benchtop multimeter repair CDA-9651 - Page 1
Forum section: electronics / testgear

author: Ddrmax
author_rank: Newbie
timestamp: March 03, 2019, 01:22:30 pm
content: Hello,I'm a owner of an old Chauvin Arnoux benchtop multimeter (french brand)This multimeter has no documentation availableThe problem I have is that after running for a long period of time, the multimeter shows in any range a severe drifting (for example it shows 1,5 V and up in the 20v range) when nothing is plugged in the banana plugsFor the time being I checked and replaced some capacitors (the small electrolytic ones) because I have seen that the two black ones are used to create a virtual groundFor now I haven't tested if the drifting occurs for nowThe thing is :- either old capacitors have their microfarads within specs but an ESR of half an ohm (the new ones for test that are crap from Amazon (rated for audio and KSC branding) have like an ESR of a tenth of a ohm ( tested with an 15$ component tester kit bought on Amazon)- after the change in cold readings it seems on par with my owon an RS multimeter in the 20 /2 volts and 200mV range- before the change it was slightly drifted (like 10mV in the 200mV range in cold readings ( the two readings were done with the multimeter shut down for about an hour)If you want I can do a reverse engineering of it since it has only one big chip that do all the crap ( other components are only passives in the first sight) other than an TL080 op-amp in the bottom corner of the first pictureSo do you think these caps could be the culprits?These caps are 100uf 25V and the white one (changed altogether) is a 220uf 25VThe original brand is soyco for the white and no recognizable brand on the two small onesThe big one is a nichon chemical 2200uf 25VAfter checking the photos the main IC is an 7107 marked TSC 7107RCPL and it's vintage is 1989 (same marking for the TL080 )Envoyé de mon Z2 en utilisant Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: March 03, 2019, 02:36:49 pm
content: The pictures are not very sharp - so I can not read the chips. Chances are the large DIP40 Chip is an ICL7107.As the meter also reads AC, the TL080 chip is likely used for AC and not as an extra input buffer. This would mean the DC input is directly from the ADC, with no more amplifier in front.Is the offset coming up, only when the input is open ?If so this would point to something like an input bias current, that could come from some protection device (diode or diode used transistor) at the input.If the offset also happens with a short, chances are the problem is with the ADC chip.A first point to check would be the supply - too much ripply could upset the ADC. I don't think the changed caps where the culprits, a few ohm ESR are likely OK for old style caps.The yellow tantalum caps are slightly suspect. Some of that look tend to go short over time. It may be possible to check them in circuit with the diode test of a DMM - just make sure to get the right polarity.

author: Ddrmax
author_rank: Newbie
timestamp: March 03, 2019, 03:47:51 pm
content: When I will have some time I will do some better photos and I will try to reverse engineer it ( it's an old style 7107) and if I recall correctly the drift stays when I short the leadsI will do the test you said but in two partsOne before powering upOne after a big warm up (if it shows up)But the problem is that I need to do the reverse engineer before and Mark with a Sharpie each components with a unique name since the PCB is has no markings ( only a few one directly on the copper near the edges of the PCB)Maybe you know if I can replace the 7107 with a newer one (CPLZ) marking instead of the actual RCPLEnvoyé de mon Z2 en utilisant Tapatalk