decade resistor, wiring, or everything.
Will investigate later.

Vgkid:
Well when I was putting everything away last night, the leads were still hooked up, I noticed that the readings became rather jumpy, I assumed it was the connection. Well I quickly discovered that the conection from the coaxial resistor to the back soldered connection was flakey, even coming loose.
Grab the soldering iron, and 1 hour later, it works. Ok that hour was exaggerated, but it took me longer than I wanted to. :-[
Now back to testing.
Ohms      3456A      Recorded      Error
1990      1989.9      1990.9         -0.05
199      199.03      199.20         -0.08
19      19.026      19.054         -0.14
For fun I hooked up my ESI SR1,1k
1000      1000.06   1000.9         .084
Something is definitely wrong with the ohms ranges. I will update this in a few.

Vgkid:
Haven't started troubleshooting it yet, but had a few musings. I ordered the 3205A manual off epay, BL wants $95.00 for it, that is the price of the meter...
I had a few musings over the source of the error. The 2000mOhms, and the 20 Ohm ranges share the same current source.At that current level, if the measured voltage is 2v, there must be another gain 100 amplifier that is off, as there is only 200mV(max) across any resistor.  I will fire up the 3456a, and the 87, and test some rails/voltages.   Mainly this is a placeholder text, and ramblings.

Vgkid:
Testing resumes
3456a      3205b         voltage
19.036      19.054         .19053            
199.04      199.20         .199183
1989.9      1991.0         .199087
Checking the highest milliohm range
1.0288      1028.8         .010294      
It looks like the voltage sensing portion is ok, but the current generation is out. If this used the voltage reference from the A2D then it is fine.   
So I went and measured the reference resistors in circuit.
1-.9923
10-9.9236
100-99.2468
1000-992.572
10000-9.92694
Mini update, messed around with the 10k range setting resistor. Desoldered it, and its parallel higher valued resistor. Hooked up the DB-655, and noticed that the output reading was 40 counts higher than it should be, Lets dial back the resistance, ended up with the decade set to 9,936K when the reading read 1000 .0Ohms, perfect, now to repeat this later, with the other ranges.

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