Amprobe AM83 repair - Page 1
Forum section: electronics / repair

author: andypandyplod
author_rank: Contributor
timestamp: August 15, 2015, 04:11:43 pm
content: Hi,I got this at a boot Fair.It was fully working then.After about a few years it started giving odd readings?. When I looked inside,an IC marked U4 (see photo),a 4011 device had blue crystals around its legs?.I thought it was IC adhesive,or old flux gone wrong? It had corroded some of the IC pins,and PCB pads. I removed the original IC carefully as was possible,and replaced it with a new one. The meter now works,but gives random results and has about + or - 25% difference,when alternating the probes on a 3v battery. This fault is also reflected when measuring mains AC. I have also lost some of the button functions. I have looked for other possible faults ,like the rotary switch, battery,and fuse connections. I had trawled the Internet for a complete circuit without success. I know its an outdated meter , and should throw it away, but I like a challenge. Any Help Please? thank you

author: Smith
author_rank: Frequent Contributor
timestamp: August 15, 2015, 05:04:53 pm
content: 1. Have you cleaned the PCB thoroughly after repair?2. Did all buttons work before the repair?3. Is the measurement stable, and repeatable?

author: mikerj
author_rank: Super Contributor
timestamp: August 15, 2015, 05:30:52 pm
content: It may be that corrosion has entered one of the vias and open circuited it.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 15, 2015, 10:57:34 pm
content: I have never used a 4011 so I had to look up the datasheet, but it states that it is 14 pin IC.Solder pads 10, 11, 12 and 13 are missing from U4? Did you repair them or trace out those pins to the next closest component? Or am I missing something here?

author: onlooker
author_rank: Frequent Contributor
timestamp: August 16, 2015, 01:07:50 am
content: Yes, apparently half the gates on 4011 are not used. In that sense, pads missing should be ok. Just hope the other pads are not weakened too much.

author: tautech
author_rank: Super Contributor
timestamp: August 16, 2015, 01:42:39 am
content: Quote from: onlooker on August 16, 2015, 01:07:50 amNo it won't be OK.As with all 4000 series IC's any unused gate input MUST be tied to VCC or GND, not left floating.

author: andypandyplod
author_rank: Contributor
timestamp: August 16, 2015, 11:16:07 am
content: Hi,answer to replies.Cleaned PCB in the area around IC, All buttons worked before it faulted.Measurement is not stable.Readings are around the correct voltage,+ or - about 15 to 20%. .Don't know what via's are?.Are they the connections through the pcb itself? The missing pads,unfortunately came off the pcb,whenI unsoldering the damaged ic. None of these pads were connected to the circuit in any way.Only half the ic is used.I am aware unused pins should be connected to vcc or ground,I have tried this and it makes no difference. I made a note of the ic circuit connections ,and repaired any missing pcb tracks.I checked for continuity of the repaired tracks. I don't know what this ic does,possibly it is part of the automatic zero circuit?I am unable to find out what the large 80 pin device is because the piezo sender is stuck to it with foam.A simular circuit using a purpose built ic may help? I have attached two photos showing replacement ic in place.Any more ideas are welcome

author: onlooker
author_rank: Frequent Contributor
timestamp: August 16, 2015, 11:37:13 am
content: QuoteWell, letting it float was the choice of the original designer of this meter. pads missing does not change anything electronically from the original design. Maybe, you meant it was a bad design?"No float input" may be a good general guideline for handling cmos ICS such as 4000 series. Though, some 4011 (e.g. Fairchild) already had internal input protection.

author: andypandyplod
author_rank: Contributor
timestamp: August 16, 2015, 11:52:27 am
content: the pads were not connected,just helping to hold the ic to the board.

author: retiredcaps
author_rank: Super Contributor
timestamp: August 16, 2015, 09:04:14 pm
content: Amprobe, in general, gets others to make meters for them. If this meter was made in Taiwan, it could be an APPA Tech meter just rebadged as Amprobe.http://www.pewa.de/DATENBLATT/DBL_AMP_AM-80erSerie_DEUTSCH.PDFA number of questions/comments/suggestions.1) Can you post a picture of the entire pcb? Both sides?2) Have you cleaned the input jacks with IPA?3) Check the solder joints on the input jacks for solder cracks.4) Do the input jacks feel solid if you wiggle them?

author: retiredcaps
author_rank: Super Contributor
timestamp: August 16, 2015, 09:26:51 pm
content: The IC that is used in the Amprobe AM87 is the KAD7001 as perhttps://www.eevblog.com/forum/testgear/is-this-amprobe-am87-trms-genuine/The AM83 might use the same?

author: andypandyplod
author_rank: Contributor
timestamp: July 11, 2017, 05:22:23 pm
content: Fault was found to be a 'via' linking the top side of the printed circuit to the bottom side (double sided PCB)It was hard to find because it was hidden under the ic 4011 .thanks to all who helped.

author: Robomeds
author_rank: Frequent Contributor
timestamp: July 14, 2017, 12:31:21 am
content: Thanks for closing out the thread with the solution!