Electronics > Projects, Designs, and Technical Stuff

10bit binary to 3 digit BCD (fixing an old nixie tube bench multimeter)

(1/4) > >>

SK_Caterpilar_SK:
I got some very nice looking school grade Tesla manufactured nixie tube multimeters. In fact three of them. The display ooks absolutely nice and the nixie face, I can only say its pure old tek pornography. Only if it was working.

In the old days they used to use a all in one 3digit 4bit BCD ADC. The C520D. This IC came in carious flavours like the soviet german C520D, the american AD2020, or the intersil CA3161, or perhaps the newest one the NTE 2054. The thing is in all the three multimeters I got the same fault occured, the ADC fried. It still converts the analog to digital, but the multiplexing order is off. Specifficaly its most signifficant digit. Interestingly this happened in all of the meter.

So here I am making a decision. While a replacement part /any of the above mentioned/ would cost 25 dollars would work, god knows how long it would. Since all the meters have the same fault I am really hesitating to try and put a new IC into it. Maybe thats why they were shelved for 15-20years in school. But back in the days the IC was basicaly available at your local drug store....not anymore. Maybe the NTE2054 with the new processes and all that work better and longer but what is the guarantee that it wont fry the second I turn the meter on.
(no it does not appear to be a design flaw in the meter.)

So instead here is my solution to the problem:
10bit paralel ADC and convert the binary into 3 BCD digits. 10bit is the closest I can get to get the full 999 range of 3 digits (that is 25 less than what 10 bit limit is)
This all sounds fun and easy and no....I am not using an arduino or FPGA or anything like that. I am a analog nerd fighting with audio stuff a lot and analog stuff in general, and I hav absolutely no interest in copying something from the internet that I wont understand at all.

SO the real problem is conversion of 10bit binary to BCD. I have found a IC (74184 and like that) which are long obsolete and unobtanium, so I guess a built up converter it is. I just dont know how at al. If anyone could somehow point me to something expandeable or draw up somethin for me with logic gates or TTLs or CMOS I would be gratefull. And please also keep in mind that I am a total noob when dealing with digital tech. Even the basic digital tech.

Thanks.

edavid:

--- Quote from: SK_Caterpilar_SK on January 30, 2020, 11:09:59 pm ---The thing is in all the three multimeters I got the same fault occured, the ADC fried. It still converts the analog to digital, but the multiplexing order is off. Specifficaly its most signifficant digit.
--- End quote ---

This is not clear - what are the exact symptoms?


--- Quote ---So here I am making a decision. While a replacement part /any of the above mentioned/ would cost 25 dollars would work, god knows how long it would. Since all the meters have the same fault I am really hesitating to try and put a new IC into it. Maybe thats why they were shelved for 15-20years in school. But back in the days the IC was basicaly available at your local drug store....not anymore. Maybe the NTE2054 with the new processes and all that work better and longer but what is the guarantee that it wont fry the second I turn the meter on.

--- End quote ---

The guarantee is that you check the voltages on all the pins to make sure there is nothing that will fry the IC.

Looks like you could get CA3162s from China for around $7.

If that's too much, it would be easier to patch in an ICL7135 than to use a binary ADC.

SK_Caterpilar_SK:
I would be gratefull If you could send me the link for that 7dollar 3162. I have tripple measure everything and everything is fine. The meters are not flawed.

MSD is out of sinc: garbage on the pin. Out of sync pulses all over, so the digits overlap. While NSD and LSD are in proper timing. I checked this with a 4channel scope with the IC separately on a breadboard. All the faulty meters have the same problem with the same IC with the same order of MSD NSD LSD. One of the meters had the transistor (which was switching the high voltage for the MSD) was fried and replaced but the problem remained no matter how I interchanged the three ADCs between the meters.

I wanted to build a complete 10bit to BCD converter because then I could just make it bomb proof and not hope it wont die again. Also if any part of it dies I can then specifficaly point to the fault point. I have a solution how to do the multiplexing that is the easy, but even if I would buy the ADCs you said cost 7 dollars I would still like to make the full circuit. Maybe one day I would decide to make a 4 or 5 digit nixie tube multimeter with custom nixies xenon filled for fun who knows.

Thanks.

floobydust:
I have a bench digital multimeter that blows a digit driver if it gets ESD or HV overload at the input jacks.\
I was puzzled, but it turns out the PCB is a bad layout, a trace close from the input switches to the decimal-point trace and it arcs over there to the digit driver.

Said another way, the multimeter (+) input jack goes to the range switch. The range switch also has a layer that goes to the display circuitry to move the decimal place. Any zap or arc from the two switch layers or across the pc board can kill the display circuitry. Does that make sense?

OP these IC's failing, could be due to ESD arc jumping over to the decoder-display driver. Have to see pictures of the PC boards and switches.
The A/D should have basic overload protection with clamp diodes, you could add them.

SK_Caterpilar_SK:

--- Quote from: floobydust on January 31, 2020, 12:48:01 am ---I have a bench digital multimeter that blows a digit driver if it gets ESD or HV overload at the input jacks.\
I was puzzled, but it turns out the PCB is a bad layout, a trace close from the input switches to the decimal-point trace and it arcs over there to the digit driver.

Said another way, the multimeter (+) input jack goes to the range switch. The range switch also has a layer that goes to the display circuitry to move the decimal place. Any zap or arc from the two switch layers or across the pc board can kill the display circuitry. Does that make sense?

OP these IC's failing, could be due to ESD arc jumping over to the decoder-display driver. Have to see pictures of the PC boards and switches.
The A/D should have basic overload protection with clamp diodes, you could add them.



--- End quote ---

yes that is a valid option aswell. Knowing it was operated by a lot of people in scool it could have gotten zapped, but the unput curcuit and power supply and digital section have their own sections on the board. If the ic would have been fried by ESD the whole board would be toast including the input side. The range switching is done on a vertical board that has no digital components to it.. I will attach pictures of the board later today (its 2 morning). I dont think it was cause by bad layout. Experience wise Tesla (czechoslovakia) built things like tanks and that included the boards. Never had one fail because of a bonehead mistake done on the board layout and I always make sure nothing is statically charged before I begin to stick probes into something (which is not common practice in schoool...). I suspect the component choice in the high voltage portion (switching the nixies on and off) because at the time component choice was very limited by cost and availability.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod