8060A Design Details and Mods

[drtaylor]

I am starting this topic as a place holder for modifications and technical questions on how to keep your 8060A operating. Please use this topic as a place to share questions and your solutions. Although primarily about the 8060A, other antique 80xx series DMMs can be covered here.

I have been asked more than once on how to extend the Diode Test Range to allow Blue/White LED's Vf to be measured. To explain why it doesn't work requires a review of the 8060A design. To measure Vf, the 8060A switches in a simple Fet based Constant Current diode that is set to 1mA. This 1mA current passes through the 1k (+-40%) PTC and the 1k input protection resistor. These alone will drop 2V at 1mA. That still leaves probably around 5V (depending on how fresh your 9V battery is) for an external Blue/White LED. So it will glow dimly. But it will exceed 2V Vf. The switches are set up so that the sense voltage goes directly into the 8060A A/D input set to the 2V range. Since the native measurement ranges of the 8060A MAC are 200mV or 2V, this limits Vf to 2V which gets most Red, Green, and yellow diodes, at least at 1mA. However GaN and other chemistry LEDs will be well above 2V, even at only 1mA excitation. It is theoretically possible to add a divider in the ohms sense line to divide the sense voltage. I'm going to experiment with means to do this, but I fear that the mod might affect other ranges. So I'll report on that later.

Remember that when the 8060A was designed, Blue LEDs did not exist. I did test it with typical LEDs of the era, and almost all would be below 2V at 1mA. I remember only a couple of brands of Green LEDs would be out of range at 1mA. This feature was primarily designed to check diodes and junctions in BJTs.

I want to stress that most LED Vf ratings are reported at 20mA and that any mod to the 8060A to provide that kind of current would put most LEDs out of the 2V range. Not to mention that the protection components would have to be made far smaller in value. So moding the diode test current, while fairly simple, would make the 8060A even less useful for LEDs. So I am going to experiment with mods to divide the ohms sense by 10. I can see a fairly easy way to do this, but I am not sure if I can get the decimal point in the right range. As is, the diode test range has enough compliance to light Blue LEDs, but the Vf will read over range...still sufficient for a go, no-go test.

If you have a constant current power supply, the superior way to measure Vf is to set the current on your power supply to 20mA or any other current you might need, and then just use the 20V range (any range > 4V) of a DMM.

[SeanB]

I agree, a lot of DMM diode test will not work with white/blue LED's. I find there are few with a high enough open circuit voltage to even light them, so that the 8060 can is quite good.

I would not really worry about that, a much better test is to simply use a spare CR2032 cell and an optional 100R resistor to test the LED, as the cell thickness is enough to fit the space between the leadouts on leaded or SMD diodes, and the voltage is enough to test single diode modules. Multichip modules can have 2 up to 30 diodes in series, so need a much higher voltage anyway.

[switcher]

Dave, do you have any ideas on feet? A few of my 8060As are missing some of the little rubber feet.
Welcome any ideas for a source.
Many thanks.

[Napalm2002]

I apologize if the questions are In The wrong thread, have you sold your ibm 8060a yet? Have you any blue housings left?
I have bought some new caps for the charge pump sections of my 2 8060a and 8062. They are my most favorite meters to use. Well I do like my fluke 289, but the 8060a has class!

Thanks Dave!

Mike m

[xwarp]

DrTaylor,

You by chance have the schematic for the U8 hybrid for the 8010/8012? Have any stashed away?