Short Circuit SDR - Now Fixed

My SDR stopped working sometime last night. I've unplugged it from the BeagleboneAI and done a little checking. The 5VE is s/c. The 5V is fine. I've also checked across the caps of the 1V, 1.8V and 3.3V. all seem high resistance so it's pretty certain not the BGA that's s/c. What does the 5vE feed? I've looked at the cct diagrams and cannot see what the 5vE feeds other than out to the BB pins (it's unplugged from the BB). There are a couple of caps either side of the T301 which all of course are essentially in parallel. They all read s/c, So, question is, what does the 5vE feed on the kiwi board other than out to the BB? Or is it one of those 4 caps that have gone s/c?

Any help gratefully received. And I'd got it all working so well with the BBAI and a temp of between 45 and 48C

Comments

  • I had assumed that 5VE (5VEXT) would be tied somewhere to 5V but apparently not. So my best guess is 5VE feeds power from the kiwi barrel connector to the BBG, then the BBG would provide the other power rails (including 5V) to the kiwi. When you did your tests was the kiwi still coupled to the Beagle? Did you apply power to the kiwi by itself and measure the power rails?

    Nick

  • Also check with a non wired barrel plug inserted, I think it may short the socket when disconnected.

  • Thanks for quick response. If I plug power into the sdr without the BB it immediately pulls my bench power supply set to 5V and 2.3A down to about 1V. I've not chosen to increase the current up (the PSU is capable of 6A). The T301 double coil gets quite warm. If I increase the current up further then I'd be likely to burn out T301. The fact the coil gets warm, if it is a cap shorted, indicates it's one of the two caps on the far side of the coil from the i/p side. However neither of those little SMT caps are getting warm. I suppose the two coils could be shorted to each other internally.

  • @njc: Correct. 5VE (external) loops power from the Kiwi DC barrel jack, through T301 and those caps, to the BBG where it is checked/switched by the BBG's power controller IC. Then it loops back to the Kiwi board on 5V (and also 3.3).

    @Gibsonmb: If you have a hard short then try and find the component responsible. Disconnect the Kiwi board from the Beagle and feed it current limited 5V. Use an IR camera (if you have one) to see who gets hot (T301 or which cap). Alternatively, coat the suspected components with isopropyl alcohol (IPA). The IPA will evaporate much faster on the one that is hot.

    T301 and the caps are not 100% essential. So they can be removed with a small soldering iron (and a steady hand and magnification). If T301 is removed just bridge the pads.

  • @Gibsonmb: Okay, you posted while I was typing. Yes, most likely one of the caps shorted. Just knock them off one-by-one with your soldering iron until you find the culprit.

  • T301 the common mode choke melted internally..... bridged for now while I try to locate a new one. thanks for help all as always. Just fyi @jks that business of being kicked out of ssh every few minutes or seconds is internal to the kiwisdr. It is happening to me on the local LAN as well.

    Worth noting for others using the BBAI that the choke appears to be at risk. I am also using the CPC BBAI fan cape... draws 190mA off of the BBAI 5v rail.

  • what physical size are those two caps (C202/C203) I just removed? So I order the correct size off eBay.

  • One melted common mode choke

  • They're 0402s, which are stupidly small for hand assembly. If it were me I wouldn't bother. I'd bridge T301 and be done with it.

    njc
  • njcnjc
    edited May 1

    I agree, those caps are not needed. The choke probably be could be omitted (shorted across) too...

  • edited May 1

    I have a somewhat similar case here, where the 3.3 has a hard short on the Kiwi board.

    I already removed C301 to no effect. Feeding it with a current limited external power supply the EEPROM stays cool and the FPGA seems to absorb all the heat. Checking resistance across the capacitors on the backside of the FPGA shows shorts on most of them.

    I guess I should be looking for someone with the tools and dexterity of njc to bring it alive again.

    Ben.

  • Ben,

    Since it seems like the problem is under the FPGA, I would try a reflow operation. No guarantees it would work but not much to lose either. If you have access to a heat gun capable of 500°F or so you could try it. Use kapton tape over the parts you don't want to move.

    Nick

  • edited May 2

    Thank you Nick for your advise.

    I will try to find a well equipped notebook service centre first that can do reflow work before trying that myself. In Taipei I would know where to go, but in Jakarta that could be a bit more difficult.

    Ben

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