Zhenxing DT-890 voltage reference upgrade

[technix]

I tried to calibrate my Zhenxing DT-890 (low battery) against a MC1403 reference chip. It was way out of calibration.

I was trying extra hard to find the reference device of my Zhenxing DT-890 and I cannot find one. Some Chinese documents I read told me that DT-890 used a resistive divider across the battery voltage as its reference. Fail.

Upgrade plan: get rid of the upper and lower fixed resistors R12 and R13, replace R12 with a short and leave R13 open, and dead bug the reference and ground pins of MC1403 directly across the trimpot.

[Vgkid]

Since it has a icl7136, that has its own internal reference.

[technix]

Since it has a icl7136, that has its own internal reference.

The schematics told me that it is not using its internal reference. I have to add an external one or rely on battery voltage.

[technix]

Putting it back for now. Calibrating it requires significant effort and tongue angle control. 

Also the trimmer is a normal trim pot, not a 10-turn pot, which makes calibrating extra difficult. Probably should send it to a cal lab for this.

[retiredcaps]

I tried to calibrate my Zhenxing DT-890 (low battery) against a MC1403 reference chip. It was way out of calibration.

I'm not sure what you are trying to do.  Please clarify for us.

If your multimeter battery is low, getting incorrect readings is expected.  If you put in a fresh battery, do you still get 2.61 for a 2.5 reference?

[Vgkid]

It looks like using a 2.5v ref would be easiest.

[technix]

Putting it back for now. Calibrating it requires significant effort and tongue angle control. 

Also the trimmer is a normal trim pot, not a 10-turn pot, which makes calibrating extra difficult. Probably should send it to a cal lab for this.

Last time Tektronix cal lab charged me 100 bucks for a Fluke 289 calibration. You sure you want to cal a 830B?

Government-owned cal labs here in Shanghai provides calibration services to general public at a very affordable price.