Tuesday, July 12, 2011

Speedtouch ST536v6 Repair

Came home one day to find my modem’s lights were all out. Abit of probing with a multimeter found the adapter to be dead. Thinking that it was just the adapter, I dug out a replacement and put it on. Things went downhill further when something on the board released it’s magic smoke and the modem wouldn’t sync properly (everything was there but the ADSL link light wouldn’t come on steady).

Visual inspection near the vicinity of the smell, I found the receiving buffer, an STmicro TS613I part visibly burnt. Oh well… I don’t really like throwing out stuff, even though I’ve got a couple more spare modems on hand I think I’ll fix this (it’s been working very well for the past 4-5yrs actually).
A search online on my favorite parts suppliers (RS/Farnell etc.) didn’t net me any results, they didn’t carry the specific part. Then I thought I’d try TI, they ship samples out fast and Burr Brown has some insane Opamps I’d like to try.

A search on TI’s keyword based system found me this.


OPA2613, it’s even listed as a TS613 replacement, aha! I thought it’d be a drop in replacement, but it looks like it’d take a slightly lower max voltage. The rest of the specs (noise, slew rate etc.) were looking better then the original part. I ordered a couple of samples and they shipped it out within 2hrs through FedEx International Priority with a tracking number.

3 days later I had the part in hand, I love the super fast shipping, thumbs up to them, and I proceeded to remove the old part, replace it, and then make modifications necessary to make it work reliably.


Here I’ve taken out the fried TS613, I think the part failed after prolonged use because the input voltage drifted over it’s max rating (the voltage source is fed from a LM7812 with a couple of external resistors to set it’s output voltage close to 14v, however one had drifted in value due to the heat or maybe it drifted because of the fried TS613? Wouldn't know for sure.) and I measured 14.5 (not a safe working voltage for sure, datasheet for the OPA posted a max rated working voltage of 12.6), I don’t know why they went with such a small margin for error, and it really seems like they were pushing the envelope (even the TS613 recommends it’s operating supply to be @ +/-6V, making it a total of 12V). I’m guessing they needed the extra voltage to make sure the output wasn’t clipped or something.

Here’s the fried TS613


Prepped the board with some flux and wicked off the excess solder. I had to remove a capacitor off the board for some clearance around the part.


Here’s everything in, OPA2613 and capacitor.


I modified the supply back to 12V (from 14) by removing the 2 external resistors and then tying shorting the GND of the 7812 to the board’s GND.


It now works, I’ve yet to experience any sorta dropout in connection yet. Will update if it does die or something again.

No comments:

Post a Comment