No all zeners are good references - the usual cheap ones are more for a supply
Of course! I use special precision zeners on 6.4 and 9.0 V. I think that good trimmer resistor don`t give perceptible drift on 4.5 digits.
Chances are fixing the old meter may still be easier than building new - as long as it does not use special unobtanium parts. Of cause this would not change things.
This multimeter called V7-38, made in USSR in 1988. Most other soviet universal voltmeters ("unversal voltmeter" is the name of multimeter on soviet standard) had metal cases, good relays with gold contact pads, very good precision resistors, military-version of ICs and transistors, some universal voltmeters had interfaces (mostly soviet version of GPIB) for connect to computers, etc. Together with that this voltmeters had great weight (metal cases!) and dimensions, and very expensive cost. V7-38 have plastic case like Gainta G7xx series, electronic components are trivial plastic DIP. By reason of simple circuit this voltmeter have some problems:
- 10M input resistance on all ranges. 0.2V and 2V ranges does not need input divider;
- Type of measurement (VDC, VAC, resistance) selected by manual switches (this switches have doubtful quality), thus no possibility change type of measurement remotely;
- Doubtful PCB tracing;
- Used a lot p-channel MOSFET for autorange switch circuits. This transistors are rary today and if they burn out, there is nothing to replace them;
- Sadly, obsolescence. Similar voltmeter can be makes on a 7135+7447, some RMS-to-DC IC converter and current generator for a resistance meter. Doubly sad that in USSR was made analog of 7135.
Fix all this bugs very difficult and quite expensive. Easier buy another multimeter... or made it
These are all fully compatible, and have no quality difference.
This was what interested me most. Thank you!
The 7135 is intended for direct and easy driving of an LED display, and I don't know any new A/D which provides this possibility.
On the other hand, microcontrollers are used everywhere today. Simple and fast autorange, connect to computer, mathprocessing, digital calibration - 7135 is not able to do any of this.
Another specialty is a 100kHz tuning fork quartz for precise 50 Hz mains suppression
When I designed my version of 7135 voltmeter, I use 2 MHz quartz with logic CMOS IC divider on 10. 200 kHz frequency should give faster a/d conversion. Is there any reason for using 100 kHz specifically?