TMK 500 Multimeter

possibly

I have a multimeter with a 12V battery, which is bound, sooner or later, to disappear from the Battery Store.

Anything that doesn't use the 9V battery, is likely doomed in some decade, to be out of luck.

Imagine trying to operate that old radio that needs a couple

Paul

...and I've often fudged obsolete camera battery replacements by stacking sufficient of the right sort of button cells together (pound shop is your friend) and holding them there by heat-shrink sleeving or other means.

Nick

I had this problem with a 70 year old AVO which was the only meter I had capablel of performing a particularly demanding measurement. I just 'seriesed-up' 10x 1.5V AA cells. Cumbersome, yes, but it worked for me.

The manual at

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See

pinnerite has brought this to us :

For the AVO, they used to make the 15v up from button cells.

You can still buy the 15v battery for AVO 8, it's a BLR-121.

mm0fmf explained :

Yep, I know, but 10 cells can be had for £1 are are fine for occaisional use.

Indeed, I bought one a few weeks ago. eBay is good.

Yes this is assumedly for the high ohms ranges. I seem to recall the old Avo

Yes but 15 or 22 v is a tough ask I think. We used to do it for model control batteries in the old days with those flat rechargable deac batteries. Brian

My AVO 8s have 15v batteries - theBLR121.

And for those not used to old multimeters it is important *not* to use the ohms ranges on modern electronics, as the voltages may be destructive.

That's a reason for owning two multimeters.

If you lost the manual for the AVO, and were worried about its usage of "hi-power ohms", you could set the second meter to volts and check what voltage comes out when the first meter is on the ohms range.

The "low-power ohms" on modern multimeters is used, to avoid forward biasing junctions. The voltage used might be around 1.0V or so, open circuit.

The "diode" range likely applies a tiny bit more.

You can check, by seeing if your modern multimeter lights a white or blue LED, but only on the "diode" range, and not on the other ohms ranges. Example of a test result...

"Diode" range:

red LED (1.8V on package): Lights green LED (2.2V on pkg): Lights, but not brightly white LED (2.5V or so): No light

That tells you the diode range is enough to forward bias a lot of bipolar junctions. But the voltage isn't very high, or the white LED would have come on.

Whereas the AVO pretty well applies the entire battery voltage, but with some current flow limitation.

Indicator LEDs have a 5V PIV, as a typical (boiler plate) value. Whereas high power LEDs, some of them specifically say in the data sheet, they are not to be reverse biased. The high power ones don't have the happy-go-lucky 5V PIV statement. (PIV stands for Peak Inverse Volts)

That's why, in a PC, if the Power LED or the HDD LED won't come on, you just rotate the two wires and "try again". The LEDs in the PC panel can be reversed on the PCB header, without damage to the LEDs. Which is very handy.

Paul

Thank you for all the helpful replies. I couldn't afford an AVO8 myself and my employers only allowed our lab an AVO7, an audio signal generator, an elderly oscilloscope (Cossor I think) a huge Eveready Leclanche cell accurate to 7 or so decomal digits and a piece of wet string. At every engieering meeting we claimed that the piece of wet string was the most accurate tool we had. Much good did it do.

For calibration we had to use the 1khz tone broadcast by the BBC after broadcasting ended, the line timebase frequency of a 405 line TV, the Leclanche cell and later the twice weekly 15 minute 19khz stereo carrier frequency sent out by the BBC. The firm went bust. |I had left by that point but who would have predicted?

Ah! Those were the days.