EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: olewales on October 14, 2015, 08:44:06 pm
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Hello, fellow eevbloggers.
I kindly ask for your help, again. I wasn't sure if I should post it here or in "repair" section, but it might be trivial so "beginners" it is.
I am trying to repair a "TL-828" rj-45 cable tester, like this one (http://www.fs.com/tl-828-a-multifunction-network-cable-and-phone-line-tester-p-11622.html) (those things are surprisingly expensive for amount of circuitry inside) and I got stuck. I am trying to identify this component (high res photo) (http://imgur.com/4kKPrhs) and possibly looking for some generic (locally available) replacement. I've done some googling on this but nothing seems to really fit. I suspect it might be some kind of comparator. Resistors above (in the photo) seem to form voltage divider feeding 2.5V to the center pin. Left pin is fed with VCC (5V), right pin is visibly connected to ground. I see no activity on the scope on neither of two pins on the other side.
Am I missing something or it can't possibly serve any purpose as 3 pin device connected like that?
I am asking here because if it is some kind of comparator it might explain behavior of device I am trying to repair. Tester seems to send test signal along all wires but does not correctly detect signals its getting back. It either report short on all wires (even with no cable plugged it) or open circuit (when I fiddled with calibration pot and set it to one of it extreme positions).
I would be thankful for any hints on this.
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What's the make/model? Does it do any analog tests or pin or polarity only? (mine is basic, using a cmos 4017b counter and a hex 40106b hex inverter)
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What's the make/model? Does it do any analog tests or pin or polarity only? (mine is basic, using a cmos 4017b counter and a hex 40106b hex inverter)
It's TL-828-A, I linked to identical product I found in google in original post. It is supposed to measure cable length. I guess that's what that multi-turn pot I mentioned is supposed to calibrate. I don't think it has any other analog measurement capability.
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What's the make/model? Does it do any analog tests or pin or polarity only? (mine is basic, using a cmos 4017b counter and a hex 40106b hex inverter)
It's TL-828-A, I linked to identical product I found in google in original post. It is supposed to measure cable length. I guess that's what that multi-turn pot I mentioned is supposed to calibrate. I don't think it has any other analog measurement capability.
So NetWork is the brand (sorry I didn't click the other link.. more interested in the chip I guess :palm:). While searching on that model, a bunch of similar basic testers come up under "NOYAFA NF-8108 Shenzhen Co" many with similar LCD and graphics. Try posting pics of the whole PCB top and bottom - it's possible to get a service pdf or schematic help from others who've had problems. What problems are you having anyway? Do some tests pass and others fail? Is it only the TDRM length tests failing? Any burn marks or shorted MOV's?
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Some sort of LDO?
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Some sort of LDO?
I saw that in a 2007 SMD-codes databook, but I think it was SOT142 - This is SOT23.
If only I had the 2014 edition (I can only find web samples of this 4000 page monster..)
(http://www.turuta.md/images/SMD2014.gif)
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So NetWork is the brand (sorry I didn't click the other link.. more interested in the chip I guess :palm:). While searching on that model, a bunch of similar basic testers come up under "NOYAFA NF-8108 Shenzhen Co" many with similar LCD and graphics. Try posting pics of the whole PCB top and bottom - it's possible to get a service pdf or schematic help from others who've had problems. What problems are you having anyway? Do some tests pass and others fail? Is it only the TDRM length tests failing? Any burn marks or shorted MOV's?
Actually, I never thought that "NetWork" may be the brand. I just read it as generic "network cable tester" :)
Here are the photos of the PCB - link (http://imgur.com/a/FK7C8)
Board was sandwiched with LCD. I found pins impossible to desolder so I had to cut them first and after desoldering what remained replaced them with some mod wire.
All "big" ICs have markings scraped off. RJ-45 connector closer to LCD pins is "main" port. Test signals are sent out of it and response is supposed to be received by it (manual says that for cable length test the other end of the cable should remain unterminated). Test signals measured on two random RJ45 pins look like this (http://imgur.com/lCDKJAD) on the scope. I don't know how they are supposed to look like but they seem pretty complex and distinct in time for each pin. The other RJ-45 connector is supposed to be just local terminator.
Crystals are oscillating but pretty much all of the circuitry on the far end from LCD connector is only powered up during measurement. At "idle" (menu screen, showing measurement result) tester consumes about 10mA, during test this value raises to about 45mA. Measured @6V (simulating 4 AAA batteries).
All of the tests fail, but only "WIREMAP" test shows anything more besides "SHORT", "OPEN" or "FAIL" message. Specifically, it shows which wires are shorted together. By fiddling with blue multi-turn pot on the PCB I was able to get following results - gallery (http://imgur.com/a/evFrr). All tests were done with no cable connected and no - "OPEN" test result does not change after connecting working cat5 UTP cable.
Also I have removed mysterious "0C8B" device using hot air to be able to see clearly if signals from two top pins are routed at all. And surprise - they are not. Photo (http://imgur.com/oyAiBB6). Unless there is a via well hidden under the pad this is a 3 pin device. It busts my voltage comparator theory. Also, after removing this IC I have not noticed ANY change in device behaviour.
Suggestion of looking for similar devices is interesting. The unit mentioned is not exactly the same (slightly different menu structure) but it indeed seems similar. I found teardown here (http://mysku.ru/blog/aliexpress/19616.html).
I am open for any additional suggestions/ideas but I am starting to suspect failure of the ADC (it has to have one... somewhere) and in that case its pretty much BER.
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That Jetson Yejing marking with TL-828 yielded little detail. The other unit seems a notch up in ability and complexity. The A/D would be on-chip but what I can't see is any TVS to protect this unit from becoming an easy "throw-away" (unless it's tsop 16 near xtal 1).
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Ok, a small update on the original question:
I searched a bit more, this time looking only for two first characters (0c) assuming that "8b" may be some kind of batch code or something like that. Combined with "SOT23-5" in query it gave me results like this:
RN5RL30 voltage regulator (http://www.aliexpress.com/item/50pcs-Original-RN5RL30AA-T1-RN5RL30-RN5RL30AA-Voltage-Regulator-SOT-153-SOT23-5-Marking-0c/32447321085.html) - datasheet (http://pdf1.alldatasheet.com/datasheet-pdf/view/37413/RICOH/RN5RL30AA-RR.html)
and
RN5VD20 voltage detector (http://www.aliexpress.com/item/100pcs-original-new-free-shipping-Voltage-Detectors-Reset-IC-RN5VD20CA-TR-RN5VD20-RN5VD20CA-SOT-153-SOT23/32429514347.html) - datasheet (http://doc.chipfind.ru/pdf/ricoh/rn5vd20.pdf)
First device does not make any sense considering its pinout and the second one is not a great fit either, but it got me thinking. It turns out that pin 1 is not directly connected to VDD but instead it is a weak pull up to +5V. When shorted to ground tester indicated low battery with an icon. This makes "0C8B" a likely candidate for some kind of voltage controller / brown-out detector. Interestingly, apart from shorting this pin 1 of this device to ground I was not able to trigger low battery warning by any other means. When playing with input voltage I was not able to see battery icon before LCD completely lost contrast. I am now wondering if it is design fault or that IC is not working correcty.
Anyway, I think I am done with trying to repair this thing. I've checked for obvious damage, checked voltages, clocks and even tried to do some reverse engineering on the circuit operation. I don't think there is much more to be learned from this exercise. Thank you all that contributed to my efforts.