DM8145/VC8145 DMM modifications

[MBY]

Dave's recent video and thread about a bench DMM with a 10,000h battery life finally made me finish upon a similar idea I had for quite some time.

Dave's idea: https://www.eevblog.com/forum/testgear/10000hr-battery-life-bench-multimeter/
General discussion about the DM/VC8145: https://www.eevblog.com/forum/testgear/vc8145-thread-it_s-here-finally/

The DM/VC8145 is a relatively cheap no-name (I don't know if 'Vici' makes them or just selling them under their name) 80,000 count bench multimeter with a decent set of functions built around the Fortune FS970X chipset. I've used it for a couple of month and so far I'm pretty happy with it. It has a simple linear PSU with one rail (9V) and a lot of unused space in the case. So there is some opportunity for mods and extensions. The case has some unused screw towers that may come in handy. A battery option mod seems obvious. There is a segment in the LCD for a "low battery" icon, but sadly it doesn't come on when the input voltage decreases.

I have already posted some data and mods in the other threads, but I feel like I was hijacking those threads, so I figured it's better to create a new one for general modifications of the 8145 in particular.

The most annoying thing with the DMM (and it seems to be a growing problems with DMMs in general) is the buzzer who signals on boot (several beeps), on function switching, on range switching (even in autorange), and it's loud and not particularly harmonic. The buzzer is simply to remove or tape over, but that hampers the mode where you actually want a buzzer. Continuity mode (which is the same as the diode mode. It may be personal preference, but I want continuity to be its own mode or at least in the low-ohm mode).

A mod which inhibit the buzzer in all modes except diode/continuity is possible and turned out to be simple. Diode/cont mode is the only mode where the relays K1 and K3 are on (the unit has four relays and different modes utilize other combos). The relays are high side switched, so an diode AND-gate turning off or on the buzzer was simple enough. Just some diodes to the high side of the coils, a pullup and a transistor.

Attached is the schematic of that mod.

In the general thread about the 8145 I said that the schottky diodes probably can be changed to whatever diodes, including LEDs. But I don't think so anymore as the gate/base of the transistor need to be at a sufficient low level when the buzzer should be inhibited. 1N4146 or 1N400x is probably okay, at least with the 2N7000 that generally have >1V in gate threshold voltage. The LED D6 is to lower the voltage at the gate and in to the "AND diodes" so current from the buzzer's high terminal which is resting at 6.66V doesn't creep into the 5V rail.

The buzzer is dynamic, 16 ohms, so if desirable, the transistor could be paralleled over the DS/CE with a resistor in order to allow for a more quiet squeak when not in diode/cont mode.

I will soon post about my battery mod...

[MBY]

...forgot crucial information in picture form on where to put the mod... 

The first picture shows where to connect the AND-diodes. Disregard the four blue LEDs I mounted. They were only used to decode which relays were turned on and when. The second picture shows how to connect to the buzzer mount. Note: You don't need to remove the shield for the analog frontend in order to connect this mod, I just forgot about putting the shield back before the photo. I removed it for general reverse engineering and before I realized that I could use the relays.

Funny thing: At least on my unit, the buzzer opening was pointing upwards, and it had a marked positive terminal. That's the opposite of what the PCB says and indeed the PCB is correct about where the positive terminal is. And yes, I damaged the poor buzzer with my iron. But of course I did...

[MBY]

And now for the battery mod...

Several ways to get an "battery option" is possible. I think that a standard 7.2 Ah 12V SLA battery might fit, and if so, it's a good choice if the transformer has enough voltage. There is likely some individual variations, and my transformer, when unloaded, after rectification gives about 14V. It's just about too low for a good headroom when charging a 12V SLA, but it may be a possibility.

Now, measuring equipment often have linear PSUs for a reason. Any source of noise should be avoided, if possible. I know this, so please don't crucify me for introducing DCDCs into this DMM. Just saying, because someone ought to tell me that that's no good idea (and rightly so) otherwise...

First I doodled some quick n' dirty li ion option, using cheap (and probably very noisy) China modules as a buck, a boost and a li ion charger. Attached is the schematics. The red circle compasses the original linear PSU. The rest is my draft for a 3.7V li ion battery option.

But, since I already had four 2V D-cell SLAs just laying around I decided to create a version with Pb acid batteries instead. One of the cells are bad, but the other three I successfully desulfated a few weeks ago. So attached is also my current working solution that I built (I made an error in my last upload in another thread).

In order to be able to switch the DMM on and off, I mounted a microswitch at a unused screw tower and glued a block (a large SMD HV-capacitor btw) on the steel rod going from the front panel to the power switch at the back.

Note: The pictures show four D-cell SLAs. I'm only using three of them. The last one is a dud, but I didn't bother to separate it from the pack. It adds a little good weight to the unit. Also, the little switch connected to the batteries near the transformer are not in the schematics. It serves no other function than to disconnect the battery if you service the thing.


---------- Some measurements and data about PSU power consumption ------------

PSU---------------------------------------
Measured with: BK 2709B DMM.
9 Vdc single rail from a 7809. About 13.5V on the unregulated side.
It has a "low bat" segment on the LCD and the case has screw towers and such
that are unpopulated. A battery option is certainly possible, but the "low bat"
segment seems NOT to be connected/used as it never comes on.
Winding down the input voltage dims the backlight and the LCD starts fading pretty soon.
The measurement seems stable and reliable down to 7.55V.
I connected a PentaRef (that haven't been calibrated for 5-6 years) set to 3.9001V and the display (Vdc range) showed a stable 3.9009V down to 7.55V, and then started drifting upwards (as expected).
-------------------------------------------

Current consumption--------------------
Measured with: uCurrent on mA range, a BK 2709B DMM.
About 11-12 mA in the following modes:
mAdc, Vdc, Adc

About 44 mA in the following modes:
Vac, mVdc, Ohm (add 0.9mA on short), Cap, Temp, Out

74 mA in:
Diode/Cont

Back-light: add 9.6 mA
-------------------------------------------

[jaroslav.rusnak@savba.sk]

Hi.
Could you help me please? I need to adjust zero on mV range of DM8145. DM8145 indicated -0.0056 mV. I could set to more close to zero. If it possible.
Thank you.
Bye.
Jaroslav
Sorry of my English.

[MBY]

Hi. Sorry for the late reply. I'm worthless at looking up old posts...

My unit shows -0.0012 mV at open circuit on the V range. Does your values change if you short circuit the COM and V terminals? Mine is closer to zero when I short circuit and I also often use the delta function, so I haven't investigated how to calibrate or adjust the meter.

[lhlad]

Hello

Tool to read the contents of the eeprom in your VC8145, via the serial port.  I have 2 of these. Serial  numbers close together.
Before anyone asks I have removed the eeprom from one and read it externally and the results match.

Can be found at https://gitlab.com/phlebos-public/vc8145eeprom.git

Needs building with qt v6 use LTS version.
Tried on Windows, Fedora and Oraccle Linux OK.
Serial port needs to be a good one I use an ftdi usb adapter.

No portable binaries as yet.

Anyway the eeproms are different in some places so possibly calibration information. 
Where they don't function mods? eg stopping the bleeping while while autoranging.

If examples of eeprom can be posted with S/N( if you want too ) maybe help see what can be twiddled.