[OMG Resistance] [Need Cal Help] Repairing and calibrating two simpson 360-2s

[iamdarkyoshi]

So I got a crapload of stuff from my college, they were just throwing it all away (including four GENUINE hakko 937's, they all work) below is a sneak peak of whats in store for possible topics in the future


But for now, I want to get these classic digital/analog multimeters working.
I did what Dave told me, and that is not to turn it on, but take it apart. So I took it apart, used my contact cleaner on all the switches and they turn so much cleaner now. The only thing I saw of concern was a resistor on the board that looked a little... toasty. It was under another resistor of the same physical size on the right side of the PCB (when looking at it from the back), above the AC/DC/Off/charging switch.

Both meters have a toasty resistor here. Other than that, everything inside looks fine. Range switches are in great shape, a small amount of corrosion on the arms of the contacts but nothing to write home about. So I put it back together and just hooked up a 5V power supply to the battery terminals, as the device is made to use four rechargeable C size cells, and I literally do not have any C cells at all. They work great on 5v.

I turned it on and measured its own power supply, and it read about 4.8v. My newest multimeter which is one of these cheap turds measured 5.2v, so at this range, the first simpson multimeter is about 7.7% out of the actual value (according to my crappy "reference")

The second one was far worse! It measured 6.5V! That is a 25% error!!

So I decided to leave them alone for now and get your feedback. I bought some USB powerbank circuits to put into these meters so I can make them rechargeable from a USB power supply, and run off of lithium. Should have pretty good battery life with a few 18650s in them.

So what do you guys think about these meters? I know they are very old, but if I can get them calibrated to at least the accuracy of my couple year old innova multimeter, I think they would be good bench multimeters. Plus they should be a neat repair project!

[iamdarkyoshi]

Made some progress!

I know this is not in the spirit of vintage tech, but I made it run off of a USB powerbank for the time being. I plan to put one inside the battery compartment and replace the charge jack with a USB jack (for charging)




My two "reference" meters are surprisingly close, in this range at least.



My test setup is a variac and 48v AC transformer with bridge rectifier and capacitor with bleed resistor. I used this to calibrate AC/DC voltage and current. For calibrating current I just used four 25w resistors in parallel as one great big load and just put all three ammeters in series with the load.

So far I have all voltage ranges (ac and dc) calibrated within +/- 1% of my "reference" EXCEPT for the HV range. Not sure how to calibrate this, because there is no seperate trimpot for it. After I get these things calibrated, I will bring them to college and compare them against the fluke meters there. There is also a nearby cal lab but they would probably just laugh at me

The 10A and 2A current ranges are also within cal, but can someone explain to me the other current ranges? It looks like all of the milliamp ranges are done through the 2A jack? Why do none of the readings make sense? All it does is move the decimal point by one place...

[iamdarkyoshi]

Well I managed to fit three 3,000mAh lithium cells inside for a total of 9,000mAh, and the powerbank PCB manages charging/discharging/voltage regulation. Works great, the meters last ages before needing a charge.

Stil have not checked the resistance measurements, have not fixed the HV range calibration, have not worked out the weird current ranges, and still have not worked on the second meter.

[iamdarkyoshi]

Oh dear me. The ohms range measures 12 ohms with a 22 ohm resistor connected. Can someone help me look for the fault?

[dacman]

Do you have a manual and have you tried the zero adjustment?

a. Connect test leads to the + and COMMON terminals.

b. Turn the function selection switch to the DC OHMS position.

c. Turn the range selector switch to the 200mV position.

d. Short the test leads together.

e. Allow the instrument to warm up for 15 minutes.

f. Using an insulated screwdriver, adjust the potentiometer through the hole located directly below the analog display for a numerical display of 00.0.

[iamdarkyoshi]

Do you have a manual and have you tried the zero adjustment?

a. Connect test leads to the + and COMMON terminals.

b. Turn the function selection switch to the DC OHMS position.

c. Turn the range selector switch to the 200mV position.

d. Short the test leads together.

e. Allow the instrument to warm up for 15 minutes.

f. Using an insulated screwdriver, adjust the potentiometer through the hole located directly below the analog display for a numerical display of 00.0.

Yup, zeroed adjustment is perfect. I have had it opened and figured out what each of the pots do, and got everything trimmed I could with them (AC/DC 2/10A current and voltage ranges are perfect, HV AC/DC is not though

Manual with schematic: http://www.simpson260.com/downloads/simpson_360-2_user_manual-1975.pdf

These boards have no silkscreen though :O