Alphatek TEK1320 environmental multimeter - Post-mortem teardown - Page 1
Forum section: electronics / general

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2012, 05:51:37 pm
content: Bought this meter faulty, thinking I could fix it.Put a new 9V battery in it and got low battery symbol. So, I knew that wasn't right. Plugged the terminals into my power supply, supplied 9V and current draw was 1A... yikes! The main globbed ASIC got very hot as did its little LDO.So, clearly the fault is in the globbed ASIC, and there's little I can do to fix that. Ah well, you win some, you lose some... My bet would be someone exceeded the maximum voltage and fried some input protection diodes.Teardown analysis. I wouldn't mind using this on low voltage circuits, but it looks like it would not meet the claimed CAT III 600V rating. The input protection consists of two TVS devices, a PTC, and a 500mA fuse. There's no fuse for the 10A range. Device is full of TL062 SO8s, I counted at least 8 and didn't check the back side. At least they only eat 400µA each, so they have chosen a low power amplifier to give the unit reasonably long battery life.Unit has more adjustments than I can tweak! Calibration by adjustments, not a good idea. If the meter is dropped or banged, they could very easily drift.RRP £55 upwards - complete rip off! Obviously as it has a globbed ASIC, it is a high volume, low cost product, and I doubt it even costs half the RRP to make.

author: SeanB
author_rank: Super Contributor
timestamp: July 27, 2012, 06:16:34 pm
content: You can buy the asic in a package and solder it on the board, after popping the old one off.

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2012, 06:35:20 pm
content: Quote from: SeanB on July 27, 2012, 06:16:34 pmWith the globbed epoxy to remove and without knowing what ASIC it actually is I'll have a lot of difficulty.

author: SeanB
author_rank: Super Contributor
timestamp: July 27, 2012, 07:17:19 pm
content: I don't remember the part number offhand but it is a cyrustek device I think that is used in a few OHL models. It is the one with the pc link built in.

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2012, 07:20:04 pm
content: Quote from: SeanB on July 27, 2012, 07:17:19 pmAny idea if it's possible to buy the Cyrustek device (is it programmed, or just configured by how its circuit is wired?) and also is it possible to remove that epoxy chip-on-board?

author: SeanB
author_rank: Super Contributor
timestamp: July 27, 2012, 07:34:43 pm
content: Must be available, i saw the info online, and my meter uses one in a globtop as well. To remove them is easy, just heat it with a hot tip ( turn to max heat) for a minute or so ( remove display from other side beforehand to keep from toasting it) and it will pop right off the board from thermal stress, leaving the pad behind and a nice clean surface.I looked at them as I wanted to add the digital output cheaply, but the dice in mine is not connected to the pads on the print, they were never bonded before encapsulation, and there are no traces to the blob.

author: tom66
author_rank: Super Contributor
timestamp: July 27, 2012, 08:25:25 pm
content: Would there be any clue on the die itself? I'll put it up again for sale as faulty in case someone knows how to fix this. But if I don't sell it I'll give it a go.

author: T4P
author_rank: Super Contributor
timestamp: July 28, 2012, 04:31:38 am
content: Quote from: SeanB on July 27, 2012, 07:17:19 pmLike... in fact this is a rebranded mastech MS8209It's the one commonly used by Mastech (V&A uses it because they copy mastech)Other than that, it costs about 77USD shipped so yes you got rippedEverything's contained in the IC and it does not have RS232 anyway

author: tom66
author_rank: Super Contributor
timestamp: July 28, 2012, 07:45:42 am
content: Quote from: DaveXRT on July 28, 2012, 04:31:38 amWell, luckily I only spent £6 on it faulty.

author: tom66
author_rank: Super Contributor
timestamp: July 28, 2012, 01:20:01 pm
content: Well, I decided to have another look in depth at this meter.I didn't notice it before but there's significant burn marks around the Volt/Ohm input jack. The PTC is fried pretty badly too. And, there are two traces that pretty bad.In addition, the inside of the back case (which I didn't think to check before) has significant burn marks and there are three burn points of interest. Under two of these are SO8s, one is a TL062, the other is unreadable. In addition, I noticed the LDO getting hot before, but just put it down to the shorted ASIC. It looks like the LDO has a small hole in it too. There's a 7660 on the board for generating a negative supply; that's outputting a positive voltage, which tells me there's a short on the -9V (if you short or overload a 7660, the output goes positive.)Tracing the circuit out, the volts/ohm jack goes into the glob-top IC through two five-meg resistors in series. That's it, no input protection except a whimsy little PTC. And these resistors are supposed to stop the full 600V+ getting into the IC.So, it's looking like there's been a pretty catastrophic event here.My guess as to the failure mode:- Somebody applies ~1000V on the input of the meter. (Rated: 600V.)- The 5 meg resistors break down causing the glob-top to see the full 1000V.- PTC rapidly burns producing most of the burn marks in the case.- Glob-top IC's input diodes conduct. Positive diode conducts first, forcing the supply rail of the meter to be briefly ~1000V.- This pulse damages at least one of the op-amps which is NOT connected through any input protection, incinerating one trace so badly the soldermask is completely missing from that area. The others might be damaged too. It also wounds the LDO, shorting its output to 9V.- The 9V going into the 3V or 5V glob-top damages it. (The surge could have also damaged it. Indeterminate...)- Internal breakdown forces many of the input protection diodes to short the outputs at 9V or 0V, forcing random segments of LCD to work (units etc.)- The surge also damages one other op-amp and incinerates another trace.The glob-top IC has many of its outputs stuck at DC 9V. This will quickly electrolyse the LCD! The 32k crystal is no longer oscillating, it's dead Jim.So, it's a pretty bad failure mode, but that's what you get for using a cheap meter...!

author: T4P
author_rank: Super Contributor
timestamp: July 28, 2012, 06:55:41 pm
content: It's not really all that cheap, it was just misusedand at 80USD it's not cheap, it's just a extreme multifunction meter that should have it's voltage and current measurements taken away for it is too riskyAfter all, mastech did note that this meter is not to be used over 600V.Meh, doesn't have a uA measurement capability anyway ... only useful for what else it throws in